Arctic Matters: How a Warming Arctic May Bring More Extreme Weather

(click here to download a pdf of this bulletin or a one-page synopsis)

by Jennifer H. Shakun

In this bulletin, we delve into an active area of climate research that is investigating how warming may affect atmospheric circulation and lead to more extreme weather. In recent years, major flooding events, heat waves, and droughts, have prompted researchers to examine the conditions that produced those extreme events and ask whether climate change might be playing a role. Meanwhile, a number of Climate Smart Land Network (CSLN) members have noted that extreme and/or persistent weather is creating challenges for forestry operations (e.g. extended wet periods that require equipment to be pulled out of the woods). Here we examine whether those weather patterns are symptomatic of an emerging trend and what to expect in the future.

The Dynamics of Extreme Weather

In a previous bulletin we described how climate change will alter the frequency and intensity of some weather and climate extremes, like record heat, heavy rains, and drought (see Climate Change and Extreme Weather Part I: Trends & Projections). Those categories of extremes are related to the physics (i.e. thermodynamics) of climate change—rising average temperatures lead to more record-breaking heat, a warmer atmosphere holds more water vapor leading to heavier rains, and so on. It is relatively straightforward to measure these types of extremes and there is a high degree of confidence regarding how they will change in the future.

But warming can also affect the mechanics (i.e. dynamics) of the climate, including aspects of atmospheric circulation like wind and pressure systems. Dynamics are the key to understanding and predicting weather at the regional level, including future changes in extremes in the mid-latitude regions where most CSLN members own and manage land. These are the kinds of changes that can lead to counterintuitive impacts like warming winters that include deep cold snaps driven by southern incursions of Arctic air (which we discuss below). Changes in atmospheric circulation are more difficult to consistently and accurately measure or model than the thermodynamically-driven changes mentioned earlier (Palmer 2013; Shepherd 2014). Nonetheless, this is an emerging area of research worth watching because, as one researcher put it: “Though the uncertainties are large, changes in atmosphere dynamics have the potential to cause rapid transitions at a regional scale leading to surprises for society” (Coumou et al. 2018).

It’s all about Persistence

The persistence of a weather system can quickly elevate a moderate event to something more extreme. A day of rain is no problem, but multiple days of rainfall become an issue when saturated soils lead to flooding. Likewise, hitting a new record high temperature may just be an interesting statistic, while a long heat wave can have real costs for energy use and human health.

Persistent extremes can happen when weather systems move more slowly or when there is atmospheric blocking that causes a system to stall in a particular location. A persistent low pressure system leads to rain and flooding (such as the record rain with Hurricane Harvey), whereas a persistent high pressure system leads to heat, drought, and wildfire (such as the conditions in California in summer 2018) (Mann 2018). One of the big questions in atmospheric research right now is whether climate change is leading to more persistent weather patterns.

The Arctic Connection

In recent years, there has been a lot of conversation in the media and elsewhere about the idea that change in the Arctic—an area that is warming twice as fast as the rest of the world (Osborne et al. 2018)—is leading to unusual weather in mid-latitude regions like the U.S. This includes the so-called “polar vortex” events, when Arctic air drifts south and causes a deep freeze over parts of the northern U.S. In fact, the polar vortex is nothing new (see box, below), but there has been a vigorous debate among scientists about whether the kind of jet stream activity that leads to these incursions of Arctic air is becoming more likely because of warming-induced changes in the atmosphere.

Colloquially, the term polar vortex has come to mean extreme cold from the influx of Arctic air over the continental U.S. (Lyons et al. 2018). In fact, the term has been around since the mid-1800’s and it is described in scientific papers beginning in the late 1940’s (Waugh 2019). Technically, it’s short for the circumpolar vortex, which is a rotating area of cold air and low pressure, high in the atmosphere above the poles. When it expands in winter it can occasionally send Arctic air south into the U.S., which has happened several times over the last few decades (NWS; Waugh et al. 2017). But winter will still be warmer on average going forward, even if we experience more cold snaps from polar vortex events. Changes in the polar vortex are a good example of how warming could actually increase climate variability in certain regions (as we discussed in a previous bulletin).

The jet streams are fast-moving “rivers” of air flowing around the planet that are driven by the temperature difference between the equator and the poles on a spinning globe. They are also the major factor driving our weather at the surface, which is one reason why there has been an explosion of science on this topic over just the last five years or so. Mathematical theory, observations of the atmosphere, and computer models are all being used to understand how accelerated warming in the Arctic might be influencing the path of the jet stream and our mid-latitude weather, including extreme weather in summer (Coumou et al. 2018; Mann 2018) and winter (Screen 2014; Rasmijn et al. 2016; Cohen et al. 2018a; Kretschmer et al. 2018; van Oldenborgh et al. 2018). There are complicated debates going on within the scientific community, but they can be broken down into essentially three questions about the influence of a warming Arctic:

  • Can it influence the jet stream?
  • Has it done so already?
  • Will it have an effect in the future?

(Barnes & Screen 2015)

Evidence suggests changes in the Arctic certainly can have an influence on the jet stream and much of the debate is centered on the question of exactly how. A variety of pathways have been proposed, including a smaller temperature difference between the equator and the poles, a jet stream with a wavier route or larger (i.e. higher amplitude) waves, changes in the location or strength of storm tracks, or weakening of the polar vortex that makes incursions of Arctic air happen more often (Barnes & Screen 2015; Taylor et al. 2017; Cohen et al. 2018b).

Many of these pathways lead to more extreme weather, especially more persistent weather systems that bring prolonged wet or dry periods. For example, the reduced temperature difference between the equator and the poles may slow the westerly wind speed of the jet stream, causing it to become highly wavy and move more slowly from west to east, which would lead to more persistent weather patterns and deeper troughs of cold air from the Arctic (Francis & Vavrus 2012; Francis & Overland 2014; Francis et al. 2017). Other research has suggested that we may increasingly be seeing conditions that cause atmospheric waves to resonate and become amplified (much like resonance increases the intensity or loudness of a sound from a musical instrument), which is also associated with more persistent and extreme weather (Petoukhov et al. 2013a; Palmer 2013; Mann et al. 2017).

There is disagreement about whether there has already been a discernable effect on the jet stream (at least in a statistical sense). This is because there are different methods being used to detect change, there is a lot of natural variability in the jet stream, and we only have the necessary observational data for the last few decades, which is too short a time period to confidently determine to what degree observed changes are related to human-induced climate change (if at all) (Barnes 2013; Barnes & Screen 2015; Screen & Simmonds 2013; Petoukhov et al. 2013b; Screen et al. 2018).

As for whether the warming Arctic will be a significant factor affecting mid-latitude weather going forward, the scientific consensus seems to lean toward the idea that it probably will, but the jury is still out (Barnes & Screen 2015; Taylor et al. 2017). There are a variety of factors in play and it is unclear whether other forces will outweigh or counteract the influence of the Arctic. Although, some research has suggested that these trends are only just beginning to emerge (Francis & Overland 2014; Mann et al. 2017), so it is likely that the science will continue to rapidly evolve on this issue.

Consequences for Working Forests

Over the last several years, a number of CSLN members have reported issues related to stagnant weather patterns that bring prolonged periods of rain or drought.

Forestry Operations

Persistently wet weather, in particular, has put a significant (and in some cases unprecedented) burden on the transportation and logging sectors in some regions. It is also affecting the number of good operating days and leading managers to plan for less predictability by developing alternative harvest plans and schedules. The ability to take a more nimble approach and “make hay while the sun shines” has become a real advantage in some circumstances. Examples include:

  • Taking advantage of drought conditions to conduct summer harvests in areas that were historically winter-only (with the added benefit of evening out harvests throughout the year)
  • Developing backup harvest plans
  • Increasing logging capacity to cut more wood during a shorter season
  • Changing the size of logging jobs to make them easier to complete in a shorter period or easier to harvest piece-meal over multiple years
  • Moving logging crews around to drier sites during wet summers

Some of these adaptation approaches have major implications for logging contractors in terms of equipment and personnel. In regions where extreme, persistent weather patterns have been reducing the number of operable days per season, this has become a real challenge for the traditional business model of the logging community. Beyond harvest operations, CSLN members describe a similar notion of preparedness and being “ready to go when conditions are good” in relation to forest fire fighting efforts, especially with longer dry spells and warmer temperatures creating conditions ripe for fire.

Forest Impacts

In a previous bulletin (Climate Change and Extreme Weather Part II: Forest Impacts), we described how “extremes play an equal or greater role [than average conditions] in shaping the distribution, survival, productivity, and diversity of plant communities.” Much of the predicted forest change will be driven by extreme events, especially when those extremes exacerbate other types of forest stress. CSLN members have already reported instances where persistent extreme weather is compounding other stressors. Examples include heavy rains that saturate soils leading to increased wind throw and forest stands with unexpected signs of decline due to the combination of drought and forest tent caterpillar.


Ultimately, the impact of a warming climate is not as simple as rising temperatures. In a fluid dynamical system like our planet, global change is complicated and will lead to some counterintuitive impacts and surprises. This includes changes in weather patterns that will affect forest health and harvest operations. There is a growing body of scientific research aimed at understanding and predicting regional weather extremes, but the inherent challenge of modeling such complex and (relatively) small-scale phenomena means that there is a lot of uncertainty. It has been suggested that the research community should move toward “a more explicitly probabilistic, risk-based approach” (Shepherd 2014), which would be helpful for decision-makers who are trying to grapple with this uncertainty. We will continue to watch for updates in this regard. In the interim, it will be important to have a robust forest monitoring system in place to track local change, given that these changes will manifest differently from one region to the next.

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Barnes, E.A. 2013. Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes. Geophysical Research Letters. 40:4734-4739. doi:10.1002/grl.50880.

Barnes, E.A. and J.A. Screen. 2015. The impact of Arctic warming on the midlatitude jet-stream: Can it? Has it? Will it? WIREs Clim. Change. 6:277-286. doi:10.1002/wcc.337.

Cohen, J., Pfeiffer, K., and J.A. Francis. 2018a. Warm Arctic episodes linked with increased frequency of extreme winter weather in the United States. Nature Communications. 9(869). doi:10.1038/s41467-018-02992-9.

Cohen, J., Zhang, X., Francis, J., Jung, T., Kwok, R., Overland, J., Tayler, P.C., Lee, S., Laliberte, F., Feldstein, S., Maslowski, W., Henderson, G., Stroeve, J., Coumou, D., Handorf, D., Semmler, T., Ballinger, T., Hell, M., Kretschmer, M., Vavrus, S., Wang, M., Wang, S., Wu, Y., Vihma, T., Bhatt, U., Ionita, M., Linderholm, H., Rigor, I., Routson, C., Singh, D., Wendisch, M., Smith, D., Screen, J., Yoon, J., Peings, Y., Chen, H. and R. Blackport. 2018b. Arctic change and possible influence on mid-latitude climate and weather. US CLIVAR Report 2018-1. 41pp. doi:10.5065/D6TH8KGW.

Coumou, D., Di Capua, G., Vavrus, S., Wang, L., and S. Wang. 2018. The influence of Arctic amplification on mid-latitude summer circulation. Nature Communications. 9:2959. doi:10.1038/s41467-018-05256-8.

Francis, J.A. and J.E. Overland. 2014. Implications of rapid Arctic change for weather patterns in northern mid-latitudes. US CLIVAR VARIATIONS. 12(3):10-13.

Francis, J.A. and S.J. Vavrus. 2012. Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters. 39:L06801. doi:10.1029/2012GL051000.

Francis, J.A., Vavrus, S.J., and J. Cohen. 2017. Amplified Arctic warming and mid-latitude weather: new perspectives on emerging connections. WIREs Clim. Change. e474. doi: 10.1002/wcc.474.

Kretschmer, M., Coumou, D., Agel, L., Barlow, M., Tziperman, E., and J. Cohen. 2018. More-Persistent Weak Stratospheric Polar Vortex States Linked to Cold Extremes. BAMS. 99(1):49-60. doi:10.1175/BAMS-D-16-0259.1.

Lyons, B., Hasell, A., and N.J. Stroud. 2018. Enduring Extremes: Polar Vortex, Drought, and Climate Change Beliefs. Environmental Communication A Journal of Nature and Culture. 12(7). doi:10.1080/17524032.2018.1520735.

Mann, M., Rahmstorf, S., Kornhuber, K., Steinman, B.A., Miller, S.K., and D. Coumou. 2017. Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events. Nature Scientific Reports. 7:45242. 12pp. doi:10.1038/srep45242.

Mann, M. 2018. “Climate Change and Extreme Summer Weather Events – The Future is still in Our Hands.” RealClimate. Available online at Last accessed Mar. 3 2019.

National Weather Service. “What is the Polar Vortex?” Available online at Last accessed mar. 19 2019.

Osborne, E., Richter-Menge, J., and M. Jeffries (Eds.). 2018. “Arctic Report Card 2018.” Available online at Last accessed Mar. 19 2019.

Palmer, T.N. 2013. Climate extremes and the role of dynamics. PNAS. 110(14):5281-5282. doi:

Petoukhov, V., Rahmstorf, S., Petri, S., and H.J. Schellnhuber. 2013a. Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes. PNAS. 110(14):5336-5341.

Petoukhov, V., Rahmstorf, S., Petri, S., and H.J. Schellnhuber. 2013b. Reply to Screen and Simmonds: From means to mechanisms. PNAS. 110(26):E2328.

Rasmijn, L.M., van der Schrier, G., Berkmeijer, J., Sterl, A., and W. Hazeleger. 2016. Simulating the extreme 2013/2014 winter in a future climate. J. Geophys. Res. Atmos. 121:5680-5698. doi:10.1002/2015JD024492.

Screen, J.A. 2014. Arctic amplification decreases temperature variance in northern mid- to high-latitudes. Nature Climate Change. 4:577-582.

Screen, J.A. and I. Simmonds. 2013. Caution needed when linking weather extremes to amplified planetary waves. PNAS. 110(26):E2337.

Screen, J.A., Bracegirdle, T.J., and I. Simmonds. 2018. Polar Climate Change as Manifest in Atmospheric Circulation. Current Climate Change Reports. 4:383-395.

Shepherd, T.G. 2014. Atmospheric circulation as a source of uncertainty in climate change projections. Nature Geoscience. 7:703-708. doi:10.1038/NGEO2253.

Taylor, P.C., Maslowski, W., Perlwitz, J., and D.J. Wuebbles. 2017. Arctic changes and their effects on Alaska and the rest of the United States. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 303-332. doi:10.7930/J00863GK.

Van Oldenborgh, G.J., de Vries, H., Vecchi, G., Otto, F., and C. Tebaldi. 29 January 2018. “A cold winter in North America, December 2017 to January 2018.” World Weather Attribution. Available online at Last accessed Mar. 19 2019.

Waugh, D.W., Sobel, A.H., and L.M. Polvani. 2017. What is the Polar Vortex and How Does it Influence Weather? BAMS. 98(1):37-44. doi:10.1175/BAMS-D-15-00212.1.

Waugh Research Group. 2019. “What is the Polar Vortex?” Johns Hopkins University, Krieger School of Arts & Sciences. Available online at Last accessed Mar. 19 2019.

Changing Hurricane Activity & Forest Risk

(click here to download a pdf of this complete article or a one-page synopsis)

By Jennifer Hushaw Shakun

Hurricanes pose a major risk to infrastructure and human safety, but they are also a significant disturbance agent in our forests, often leaving impacts that persist for decades. The 2017 Atlantic hurricane season included a number of record breaking storms that were linked to above-average ocean temperatures (Murakami et al. 2018) and, since that particularly active hurricane season, there has been increasing discussion about changing hurricane risk in the context of climate change. In this bulletin, we summarize various aspects of hurricane activity and how they are projected to change (or not) in the future, with implications for forest health and timber value.

Recipe for a Hurricane

Hurricanes, which are more generally known as tropical cyclones (see box below), are massive storms fueled by warm, moist air over the tropical ocean waters near the equator, with wind speeds of at least 74 miles per hour. As warm, moist air rises it leads to areas of high and low pressure that ultimately create a spinning storm system of clouds and wind. In the simplest sense, the two most important factors in the development and intensification of hurricanes are (1) warm ocean waters and (2) wind shear (changes in wind direction and/or speed with height). These factors work in opposing ways—warm water provides hurricanes with their energy, while wind shear puts on the brakes by tearing storm systems apart (NOAA GFDL 2018a).

Determining how climate change might affect tropical cyclone activity involves figuring out how these two major factors will change, along with a few additional variables, such as the temperature of the upper atmosphere and relative humidity (NOAA GFDL 2018a). Over 90% of the global warming we have experienced to date has been absorbed by the oceans (Rhein et al. 2013), but warming ocean waters alone will not increase the number of hurricanes each year—it will, however, mean more fuel for them when they do form (Climate Central 2018a; Climate Central 2018b).

How is hurricane activity or risk changing, in terms of…


Detecting changes in hurricane frequency is challenging because records of hurricane activity are less reliable and less complete before the mid-1970’s (after which we have more consistent observations from satellites and other sources). When studies account for these data limitations they find there has been no increase in the global number of hurricanes since the 1800’s (Landsea et al. 2010; NOAA GFDL 2018b).

The number of hurricanes making landfall in the U.S. has not changed significantly either, but there has been an increase in hurricane activity over the entire Atlantic Ocean basin since the 1970’s (NOAA 2012). However, too little time has elapsed to say whether that increase is part of an on-going trend related to human-induced climate change or whether it is within the realm of natural variability (Kossin et al. 2017).

There is still some debate about whether we will have more or fewer tropical cyclones in the future as the climate changes. While there are modeling studies that suggest the frequency will increase (Emanuel 2013), the general consensus is that, globally, the total number of hurricanes will stay about the same or perhaps decrease by up to a third (Knutson et al. 2010; NOAA 2012; Kossin et al. 2017). Although, research suggests we are likely to have an increasing number of the most intense storms, even if overall numbers go down (see section on Intensity, below). In the Atlantic Ocean, in particular, there is currently no consensus about how hurricane frequency will change (NOAA 2012).


The intensity or strength of tropical cyclones is usually measured in terms of wind speed, such as the well-known categories of the Saffir-Simpson Hurricane Wind Scale. The maximum intensity a hurricane can achieve is determined by the temperature of the surface ocean and the thermodynamics of the atmosphere above it (Emanuel 1986; Emanuel 1995; Emanuel 1997). All else being equal, rising ocean surface temperatures will increase the potential intensity of tropical cyclones.

This is borne out by the latest scientific research, which indicates that we will most likely see an overall increase in tropical cyclone intensity, including an increase in wind speed between 1 and 10%, as well as a greater number of category 4 and 5 storms in a warmer world (Kossin et al. 2017; Bender et al. 2010; Knutson et al. 2015; NOAA GFDL 2018b). As one recent publication put it: “We thus expect tropical cyclone intensities to increase with warming, both on average and at the high end of the scale, so that the strongest future storms will exceed the strength of any in the past” (Sobel et al. 2016).

In fact, there is evidence this is already happening. Across the globe, there have been increases in the strongest tropical storms, especially in the North Atlantic where the strongest storms are getting stronger (Kossin et al. 2013; Elsner et al. 2008). There is also at least one study that suggests tropical cyclones are intensifying faster (Kishtawal et al. 2012). In a recent essay, several leading researchers in this field also noted that: “Of these seven [major tropical cyclone] regions, five had the strongest storm on record in the past five years, which would be extremely unlikely just by chance” (Rahmstorf et al. 2017).

There are other measures, beyond wind speed, that give a more holistic assessment of hurricane activity, such as the accumulated cyclone energy (ACE) index or the power dissipation index (PDI). These are functions of wind speed AND storm duration, which are accumulated for a particular region to estimate the overall intensity of the hurricane season (NOAA NWS CPC 2017; Emanuel 2005).

One study found that PDI more than doubled in the Atlantic and increased 75% in the western North Pacific between the mid-1970’s and the early 2000’s. The increase was due to a combination of longer-lasting storms and faster winds (Emanuel 2005). Although, a recent update indicates that annual PDI has actually fallen in the North Atlantic since that study was published (see EPA Climate Change Indicators: Tropical Cyclone Activity, Figure 3). Another recent study (Lin and Chan 2015) found a ~35% drop in PDI from 1992 to 2012 in the western North Pacific. In that case, storm intensity increased, but it was offset by having fewer storms that were not as long-lasting. This is an example of the way indices like PDI can be useful for understanding the interplay between changing intensity, frequency, and duration.


Often times, it is the incredible volume of rain associated with hurricanes that does the most damage. All the resesarch to date points toward increasing rainfall rates in a warmer world—on the order of 10-20% by the end of the century (NOAA GFDL 2018b; Knutson et al. 2010). This is true on a global scale and for the Atlantic basin in particular. Rainfall rates are expected to increase because a warmer atmosphere will hold more water vapor and because these storms may move more slowly, dropping more precipitation in a given location (see section on How Fast They Move, below). As a recent example, the evidence suggests human-induced climate change contributed to the historic rainfall totals from Hurricane Harvey, making the heavy downpours 3 times more likely and 15% more intense (van Oldenborgh et al. 2017; Climate Central 2017).


Sea levels have been rising and will continue to do so for the foreseeable future. This is because ocean water expands slightly as it warms and warming temperatures are releasing large volumes of water that were previously frozen in mountain glaciers and polar regions. All else being equal, rising sea levels will increase the height of hurricane storm surges and the vulnerability of coastal communities to that type of flooding (Knutson et al. 2010; Kossin et al. 2017; NOAA GFDL 2018b).


One measure of tropical cyclone activity that is easier to accurately pin down is when a storm reaches its peak intensity. This is more straightforward than metrics like duration or absolute intensity because you only need to know when a storm is at its relative strongest, without needing to know the absolute wind speed in miles per hour. When researchers looked at global records of tropical cyclone activity for the last 30 years, they found these storms now reach their maximum strength about 200 miles farther from the equator (that is a trend of around 72 (+/- 43) miles per decade). This poleward migration appears to be happening globally, albeit at different rates in different regions, and it is consistent with the expansion of the tropics that has been linked to human-induced climate change (Kossin et al. 2014). If this movement in the location of peak storm intensity continues, it will change the historic patterns of storm risk across different regions.


Recent research indicates that tropical cyclone are moving slower—translation speed (i.e. forward-moving speed, as opposed to rotation speed) has decreased globally by 10% since 1950 (Kossin et al. 2018). The slowdown has been even more dramatic in some regions, with tropical cyclones moving 30% slower over land areas near the western North Pacific and 20% slower over land areas near the North Atlantic. This has big implications for storm-related rainfall because a slower moving tropical cyclone will drop much more water in one region (the historic rainfall totals from Hurricane Harvey were an example of this). Tropical cyclones move within large-scale atmospheric circulation patterns that are affected by climate change, but the observed slowdown cannot be confidently linked to human-induced warming at this time (Kossin et al. 2018).

Forest Impacts from Hurricane Activity

It is estimated that, on average, hurricanes affect almost 3 million acres of forest and cost around $700 million dollars each year in the U.S. (Dale et al. 2001). By that measure, some recent hurricane seasons certainly qualify as above average. The Florida Forest Service puts the timber damage from Hurricane Michael in October of this year at $1.3 billion over 3 million acres of forestland (FDACS 2018). Another estimate of the damage from both Hurricane Michael and Hurricane Florence (which made landfall a month earlier), puts the total loss of timber value around $1.6 billion over 5 million acres across Florida, Georgia, and Alabama (SAF 2018).

This loss of value is also related to loss of aboveground carbon. A study of the impact of Hurricane Katrina on forests in the Gulf Coast estimated there was mortality or damage to ~320 million large trees, which represents a significant portion of the annual U.S. forest tree carbon sink. That same study noted that the predicted increase in storm activity “will reduce forest biomass stocks, increase ecosystem respiration, and may represent an important positive feedback mechanism to elevated atmospheric carbon dioxide” (Chambers et al. 2007). In other words, we expect to have stronger, slower-moving storms in the future that drop more rain, so it is reasonable to expect that the risk to timber resources may change, including loss of value and forest carbon stocks in some places.

In the aftermath of a major storm event, there is an immediate loss of merchantable value due to structural damage or tree mortality, but there are also longer-term impacts to consider—wounded and stressed trees are more vulnerable to attack from insects and pathogens, the increased volume of downed wood can provide additional fuel for wildfires, and there may be infrastructure and access-related issues if forest roads, culverts, etc. have been damaged.

Importantly, surviving trees may also experience growth impacts that can persist for a while after the storm event. One study of coastal forests in Virginia found a decline in radial growth that lasted for up to 4 years after an extreme storm before beginning to recover. They also found that there was a strong correlation between the amount of growth decline and the strength of the storm (as measured by wind speed or storm surge height) (Fernandes et al. 2018). This suggests that the projected increase in hurricane intensity may have corresponding impacts on forest growth in impacted areas.

Things to Do

Of course it is not possible to prevent trees from being severely damaged or uprooted in the strongest winds of category 4 and 5 hurricanes, but there are steps you can take to build windfirmness in forest stands and help them withstand lower intensity storm systems. We recommend re-visiting two of our earlier bulletins on that topic for more detailed information about the factors that increase the risk of wind damage (see Part 1) and the management actions that can maximize resilience to wind-related disturbance (see Part 2).

Other key actions are to maintain forest access and have systems in place for carrying out rapid assessments of forest damage after storm events (with field surveys and/or aerial/satellite imagery). This will help prioritize salvage efforts, which can be time-sensitive if the goal is to limit additional loss of value due to rot, pests, or disease. In some cases, it may also be worthwhile to consider acquiring insurance to hedge against catastrophic timber loss due to extreme wind events.

There are useful resources on this topic that are geared toward urban forestry, but which may have some useful insight, such as this series from the University of Florida. One of the publications in that series (see link under Recommended Resources) outlines a number of lessons learned from 10 hurricanes that hit the Gulf Coast and Puerto Rico between 1995 and 2005, including:

  • The higher the wind speed of the hurricane, the more likely trees will fail.
  • Trees in groups survive winds better than trees growing individually.
  • Some species resist wind better than others.
  • Pines may show no immediate visible damage after hurricanes but may decline over time.
  • Trees that lose all or some of their leaves in hurricanes are not necessarily dead.
  • Native trees survived better in South Florida hurricanes.
  • Older trees are more likely to fail in hurricanes.
  • Unhealthy trees are predisposed to damage.
  • Trees with poor structure or bark inclusions are more vulnerable in the wind.
  • Trees with more rooting space survive better.
  • Damaged root systems make trees vulnerable in the wind.

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Native Pests in Novel Places: The Southern Pine Beetle Example

(click here to download a pdf of the full bulletin or a one-page synopsis)

By Jennifer Hushaw Shakun

Climate-driven changes in pests and disease are already causing significant near-term impacts on forest health—a reality that we highlighted in an earlier bulletin on Forest Pests and Climate Change. Notably, several important native pests, including spruce budworm (Gray 2013), mountain pine beetle (Cullingham et al. 2011; de la Giroday et al. 2012), and southern pine beetle (Dodds et al. 2018), have shown signs that their range is expanding with warmer temperatures or is predicted to do so in the near future. They are prime examples of a phenomenon that is likely to become more common in the future, i.e. native pests that are well-known in one region move into new geographies where they were previously unknown.

The novelty creates some immediate challenges for forest managers (Aoki et al. 2018). The good news is that, unlike with most alien invasive species, we have far more information (both research and institutional knowledge) about these organisms. We also have examples of systems and best practices that have worked (or failed) in other regions. With the right networks in place for sharing information across jurisdictions (e.g. the CSLN), the learning curve for managers encountering a particular pest for the first time will not be as steep and the likelihood of an effective and rapid response is increased (Ayres & Lombardero 2018; Morris et al. 2018).

Southern Pine Beetle: An Overview

Southern pine beetle (Dendroctonus frontalis Zimmermann) (SPB) is one of the most destructive pests in the southern U.S. (Anonymous 1989; Coulson & Klepzig 2011), with a loss to timber producers of an estimated $43 million per year (Pye et al. 2011). Its historic range in the U.S. extends from Florida into southern New Jersey and as far west as central Arizona (Hain et al. 2011). Although, “given its wide host range, genetic plasticity, and ability to sustain epidemics in nontraditional species, it appears that the geographic range of the SPB is only constrained by host availability and climatic conditions” (Hain et al. 2011).

The small adults (~ 3 mm long) bore into the tree and create S-shaped galleries in the inner bark where they lay their eggs. The larvae develop in the inner bark and move into the outer bark to pupate before emerging as adults. The entire life cycle can take place in just 30 days under ideal conditions, which allows SPB to have three to seven generations per year (Anonymous 1989).

While SPB “infests and kills all pine species in its range” (Hain et al. 2011), its preferred hosts are southern yellow pines such as loblolly, shortleaf, Virginia, pond, and pitch pines (Anonymous 1989). Pioneer beetles will often infest weakened or stressed trees and begin producing aggregation pheromones that attract additional beetles, which ultimately overwhelms the tree’s defenses. At this stage, the beetles can reproduce within the infested tree and attacks begin on neighboring trees when the new adults emerge, forming the distinctive “spots” of an SPB outbreak (Hain et al. 2011). Pitch tubes (about the size of a piece of popcorn) on the bark and discoloration of foliage are two noticeable indications of an infestation (Anonymous 1989). SPB’s rapid life cycle and its ability to attack and kill healthy trees make it particularly destructive (Dodds et al. 2018).

Over the past several decades, there has been a decline in major SPB outbreaks within its historic range in the southeastern U.S. There are a number of hypotheses for why this reduction has happened, but recent research suggests an “increase among several variables associated with intensive pine silviculture and genetic tree improvement efforts” is a possible explanation (Asaro et al. 2017). Forest management and SPB suppression efforts are known to have had an important influence on the frequency, intensity, and extent of SPB outbreaks (Clarke et al. 2016)—an important insight for managers encountering this pest in new regions.

Southern Pine Beetle Outbreak Risk

As with most forest pests, the risk that an infestation will become a severe outbreak depends on the size and location of the pest population and the susceptibility of the host (Aoki et al. 2018) at the tree, stand, or landscape level. One well-established method for assessing SPB risk relies on one of these two components—using the number of SPB caught per day in traps, along with the ratio of SPB to one of its major predators—to predict infestation trends (Billings & Upton 2010). Alternatively, southern pine beetle hazard maps generated by the US Forest Service use variables related to average diameter, basal area/density, and proximity to the infestation (Krist et al. 2014).

Weakened or stressed trees, e.g. those struck by lightning, are a risk factor (Hain et al. 2011) because they are often the “patient zero” of SPB spots during an outbreak. There are also a variety of stand-level characteristics that affect the vulnerability of host trees by influencing resource availability, density, stand age/size, and the dispersal of beetle pheromones (see Table 1; Aoki et al. 2018). High stand density, in particular, is known to increase the risk of an SPB outbreak (Clarke & Nowak 2009; Guldin 2011), by making it easier for the beetles to attract each other with pheromone plumes and to travel from the infested tree to a new host.

Southern Pine Beetle in the Northern Context

Warmer winter temperatures have facilitated the northern expansion of the SPB range (via improved over-winter survival) and model projections suggest that the climate will become progressively more suitable for SPB in these areas over time (Lesk et al. 2017). But it is more difficult to predict the impact of SPB in this new environment because its population dynamics may be altered and the susceptibility of northern pine species is not completely understood.


Site index and site moisture can be useful predictors of SPB infestation risk in the southern U.S., but they appear to be less important in new areas like the New Jersey Pinelands (Aoki et al. 2018). However, the same study found that “stands with high percentage pine and high pine basal area were more susceptible. Stands composed of smaller, closer together, shorter, and younger trees, with lower percent live crown, were also more susceptible,” i.e. SPB risk seems to vary with stand characteristics in the newly invaded areas in much the same way it did in well-studied systems farther south (Aoki et al. 2018). If that holds true, it may be easier to successfully predict SPB infestations in the Northeast going forward.


SPB primarily attacks hard pines (preferably loblolly and shortleaf) in the Southeast, but it will successfully attack “nontraditional” host species, such as eastern white pine, red spruce, Norway spruce, eastern hemlock, and others, when outbreak populations are large enough (Hain et al. 2011). Pitch pine has proven to be a suitable host in the recent outbreaks in New Jersey and Long Island, while red and Scots pine were identified as hosts in the Connecticut infestation (Dodds et al. 2018). The potential threat to eastern white pine is top of mind for many in the Northeast and, fortunately, successful reproduction of SPB has not be documented in that species so far. Although, both white and jack pine (farther north) have the potential to act as suitable hosts (Hain et al. 2011; Dodds et al. 2018).


Many northeastern pine species may be susceptible to SPB, but it is possible that the composition of northeastern forests (specifically the absence of many pure pine stands and more isolated and disbursed populations of potential hosts) will reduce the risk of severe outbreaks at the landscape level. The risk may be greatest in certain unique, pine-dominated ecosystems, including pitch pine barrens and natural red pine stands. Pitch pine barrens, in particular, are often unmanaged and lack the regular occurrence of fire and management intervention necessary to reduce overstocking and the dense canopy conditions that are conducive to SPB (Dodds et al. 2018). Areas where potential host species occur in relatively pure, high density stands across large areas (such as red spruce) will also be at high risk for SPB infestations.



A new study (Lesk et al. 2017) maps where and when the climate will likely become suitable for southern pine beetle in the northeastern U.S. and southeastern Canada.

Winter minimum temperatures between 7 and -4 ⁰F are sufficient to kill SPB while they overwinter within the inner bark of trees. This limited cold tolerance restricts the northern extent of their geographic range. Bark also has an insulating effect that provides the beetles with some protection and buffers cold extremes by 4-7 ⁰F, which researchers accounted for in this study.

They found that, currently, the northernmost SPB sites are generally at latitudes where the lowest winter temperature in the inner bark is 14 ⁰F. They estimate that an “SPB-suitable” climate will eventually develop in other areas if conditions warm up enough to keep the winter inner bark temperature above 14 ⁰F for ten consecutive years. Using these parameters and the results from two dozen global climate models (which project an increase in annual minimum air temperature of 6-13 ⁰F by mid-century), researchers modeled the emergence of SPB-suitable climate over this century.

On average, they found that by 2090 all of the northeastern U.S. and large areas of southeastern Canada will be hospitable for SPB (see Figure 2a). Although, they also noted that there is considerable uncertainty, with a spread of several decades between the low- (see Figure 2b) and high-end (see Figure 2c) of estimates. The results suggest that several likely northern host species will be vulnerable over large portions of their range by mid- to late-century—by 2050, an estimated 78% of the pitch pine range will be suitable for SPB and by 2080, 71% of red pine and 48% of jack pine range will be suitable.


SPB Management: Things to Do

For managers encountering this pest for the first time, there is extensive literature and practice to draw on for determining the best response. Within its historic range, managers have developed and honed management strategies that control the spread and severity of infestations when they occur and reduce the risk of future outbreaks.

The “things to do” include:

  • prevention (i.e. thinning high hazard areas),
  • landscape prioritization and hazard models (i.e. assessing susceptibility based on stand characteristics to identify priority areas for treatment),
  • detection and monitoring (i.e. aerial surveys and pheromone-baited traps), and
  • evaluation and direct control (i.e. cutting infested trees and a green tree buffer using the cut-and-leave or cut-and-remove method).

(Dodds et al. 2018)

The management tactics used in the southern U.S. appear to have worked equally well in the recent northern outbreaks. They included early monitoring, rapid treatment of initial SPB “spots,” and preventative thinning (Aoki et al. 2018). In fact, thinning (and other methods of reducing density) in pine-dominated stands has proven to be the most effective tactic for reducing susceptibility to SPB. As one source puts it: “The best silvicultural defense against SPB is to manage forest stands so that individual trees are vigorous and stands are not overstocked” (Guldin 2011). Evidence for the effectiveness of this approach can be found within the historic SPB range where there are examples of outbreaks that occurred almost entirely on unthinned stands (Nowak et al. 2015). Although, it is worth noting that sufficient markets and timber harvesting infrastructure must be present to make such preventative treatments feasible and cost-neutral (at best). That may be more of an impediment for northern landowners than in the southeastern U.S. where SPB suppression has been very successful
(Ayres & Lombardero 2018; Morris et al. 2018).




The general management recommendations described above can apply to any pest infestation, not just southern pine beetle—be proactive where possible, watch for change, and plan for control efforts in susceptible areas. As some native pests move into new areas, it will be important to draw on the existing knowledgebase from their historic range, but it is also likely that traditional host species, risk factors, and effective management may be different because of changes in the biology of the pest or differences in host susceptibility. The mountain pine beetle (MPB) in the West provides a recent example, with evidence that MPB enjoyed greater reproductive success in lodgepole pine as it encountered “naïve” host trees in areas where the climate was previously unsuitable—a factor that likely contributed to the unprecedented scale of recent outbreaks in western Canada (Cudmore et al. 2010). This highlights the importance of staying attuned to the expanding range limits of particularly destructive pests, like SPB, and closely monitoring reports from newly affected areas to learn whether there are notable changes in pest behavior or outbreak patterns.



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Anonymous. 1989. “Southern Pine Beetle. In: Insects and Diseases of Trees in the South. USDA Forest Service. Protection Report R8-PR16. 98 pp.

Aoki, C.F., Cook, M., Dunn, J., Finley, D., Fleming, L., Yoo, R., and M.P. Ayres. 2018. Old pests in new places: Effects of stand structure and forest type on susceptibility to a bark beetle on the edge of its native range. Forest Ecology and Management. 419-420:206-219.

Asaro, C., Nowak, J.T., and A. Elledge. 2017. Why have southern pine beetle outbreaks declined in the southeastern U.S. with the expansion of intensive pine silviculture? A brief review of hypotheses. Forest Ecology and Management. 391:338–348. doi:

Ayres, M.P. and M.J. Lombardero. 2018. Forest pests and their management in the Anthropocene. Canadian Journal of Forest Research. 48: 292–301

Ayres, M.P., Martinson, S.J., and N.A. Friedenberg. 2011. Southern Pine Beetle Ecology: Populations within Stands. In: Coulson, R.N.,; Klepzig, K.D. 2011. Southern Pine Beetle II. Gen. Tech. Rep. SRS-140. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 75-89.

Billings, R.F. and W.W. Upton. 2010. A methodology for assessing annual risk of southern pine beetle outbreaks across the southern region using pheromone traps. In: Pye, J. M., Rauscher, H.M., Sands, Y., Lee, D.C., Beatty, J.S. (Eds.), Advances in Threat Assessment and Their Application to Forest and Rangeland Management. Gen. Tech. Rep. PNW-GTR-802. U.S. Department of Agriculture, Forest Service, Pacific Northwest and Southern Research Stations, Portland, OR, pp. 73–85.

Clarke, S.R. and J.T. Nowak. 2009. Southern Pine Beetle. USDA Forest Service Forest Insect & Disease Leaflet 49. USDA Forest Service, Pacific Northwest Region, Portland, OR.

Clarke, S.R., Riggins, J.J., and F.M. Stephen. 2016. Forest Management and Southern Pine Beetle Outbreaks: A Historical Perspective. Forest Science. 62(2):166-180. doi:

Coulson, R.N. and K.D. Klepzig. 2011. The Southern Pine Beetle. USDA Forest Service, GTR, SRS-140. 512.

Cudmore, T.J., Björklund, N., Carroll, A.L., and B.S. Lindgren. 2010. Climate change and range expansion of an aggressive bark beetle: evidence of higher beetle reproduction in naïve host tree populations. Journal of Applied Ecology. 47:1036–1043. doi:10.1111/j.1365-2664.2010.01848.x.

Cullingham, C.I., Cooke, J.E.K., Dang, S., Davis, C.S., Cooke, B.J., and D.W.Coltman. 2011. Mountain pine beetle host‐range expansion threatens the boreal forest. Molecular Ecology. 20:2157-2171. doi:

de la Giroday, H.-M.C., Carroll, A.L., and B.H. Aukema. 2012. Breach of the northern Rocky Mountain geoclimatic barrier: initiation of range expansion by the mountain pine beetle. Journal of Biogeography. 39:1112-1123. doi:

Dodds, K.J., Aoki, C.F., Arango-Velez, A., Cancelliere, J., D’Amato, A.W., DiGirolomo, M.F., and R.J. Rabaglia. 2018. Expansion of Southern Pine Beetle into Northeastern Forests: Management and Impact of a Primary Bark Beetle in a New Region. Journal of Forestry. 116(2):178-191. doi:10.1093/jofore/fvx009.

Gray, D.R. 2013. The influence of forest composition and climate on outbreak characteristics of the spruce budworm in eastern Canada. Canadian Journal of Forest Research. 43(12):1181-1195, doi:

Guldin, J.M., 2011. Silvicultural considerations in managing southern pine stands in the context of southern pine beetle. In: Coulson, R.N., Klepzig, K.D. (Eds.). Southern Pine Beetle II. USDA For. Serv. Gen. Tech. Rep. SRS-140, pp. 317–352.

Hain, F.P., Duehl, A.J., Gardner, M.J., and T.L Payne. 2011. Natural History of the Southern Pine Beetle. In: Coulson, R.N.; Klepzig, K.D. 2011. Southern Pine Beetle II. Gen. Tech. Rep. SRS-140. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 13-24.

Krist, F.J., Ellenwood, J.R., Woods, M.E., McMahan, A.J., Cowardin, J.P., Ryerson, D.E., Sapio, F.J., Zweifler, M.O., and S.A. Romero. 2014. 2013–2027 National Insect and Disease Forest Risk Assessment. FHTET 14-01, USDA Forest Service Forest Health Technology Enterprise Team, Fort Collins, Colorado, USA.

Lesk, C., Coffel, E., D’Amato, A.W., Dodds, K., and R. Horton. 2017. Threats to North American forests from southern pine beetle with warming winters. Nature Climate Change. 7:713-717. doi:10.1038/NCLIMATE3375,

Lorio, Jr., P.L. 1986. Growth-Differentiation Balance: A Basis for Understanding Southern Pine Beetle-Tree Interactions. Forest Ecology and Management. 14:259-273.

Morris, J.L., Cottrell, S., Fettig, C.J., DeRose, R.J., Mattor, K.M., Carter, V.A., Clear, J., Clement, J., Hansen, W.D., Hicke, J.A., Higuera, P.E., Seddon, A.W.R., Seppä, H., Sherriff, R.L., Stednick, J.D., and S.J. Seybold. 2018. Bark beetles as agents of change in social-ecological systems. Front. Ecol. Environ. 16(S1):S34-S43. doi:10.1002/fee.1754,

Nowak, J.T., Meeker, J.R., Coyle, D.R., Steiner, C.A., and C. Brownie. 2015. Southern Pine Beetle Infestations in Relation to Forest Stand Conditions, Previous Thinning, and Prescribed Burning: Evaluation of the Southern Pine Beetle Prevention Program. Journal of Forestry. Journal of Forestry. 113(5):454–462. doi:

Pye, J.M., Holmes, T.P., Prestemon, J.P., and, D.N. Wear. 2011. Economic Impacts of the Southern Pine Beetle. In: Coulson, R.N.; Klepzig, K.D. 2011. Southern Pine Beetle II. Gen. Tech. Rep. SRS-140. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 213-222.

Reeve, J.R., Ayres, M.P., and P.L. Lorio, Jr. 1995. Host suitability, predation, and bark beetle population dynamics. In: N. Cappuccino and P.W. Price (Eds.) Population dynamics: New approaches and synthesis. Academic Press, San Diego, CA. Pp 339-357.





Forest Disturbances in a Changing Climate

(click to download a formatted pdf of this complete article or a one-page synopsis)

By Jennifer Hushaw

Disturbance shapes the character and composition of ecosystems and it is “a pervasive feature of forests” (Perry 1994). Wildfires, blowdowns, pests and other disturbance agents affect the spatial patterns of vegetation and ecosystem processes, creating a diversity of conditions across the landscape, and they can leave an imprint that shapes forest dynamics for decades to centuries after the initial disturbance event (Turner 2010).

There is an increasing amount of research on forest disturbance in a changing climate, including a number of studies that suggest certain types of disturbance (e.g., wildfire or insect outbreaks) are increasing over time (as noted in previous bulletins). In a recent review in the journal Nature Climate Change, researchers noted that increases in the occurrence and severity of forest disturbance have been documented worldwide and they provide a comprehensive analysis of these changing dynamics by elucidating global trends from hundreds of separate studies (Seidl et al., 2017).


Researchers synthesized results from over 670 studies (published from 1990 to the present) that assessed how forest disturbance changed in response to a change in climate, focusing on six disturbance agents (fire, drought, wind, snow and ice, insects, and pathogens). From these results, they isolated over 1,600 observations for further analysis.

For each type of disturbance, they looked at the evidence for direct, indirect, and interaction effects of climate change. They determined whether climate change had a predominantly amplifying or dampening effect on disturbance, as well as the relative size of that effect. This allowed them to assess the degree of climate sensitivity. In particular, they examined whether disturbance was likely to increase or decrease under either (1) warmer and wetter or (2) warmer and drier conditions.

Note: The disturbance agents included in this study have been discussed in previous CSLN bulletins, including wildfire (Part I & Part II), drought (here & here), wind, snow and ice (Part I & Part II), and pests/pathogens. Refer to those publications for a more in-depth discussion of climate change effects.  


Climate Shapes Disturbance Regimes

  • Climate has a “substantial influence” on forest disturbance – direct effects were most common (~57% of observations), followed by indirect (25%) and interaction (~18%) effects.
  • Temperature had a greater influence on disturbance at higher latitudes (most important in boreal regions), while water availability was more influential at lower latitudes (most important in the tropics).
  • Interaction between agents tended to increase disturbance – posing an increased risk of crossing ecological tipping points.
    • “In particular, disturbances by drought and wind strongly facilitate the activity of other disturbance agents, such as insects and fire […].”
  • Indirect climate effects commonly had a dampening influence by reducing ecosystem vulnerability to disturbance over the long-term,
    • g. a climate-mediated shift toward more drought tolerant tree species reduces potential for drought-induced forest mortality.
  • It can take years to centuries for the disturbance regime to respond to the change in climate.
    • Interaction effects resulted in the fastest response time (< 6 years in 81% of cases),
      • g. drought weakens tree defenses, leading to an increased risk of bark beetle outbreak within a few years.
    • Indirect effects resulted in the slowest response time (> 25 years in ~45% of cases),
      • g. a forested area becomes progressively drier over time, leading to self-thinning and a reduction in stand density (after many decades), which reduces risk of wildfire.

Forest Disturbance Will Likely Increase in the Future

  • Recently documented increases in disturbance are “likely to continue in the coming decades as climate warms further.”
    • Studies indicate disturbance activity will increase in all biomes and more for conifer forests than broadleaved and mixed forest types.
  • Overall, disturbances from fire, drought, wind, insects, and pathogens are likely to increase, while disturbances from snow and ice are likely to decrease.
    • Fire, insects, and pathogens will increase “regardless of changes in water availability.”
    • Drought, wind, and snow will be “strongly contingent on changes in water availability.”
  • Under warmer and drier conditions, most studies show:
    • ↑ fire
    • ↑ drought
    • ↑ insect activity
  • Under warmer and wetter conditions, most studies show:
    • ↑ wind disturbance
    • ↑ pathogen disturbance


  • Longer term effects of climate change on disturbance regimes may be underestimated because the analysis is limited by the length of the observational period used in the original studies.
  • The predominance of direct climate effects may be partially due to the fact that they are easier to detect and measure than indirect or interaction effects.
  • The majority of observations they analyzed were from ecosystems in North America and Europe, making it unclear whether some of the observed trends (e.g. greater impacts on disturbance in boreal regions) are due to the degree of climate change and ecosystem characteristics or (at least partially) the result of publication bias.
  • The scientific literature focused more on fire, drought, insects, and pathogens than the other disturbance agents.
  • Invasive alien pests were not considered in their analysis, but will likely play a part in future disturbance change.

Things to Do

Forest management can ameliorate negative impacts from increasing disturbance, by actively promoting the characteristics of resilient forests, and shifting species composition, stand density, and other features in a way that reduces vulnerability to disturbance and ensures that when disturbance (inevitably) happens the damage is minimized.


Addressing changes in forest disturbance is an important part of climate change adaptation. In fact, “Plan for and respond to disturbance” is one of the ten major adaptation strategies proposed by the USDA Forest Service in Forest Adaptation Resources: Climate Change Tools and Approaches for Land Managers. The approaches they suggest include:

  • Prepare for more frequent and more severe disturbances
  • Prepare to realign management of significantly altered ecosystems to meet expected future environmental conditions
  • Promptly revegetate sites after disturbance
  • Allow for areas of natural regeneration after disturbance
  • Maintain seed or nursery stock of desired species for use following severe disturbance
  • Remove or prevent establishment of invasives and other competitors following disturbance (Butler et al., 2012)

In previous bulletins we have discussed various management actions that can be taken to reduce the effects of specific types of disturbance. For example…



In addition to active management that reduces the likelihood, severity, or extent of forest disturbance, it will be increasingly important to monitor changes in disturbance regimes such as those outlined in the Seidl et al. (2017) study. We are addressing this need through the Resiliency Assessment Framework, which is currently under development. The major categories of monitoring information that will be collected are related to forests, climate, disturbance, and operations. Potential research questions and example monitoring metrics for disturbance agents are listed below:

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Butler, P.R., Swanston, C.W., Janowiak, M.K., Parker, L.R., St. Pierre, M.J., Brandt, L.A. 2012. Adaptation Strategies and Approaches. In: C.W. Swanton and M.K. Janowiak, editors. Forest Adaptation Resources; Climate change tools and approaches for land managers. Gen. Tech. Rep. NRS-87. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station, p. 15-34.

Perry, D.A. 1994. “Disturbance in Forest Ecosystems.” In: Forest Ecosystems, p. 127. Baltimore, Maryland: Johns Hopkins University Press.

Seidl, R., Thom, D., Kautz, M., Martin-Benito, D., Peltoniemi, M., Vacchiano, G., Wild, J., Ascoli, D., Petr, M., Honkaniemi, J., Lexer, M.J., Trotsiuk, V., Mairota, P., Svoboda, M., Fabrika, M., Nagel, T.A., Reyer, C.P.O. 2017. Forest disturbances under climate change. Nature Climate Change. 7: 395-402.

Turner, M.G. 2010. Disturbance and landscape dynamics in a changing world. Ecology. 91(10): 2833–2849.

New Evidence of Tree Species on the Move

(click to download a pdf of this complete article or download a one-page synopsis)

By Jennifer Hushaw

There has been significant debate and research about where, how quickly, and to what degree our forests will shift with the changing climate. These questions have primarily been tackled through modeling (e.g., see Modeling Future Forests bulletin) or studying plant response in the distant past through paleoecology. Quite a bit of uncertainty remains because we do not have complete information about the physiological limits, tolerances, and life history traits of all species, or a good understanding of how changes in relative competiveness will play out (Corlett & Westcott 2013). The paleo record offers us a few examples of vegetation response to abrupt climatic change, but we are generally limited to episodes that were far more gradual than the relatively rapid warming we see today (Williams & Burke in press). There is also the confounding factor of human influence on the landscape, via land use, forest management, wildlife management, introduction of new species, and so on.

Given this uncertainty, researchers have used recent observational data to detect early signs of tree species migration and ecosystem shifts. The results have been mixed, as to whether large-scale shifts are underway and whether they are due to climate change. In this bulletin, we highlight a noteworthy new study in the eastern U.S. that shows evidence of significant changes in abundance for a number of tree species. We look at how these results compare with previous research, highlight some overarching themes, and conclude with action items for managers.

New Evidence of Tree Species Shifts  

A recently published study by Fei et al. (2017), entitled Divergence of species responses to climate change, found “prominent westward and poleward shift in abundance for most tree species in the eastern United States during the last 30 years.” The study provides evidence that eastern tree species are responding to recent changes in climate, and the details of their findings highlight the individual responses of different species groups.


Researchers used tree abundance data from the U.S. Forest Inventory & Analysis (FIA) program for 86 species/groups in the eastern U.S. They looked at shifts in abundance between two inventory periods: 1980-1995 (depending on the state) and 2015 (the most recent completed year)—with an average window of about 30 years. They analyzed the relationship between observed species shifts and climate (specifically, mean annual temperature (MAT), total annual precipitation (TAP), and Palmer Drought Severity Index (PDSI)), as well as forest succession status. They also looked at whether there were differences in species response depending on functional traits (drought tolerance, wood density, and seed weight) or evolutionary lineage.

  • Distinct spatial patterns
    • 73% of species shifted their abundance centers westward *
    • 62% of species shifted their abundance centers poleward **
    • Shifts were primarily due to changes in subpopulations at the leading edges
    • There were regional differences in the primary direction of abundance shifts:
      • Northern Hardwood = poleward
      • Central Hardwood = westward
      • Southern Pine-Hardwood = westward
      • Forest-Prairie Transition = westward
    • Sapling abundance shifted in higher proportions (and farther) than adult tree abundance
    • Longitudinal shift was 1.4 times faster than latitudinal
    • Observed a poleward shift rate of 11.0 km per decade, similar to estimates from previous studies
  • Influence of climate and succession
    • Changes in abundance were more strongly related to moisture (precipitation and drought index) than temperature
      • “…changes in mean annual precipitation alone explained about 19% of the variability in species abundance change and spatial shift.”
    • Early successional forests had more gains in abundance than forests in late successional stages
  • Different shifts depending on traits and evolutionary lineage
    • Influence of physiological tolerance and dispersal ability
      • Generally, species that shifted westward had larger seed size and higher wood density than species that shifted eastward
      • Species with medium to high drought tolerance shifted westward at a faster rate than those with low drought tolerance
      • Species that shifted northward were associated with lower annual precipitation and lower wood density than southward shifting species
      • Wind-pollinated species primarily shifted northward, while animal-pollinated shifted southward ****
    • Influence of phylogeny (evolutionary lineage)
      • 5% of angiosperms (hardwoods/flowering plants) shifted westward ***
      • 4% of gymnosperms (softwoods/non-flowering plants) shifted northward, along with all poplar and most birch species
* 65% of these were statistically significant

** 55% of these were statistically significant

*** 52.3% of these were statistically significant

**** Similar northward shifts occurred in New England 10–8K years ago (during the early Holocene)


The results of this study suggest some interesting implications for tree species migration in the eastern U.S., including the following highlights from the discussion section:

  • Vegetation dynamics appear to be a more sensitive to moisture than temperature, at least in the near-term.
  • While the western portion of the study area is drier than the east, it experienced an increase in total annual precipitation over the study period, suggesting that drought tolerant species shifted westward because they could more readily take advantage of the increased moisture.
  • Poleward shifts were more prominent at higher latitudes, where the greatest warming has taken place to date.
  • The stronger trend observed for saplings supports the idea that saplings will respond more quickly to climatic change and exhibit greater sensitivity to drought than adults.
  • Results support the idea that initial changes will be most prominent at species range margins, particularly the leading edge.
  • Gymnosperms have less efficient water transport systems that lead to lower maximum growth rates (compared with angiosperms), so their primarily northward shift may be due to a lack of competitiveness in the drier western region of the study area.
  • Seed size may be an important factor because it is linked to different colonization, tolerance, and competitive strategies used by different species.
  • Higher wood density is often associated with greater survival, which may explain the preferential shift of high wood density species into the droughty (southern) and relatively dry (western) portions of the study area.
  • Non-climatic factors also played a role, including successional processes, forest densification related to fire suppression, and (potentially) infestations (of pests, plants, and pathogens), forest conservation, and plantation efforts.

Previous Research

Numerous studies have documented species range shifts that are consistent with what we would expect in a warming world, namely upward shifts in elevation and latitude, for plants and many other types of organisms (Root et al. 2003; Parmesan & Yohe 2003; Parmesan 2006; Chen et al. 2011). For tree species, in particular, these types of elevational or poleward shifts have been previously documented in temperate (Lenoir et al. 2008; Beckage et al. 2008; Woodall et al. 2009; Wright et al. 2016), boreal (Soja et al. 2007), and even tropical (Feeley et al. 2011) biomes.

Although, the studies that have detected changes (including Fei et al. 2017) show discernible shifts for only some fraction of species. Not all tree species are responding (yet), and some are responding in ways we might not expect, including instances of downhill shifts (Crimmins et al. 2011) and the expansion of some species into more southerly areas (Woodall et al. 2009). This is due to the complex array of factors that influence where and how tree species will grow, and researchers have cited a number of these to explain the apparent lack of movement or counterintuitive shifts among some species, including competition from established species, changes in moisture availability, adaptation, and so on.

In some cases, we also see contradictory results. For example, a 2012 study by Zhu and colleagues examined FIA data for over 90 species in the eastern U.S. and they found that only ~20% of species showed a pattern of northward shift (with close to 60% exhibiting evidence of range contraction) and no apparent relationship between these observed patterns and seed size, dispersal characteristics, or degree of climate change, which is in contrast to the findings of Fei et al. (2017). However, the two studies used different methodologies, which may explain the difference. In fact, the issue of methodology has been pointed to as a partial explanation for the lack of evidence of species shifts to date. Shifts may be happening that our methods can’t sufficiently detect because of challenges with accurately identifying species range margins and data availability across entire species’ distributions (Jump et al. 2009).

In fact, previous studies have used a variety of a different approaches to determine whether species are on the move, including comparing the average latitude of tree biomass with the average latitude of seedlings for a given species (Woodall et al. 2009), comparing the 5th and 95th percentile latitudes for seedlings and trees (Zhu et al. 2012), assessing change in the average elevation of recruitment over time (Wright et al. 2016), and others. Fei et al. (2017) is different from many preceding studies in using abundance data throughout the species range, rather than focusing solely on range margins for detecting changes.

Take Homes

Despite the inherent uncertainty in forecasting future forests, current scientific understanding suggests some general rules-of-thumb regarding tree species shifts, such as:

  • Tree species will respond independently, not as cohesive forest types
  • Significant time lags are likely
  • There is potential for faster change with mortality from extreme events
  • Changing moisture availability will likely be more important than changing temperature for driving species shifts in the near-term
  • Most species will experience climate conditions that are novel for that species (in some portion of their range)
  • Look for initial forest composition changes at range margins
  • Look for initial changes in regeneration, rather than the overstory
  • Uncertainty is not if tree distributions and abundance will shift, but exactly where and when
  • Generally, we expect…
    • Range expansion at the leading edge (northern and higher elevations)
    • Range contraction at the trailing edge (southern and low-altitudinal limits)

The Fei et al. (2017) study supports many of these and also provides a useful illustration of how to think about these types of changes. This is often framed as a question of whether a particular species will disappear from the landscape in the next 50 to 100 years (e.g. Will all the sugar maple disappear from New England?), but the more relevant question(s) should be things like:

  • Will the species become more or less prevalent (in terms of relative abundance)?
  • How will changes in relative competitiveness manifest?
  • Will the species experience growth declines due to less optimal climate conditions and/or a reduced capacity to fend off threats, such as insect infestation?
  • Is the overall species range likely to shift, contract, or expand in the long-term?

Things to Do

Two important things managers can do are (1) monitor and (2) promote regeneration. Monitoring includes watching for changes in the forests you manage (e.g. changes in moisture availability or in regeneration success, relative abundance, and/or competitiveness among species) and keeping an eye on the results of large-scale studies like the new research highlighted here. This is especially important in light of the distinct regional differences observed by Fei and colleagues. This will also be an important component of the Resiliency Assessment Framework that is currently under development, which is designed to provide informative metrics for detecting the impacts of climate change on North American forests.

Regeneration is the phase when species have the best opportunity to express adaptation to new climate conditions through phenotypic plasticity and it is the mechanism of species migration, so promoting regeneration can be the first step in moving a forest stand toward a condition/composition that may be more resilient in the face of changing climate conditions.


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Beckage, B., Osborne, B., Gavin, D.G., Pucko, C., Siccama, T., Perkins, T. 2008. A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont. PNAS. 105(11): 4197-4202.

Chen, I-C., Hill, J.K., Ohlemüller, R., Roy, D.B., Thomas, C.D. 2011. Rapid Range Shifts of Species Associated with High Levels of Climate Warming. Science. 333: 1024-1026.

Corlett, R.T., Westcott, D.A. 2013. Will plant movements keep up with climate change? Trends in Ecology & Evolution. 28(8): 482–488.

Crimmins, S.M., Dobrowski, S.Z., Greenberg, J.A., Abatzoglou, J.T., Mynsberge, A.R. 2011. Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations. Science. 331(6015): 324-327.

Feeley, K.J., Silman, M.R>, Bush, M.B., Farfan-Rios, W., Cabrera, K.G., Malhi, Y., Meir, P., Salinas, N., Raurau-quisiyupanqui, M.N., Saatchi, S. 2011. Upslope migration of Andean trees. Journal of Biogeography. 38(4): 783-791.

Fei, S., Desprez, J.M., Potter, K.M., Jo, I., Knott, J.A., Oswalt, C.M. 2017. Divergence of species responses to climate change. Science Advances. 3: e1603055.

Jump, A.S., Mátyás, C., Peñuelas, J. 2009. The altitude-for-latitude disparity in the range retractions of woody species. Trends in Ecology & Evolution. 24(12): 694-701.

Lenoir, J., Gégout, J.C., Marquet, P.A., de Ruffray, P., Brisse, H. 2008. A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century. Science. 320: 1768-1771.

Parmesan, C. 2006. Ecological and Evolutionary Responses to Recent Climate Change. Annu. Rev. Ecol. Evol. Syst. 37: 637-669.

Parmesan, C., Yohe, G. 2003. A globally coherent fingerprint of climate change impacts across natural systems. Nature. 421: 37-42.

Root, T.L., Price, J.T., Hall, K.R., Schneider, S.H., Rosenzweig, C., Pounds, J.A. 2003. Fingerprints of global warming on wild animals and plants. Nature. 421: 57-60.

Soja, A.J., Tchebakova, N.M., French, N.H.F., Flannigan, M.D., Shugart, H.H., Stocks, B.J., Sukhinin, A.I., Parfenova, E.I., Chapin III, F.S., Stackhouse Jr., P.W. 2007. Climate-induced Boreal Forest Change: Predictions versus Current Observations. Global and Planetary Change. 56(3-4): 274-296.

Williams, J.W. & Burke, K. (in press). Past abrupt changes in climate and terrestrial ecosystems. In Climate Change and Biodiversity (eds T. Lovejoy & L. Hannah)

Woodall, C.W., Oswalt, C.M., Westfall, J.A., Perry, C.H., Nelson, M.D., Finley, A.O. 2009. An indicator of tree migration in forests of the eastern United States. Forest Ecology and Management. 257: 1434-1444.

Wright, D.H., Nguyen, C.V., Anderson, S. 2016. Upward shifts in recruitment of high-elevation tree species in the northern Sierra Nevada, California. California Fish and Game. 102(1):17-31.

Zhu, K., Woodall, C.W., Clark, J.S. 2012. Failure to migrate: lack of tree range expansion in response to climate change. Global Change Biology. 18: 1042-1052.

Shifting Phenology in a Changing Climate

(click here to download a pdf of this full article or a one-page synopsis)

By Jennifer Hushaw

Phenology is the study of the seasonal rhythms of plants and animals, especially the timing of natural cycles as related to weather and climate. It is a sensitive indicator of climate change, with far reaching implications for ecosystem processes, productivity, and even the global carbon budget.

In previous bulletins, we discussed how phenology is expected to shift due to a warming climate, leading to a whole host of direct and downstream impacts. In this bulletin, we delve into more detail and reveal how phenology of boreal and temperate trees, in particular, has already shifted and is likely to continue changing, as well as the potential ramifications for forestry. We also discuss some of the major questions that still remain to be answered, such as which species are most well-suited to track warming trends and maintain optimal phenology in the future.



Phenology is sometimes described as “the pulse of the planet” because of the way it mediates seasonal and annual processes related to carbon, water, and nutrient cycling. By controlling the timing and extent of leaf area, flowering, leaf fall and other developments, phenology directly influences productivity, growth, evapotranspiration, runoff, decomposition, and mineralization (Richardson et al. 2013). It is also relevant on a global scale because it influences vegetation-related feedbacks to the climate system, such as:

  • Albedo, e.g. changes in reflected solar radiation when deciduous forests move from leaf-off to leaf-on conditions
  • Canopy conductance, e.g. changes in the amount of leaf area that affect transpiration rates and CO2 uptake
  • Flows of water and energy, e.g. increased transfer of water vapor to the lower atmosphere following leaf-out
  • CO2 fluxes, e.g. changes in the balance between forest canopy photosynthesis and ecosystem respiration

Through these feedbacks, phenology not only influences regional weather patterns, but can also affect long-term global climate (Richardson et al. 2013). All this means that phenology has massive implications for global change science, ecosystem processes, and land management (including forestry).

For a more detailed description of climate feedbacks, including those associated with forests, revisit the June 2015 bulletin, Uncertainty in Climate Change and Forest Response: Part I.


Temperate and boreal trees go into dormancy every winter to protect their tissues against damage from cold temperatures, creating an annual cycle where dormancy is induced in the fall and released in the spring. These phenological shifts are cued and mediated by four primary factors:

  • Degree of warming in spring
  • Onset of cold temperatures in fall
  • Degree and duration of winter chilling
  • Photoperiod (i.e. day length relative to night length)

(Way & Montgomery 2015)

The diagram in Figure 1 illustrates how this process generally unfolds. In autumn, shorter days and lower temperatures induce endodormancy (an internally, genetically controlled, set state of inactivity), where growth ceases. Trees can only resume growth in the spring after they receive a signal that winter has ended, in the form of exposure to cool, non-freezing temperatures (also known as ‘chilling’). Although the amount of chilling required varies from species to species, it is a necessary prerequisite to move the tree into ecodormancy (a state of inactivity imposed by unfavorable environmental conditions), which is when they become sensitive to temperature and photoperiod cues. Once a certain amount of warming (i.e. degree-days) have been accumulated or certain photoperiod thresholds are met, the plant is released from ecodormancy and experiences the onset of bud burst, leaf unfolding, flowering, etc. (Basler & Körner 2014).

Clearly, much of this process is strongly mediated by temperature, including the rate at which buds and leaves develop after dormancy, but photoperiod and chilling are critical controls as well. As we discuss in a later section, the degree to which particular species are sensitive to chilling and/or photoperiod has the potential to constrain how well they track warming temperatures and adapt to changes in climate.


Complexity arises because the relative importance of the four factors listed above varies by species, genetic makeup, gene expression (i.e. phenotype), successional strategy, and region of origin (Way & Montgomery 2015; Basler & Körner 2014; Körner & Basler 2010; Kramer et al. 2017; Laube et al. 2014; Rohde et al. 2011). It also depends whether we are considering spring or fall phenology, since the latter is generally more sensitive to photoperiod than the former (Way & Montgomery 2015).

For example, one key factor is sensitivity to photoperiod. Trees that rely strongly on day length to signal phenology, rather than temperature cues, have certain advantages and disadvantages. Relying on photoperiod can help trees guard against leafing out too early and experiencing late season frosts, but responding to temperature gives trees the flexibility to take advantage of the extended period for photosynthesis (and the associated growth increase) offered by earlier onset of the frost-free season. As a result, species that are sensitive to photoperiod are less likely to experience earlier leaf out in response to warming. A common example is Fagus sylvatica (European beech), which is known to be particularly sensitive to photoperiod (less so to temperature) and has demonstrated a low level of variability in the timing of leaf unfolding from year to year, despite variability in temperature (Basler & Körner 2014). See Table 1 (below) for the categorization of some common species from across the globe.

Phenology: An Indicator of Change

People have been recording the timing of the seasons through plant phenology for centuries. The longest known records date back to the 9th century and describe the flowering of Japanese cherry trees, which are now blooming earlier than at any point in the last 1200 years (Primack et al. 2009). Over the past few decades, phenology has increasingly been recognized as a useful indicator of long-term ecosystem change (Richardson et al. 2013) and is now a prominent part of efforts to track the impact of a warming climate. In fact, the U.S. Global Change Research Program (USGCRP) includes the start of spring as one of their 14 initial indicators of climate change.


Spring phenology is well-understood and has been more extensively studied than autumn, in large part because it is easier to measure and detect the changes associated with phenomena like leaf out and flowering than the gradual process of fall senescence.

Evidence from ground-based and satellite studies (mostly in the Northern Hemisphere) shows spring advancement for “hundreds of plant and animal species in many regions” and, globally, spring has been advancing earlier at an average rate of around 3.3 (± 0.87) days per decade for tree species, with larger changes generally at higher latitudes (Settele et al. 2014). Indeed, a recent analysis of 25 years of satellite data detected an advance of 14.5 days in the start of the growing season in northern high latitude areas (> 45⁰N) (Jeganathan et al. 2014). Advance in the timing of spring onset in temperate trees over the last four decades can be attributed to warming temperatures (Richardson et al. 2013).

In the U.S., there has been a general trend toward earlier springs since 1984 (USGCRP). In fact, the year 2012, which was the hottest year on record for the U.S., stands out as the earliest spring start (Figure 2). Although, that record may soon be broken because “2017 is shaping up to be two to three weeks earlier than 2012 in many parts of the country” (NPN 2017) and up to three weeks earlier than normal (compared to 1981-2010) in some locations in the southeast (Figure 3).


The effect is less pronounced than for spring, but there have been documented delays in autumn senescence in European and North American temperate forests of 3-4 days per decade since 1982 (Rosenzweig et al. 2007; Richardson et al. 2013). The work by Jeganathan et al. (2014), mentioned above, also detected a 16-day delay in the end of the growing season in high northern latitudes over the last several decades.


These shifts in phenology, combined with a lengthening of the frost-free season, have increased the length of the growing season in many places (as we discussed in a previous bulletin). In the U.S., it has increased by as much as ten days since the 1980’s (EPA; Figure 4), with some parts of the country experiencing increases of up to 50 days in the period since 1895 (EPA; Figure 5). This has the potential to be a boon for the productivity of many ecosystems, including forests, e.g. research suggests lengthening the growing season by 5-10 days may increase annual net primary productivity of forest systems, by as much as 30% (Jackson et al. 2001) and other studies have shown that a difference of just one week in the timing of canopy development can mean a 20% difference in photosynthetic production from year to year (Myneni et al. 1997).

Shifting Phenology in a  Warmer World

Given the importance of temperature for signaling tree species phenology, what are the implications of climate change for our forests? This is an important question because changes in phenology have implications for productivity, survival, inter-species competition, pest and disease impacts, wildlife, and more. A review of the latest science suggests the following:

  • Overall extension of the growing season will increase forest productivity
  • Pioneer species (which have lower chilling requirements) may benefit from warmer winters
  • Many species demonstrate phenotypic plasticity, or an ability to shift their phenology to take advantage of warmer temperatures
  • Most species are likely to experience decreases in frost damage over time (on average)
  • Many species are likely to experience increased frost damage in some part of their distribution, particularly on the margins (even if they see a decrease on average)
  • Invasive species tend to have lower chilling requirements and less sensitivity to photoperiod, so they will benefit from warmer winters and take advantage of earlier spring warmth
  • Species composition may change due to phenologically-induced changes in understory light conditions that influence seedling survival
  • Certain characteristics, such as sensitivity to photoperiod, appear to be genetically determined, so some species will be limited in their ability to response to warming temperatures (especially in autumn when phenology is more strongly controlled by photoperiod)
  • For some species, milder winters will make it difficult to meet chilling requirements—leading to delays in spring phenology that may reduce their competitive advantage and growth potential, but also reduce their risk of late season frost
  • There will be changes in competitive advantage between species, based on varying ability to track warmer temperatures and take advantage of a longer growing season

(Laube et al. 2014; Kramer et al. 2017; Morin & Chuine 2014; Körner & Basler 2010; Basler & Körner 2014; Fu et al. 2015; Chen et al. 2017)

Emerging Research & Remaining Questions

The relationship between a warming world and forest phenology may seem straightforward—warmer temperatures, longer growing season, increased growth and productivity. However, a review of the current science reveals that it’s not quite that simple. Tree species, and even particular provenances, have unique sensitivities to the various seasonal cues, which makes it challenging to anticipate exactly how the timing of phenological events will shift on a local scale. Since there are many factors at play in determining how a particular species or site will change, it will be important to watch your own forest carefully to see which species are responding most effectively to warming temperatures.

As new research emerges, we will be better able to accurately pin down likely changes and identify the potential impacts for forest health, productivity, and composition. The following lists major uncertainties in the science, on-going research needs, and key questions that remain to be answered:

  • Relative importance of photoperiod versus temperature
  • Species-specific responses
  • Degree of phenotypic plasticity of particular species
  • Change in likelihood of frost damage (overall)
  • Better understanding/more research into climate change impacts on autumn phenology
  • Tension between scientific evidence for constraints on phenology (e.g. photoperiod sensitivity) and demonstrated species plasticity
  • Potential role of air humidity as a control on phenology
  • Improved phenological models that are more generalizable
  • Improved representation of phenological processes in terrestrial ecosystem models
  • Extremes can fundamentally alter phenological response—posing a challenge for prediction
  • Effective temperature range for chilling is only vaguely known for forest trees
  • Potential role of soil water in mediating phenology
  • Lack of an underlying ecological or physiological scheme that differentiates between photoperiodically sensitive and insensitive trees species—to facilitate prediction under future climate

(Basler & Körner 2014; Way & Montgomery 2015; Tansey et al. 2017; Kramer et al. 2017; Morin & Chuine 2014; Richardson et al. 2013; Laube et al. 2014b; Delpierre et al. 2016; Carter et al. 2017; Delpierre et al. 2017)

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Rohde, A., Bastien, C., Boerjan, W. 2011. Temperature signals contribute to the timing of photoperiodic growth cessation and bud set in poplar. Tree Physiology. 31: 472-482.

Rosenzweig, C., G. Casassa, D.J. Karoly, A. Imeson, C. Liu, A. Menzel, S. Rawlins, T.L. Root, B. Seguin, P. Tryjanowski, 2007: Assessment of observed changes and responses in natural and managed systems. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 79-131.

Settele, J., R. Scholes, R. Betts, S. Bunn, P. Leadley, D. Nepstad, J.T. Overpeck, and M.A. Taboada, 2014: Terrestrial and inland water systems. In: Climate Change 2014: Impacts,Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 271-359.

Tansey, C.J., Hadfield, J., Phillimore, A.B. 2017. Estimating the ability of plants to plastically track temperature-mediated shifts in the spring phenological optimum. Global Change Biology. Available from, DOI: 10.1111/gcb.13624.

Way, D.A., and Montgomery, R.A. 2015. Photoperiod constraints on tree phenology, performance and migration in a warming world. Plant, Cell and Environment. 38: 1725-1736.



Attributing Extremes to Climate Change

(click here to download a formatted pdf of this full article or a one-page synopsis)

By Jennifer Hushaw

In previous bulletins, we described how climate change is altering the frequency and intensity of certain types of extreme weather (see Climate Change and Extreme Weather: Part 1) and how these extremes shape plant communities, forest health, and the global carbon budget (see Climate Change and Extreme Weather: Part 2). This is still front and center, as there have been many record-breaking extremes across the globe in the last few years (Otto 2016) and an increasing prevalence of heatwaves and/or heavy rain events in many regions.

In this bulletin, we are highlighting the science of extreme event attribution—an emerging field of research aimed at estimating exactly how much human-induced climate change contributes to individual extreme events. In some cases, scientists can now go beyond the often repeated line that “no individual weather event can be attributed to climate change” and instead provide quantitative estimates of how much the likelihood, frequency, or magnitude was influenced by anthropogenic warming. This information improves climate risk assessment and can help determine the appropriate management response, by differentiating between truly rare events and those that represent part of an on-going trend that warrants a shift in management.

Event Attribution

Over a decade ago, many thought it was essentially impossible to attribute individual weather events to climate change with any degree of certainty, but a new field of inquiry was sparked after a commentary in 2003 raised the question of liability for damages from extremes. The 2003 European heatwave was the first event subjected to this type of attribution analysis and researchers found climate change more than doubled the risk of such extreme heat (Stott et al. 2004). Since then, the field has taken off and the number of submitted studies about attribution grew by a factor of five between 2012 and 2015 (National Academies 2016).

There are a number of cooperative efforts working on this type of research, including the World Weather Attribution project, which is an international partnership providing rapid, near-real time analysis of major extreme events. Another is the [email protected] project, which uses regional climate models (run through a volunteer computing network that is part of the platform) to determine how climate change affects weather on a smaller scale.


There are two general approaches in this type of research—utilizing the historical record or modeling—and a hybrid approach is most common. In the former, researchers look at the historical context to see whether the rarity of an observed event (e.g. heatwave, heavy downpour, etc.) has changed over time. They may also look to see whether the recent extreme is different in any way from similar meteorological events in the past. In the latter, researchers run model simulations of the climate with and without human influence to see whether anthropogenic carbon emissions changed the probability or intensity of a particular type of extreme.

With the modeling approach, there are a number of metrics that quantitatively describe the change in risk. A common one is Fraction of Attributable Risk (FAR)—the portion of the risk of a particular extreme that is attributable to human influence, where FAR = 1 means all of the current risk is attributable to human-induced climate factors and FAR = 0 means all of the risk is attributable to natural factors (Pall et al. 2016).  For example, the Australian summer of December 2012 to February 2013 was the hottest on record and an attribution study revealed the FAR for average summer temperatures was 0.72, meaning there was a more than threefold increase in the risk of record summer temperatures as a result of human influence on the climate (Lewis and Karoly 2013).


There has been rapid progress over the last five years, both in the sophistication of the science and the speed with which attribution assessments can be done. In many cases, an assessment can be made within weeks, as compared to months (and publication of results long after the event). This is thanks to improved scientific understanding of the climate and weather mechanisms that produce extremes, advances in methodology, and increases in computing power (National Academies 2016; Pall et al. 2016). As an example, researchers with the World Weather Attribution project were able to turn around an attribution assessment for the May 2016 European flooding less than two weeks after the event.


Some events can be attributed to climate change more confidently than others. We have the greatest confidence in studies with the following:

  • A solid understanding of the physical mechanisms underlying the extreme event.
  • Consistent evidence from a high-quality observational record.
  • Climate models that accurately simulate and reproduce the class of extreme event.

(National Academies 2016; Hassol et al. 2016)

Meeting these criteria yields the most reliable results, but it is more difficult to “check all the boxes” for some classes of extremes, which is why uncertainty often depends on the type of event in question. There is more confidence in attribution studies about extremes directly linked to temperature (e.g. heatwaves) than about extremes indirectly related to temperature that are driven by multiple factors (e.g. convective storms or wildfires). Generally, the more direct the link with temperature, the easier it is to determine attribution with a great degree of confidence (Climate CIRCulator 2016; National Academies 2016).

This hierarchy is reflected in Figure 1 (below), which ranks various extremes in terms of the criteria mentioned above. Extremes directly related to temperature are toward the left and as you move right the extremes are more indirectly related to temperature and more complex in terms of the underlying mechanisms. Wildfire, for example, is a complex phenomenon that falls toward the right of the figure in terms of attribution confidence because the intensity and occurrence is not directly related to temperature, but is instead related to many factors, including humidity, antecedent conditions, fuel loads, and others (as we discussed in a recent bulletin on wildfire).

A Recent Example

Texas experienced a record-setting drought and heatwave in 2011 that had far reaching impacts, including on the forest industry. Not only were there a record number of acres burned in wildfires that year, but commercial timber losses from the drought were estimated at $755 million (with only 13% of that due to fire), according to the Texas Forest Service (Hoerling et al. 2013). Analysis revealed that the biggest factor contributing to the intensity of the heatwave was a precipitation deficit driven by changes in sea surface temperature, including a La Niña event. This natural variability explained about 80% of the temperature increase associated with the heatwave, but an additional 20% was attributable to human-induced climate change, specifically summertime temperature trends (Hoerling et al. 2013). This is an example where an extreme was largely caused by natural processes, but anthropogenic warming likely made the overall impact and damages worse than they may otherwise have been.

Using Attribution Information to Inform Forest Management

Extremes can have a huge influence on forest health, composition, and productivity (as we described in a previous bulletin) and event attribution research will help more accurately pin down the role of climate change in these events. This improves our understanding and helps avoid over- or under-estimation of the risks.

This is important because the way we perceive and frame the causes of an event influences how we respond to it, including post-disaster actions, preventive practices, and adaptation or mitigation initiatives (Lidskog & Sjödin 2016). If attribution studies suggest a particular extreme event is part of an on-going trend (rather than a rare, one-off occurrence), it makes sense to take action that reduces that risk in the future. For example, a homeowner whose property is severely damaged by a storm surge may decide whether to rebuild or relocate based on this type of information because it sheds light on whether they can expect a repeat in the future.

The following are some example forest management scenarios that illustrate how you might apply this idea in practice:

  1. A major storm event causes numerous blow downs in forest stands. A subsequent study suggests that a storm of that magnitude was twice as likely because of climate change. With the expectation that these types of storm events may become more frequent, you divert resources toward pro-active management that develops increased wind-firmness in intact and newly established stands, rather than focusing solely on salvage efforts.
  2. A mild drought that would historically leave stands largely unaffected results in some tree mortality due to the subsequent onset of a prolonged heat wave. Studies indicate the extreme heat was largely due to human-induced climate change (e.g. FAR = 0.8) and similar heat events will become more likely in the future. You consider planting more drought-tolerant variants or reducing density in established stands because evidence indicates that particular site will be more prone to drought-related forest mortality going forward.
  3. A severe winter storm leads to extensive ice damage in forest stands, but subsequent research indicates the storm was largely the result of natural climate forces and, while rare, icing events of similar magnitude were experienced in the past. You undertake any necessary salvage, but otherwise refrain from changing your management because evidence suggests the storm was an infrequent, but not unusual weather event.
  4. Culverts on a forest road, that were sized following the typical standard, wash out during an unusually heavy downpour. Analysis shows the rain event was more intense as result of warming trends. When replacing the culverts, you upsize in anticipation of larger peak flows in the future.

Manomet’s Resiliency Assessment Framework, which is currently under development, lays the groundwork for an enhanced forest monitoring system that will improve the quality and length of the observational record for key variables—a necessary component of robust attribution assessments (as discussed above). Our ultimate goal is to discern signal from noise in forest trends, by combining the data from this monitoring framework with the rapidly advancing science of attribution, which will enable more effective management that reduces forest vulnerability and capitalizes on potential advantages.

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Allen, Myles. 2003. Commentary: Liability for Climate Change. Nature. 421: 891-892.

Climate CIRCulator. Linking Extreme Weather to Climate Change. June 30, 2016. Accessed at:

Hassol, S.J., Torok, S., Lewis, S., Luganda, P. 2016. (Un)Natural Disasters: Communicating Linkages Between Extreme Events and Climate Change. World Meteorological Organization. Bulletin, Vol. 65(2). Accessed online at:

Hoerling, M., Kumar, A., Dole, R., Nielsen-Gammon, J.W., Eischeid, J., Perlwitz, J., Quan, X., Zhang, T., Pegion, P., Chen, M. 2013. Anatomy of an Extreme Event. Journal of Climate. 26: 2811-2832.

Lewis, S.C. and Karoly, D.J. 2013. Anthropogenic contributions to Australia’s record summer temperatures of 2013. Geophysical Research Letters. 40: 3705-3709.

Lidskog, R. and Sjödin, D. 2016. Extreme events and climate change: the post-disaster dynamics of forest fires and forest storms in Sweden. Scandinavian Journal of Forest Research. 31(2): 148-155.

National Academies of Sciences, Engineering, and Medicine. 2016. Attribution of Extreme Weather Events in the Context of Climate Change. Washington, DC: The National Academies Press. Doi: 10.17226/21852

Otto, F.E.L. 2016. News & Views, Extreme Events: The art of attribution. Nature Climate Change. 6: 342-343.

Pall, P., Wekner, M., Stone, D. Chapter 3: Probabilistic extreme event attribution. In: Dynamics and Predictability of Large-Scale, High-Impact Weather and Climate Events. Special Publications of the International Union of Geodesy and Geophysics. [Li, J., Swinbank, R., Grotjahn, R., Volkert, H. (eds.)] Cambridge University Press, United Kingdom, 2016.

Stott, P.A., D.A. Stone, & M.R. Allen, 2 December 2004. Human contribution to the European heatwave of Nature. 432: 610–614.

Wildfire in a Warming World: Part 2

(click here to download a pdf of this complete article or a one-page synopsis)

By Jennifer Hushaw & Si Balch

In Part I, we discussed the recent rise in U.S. wildfire, the evidence suggesting climate is a major driver of that increase, and the reality that future increases in temperature and drought frequency (in some regions) will lead to greater fire potential, especially in moisture-limited ecosystems. There is no question that wildfire risk has changed (and will continue to change) research-update-boxas a result of on-going climate change. Importantly, the anticipated shifts in fire will have big implications for commercial forests and conservation lands alike, as well as implications for the climate system itself because wildfire acts as a positive feedback that accelerates terrestrial carbon emissions. Severe disturbance caused by novel fire regimes may also hasten the species shifts expected with climate change, making fire an important driver of ecosystem change in both the near- and long-term.

In Part II, we describe the latest research on future changes in fire frequency, extent, and/or severity, as well as discussing management strategies and outlining some useful information sources.

Overview of Changing Fire Risk


Fire potential and the length of the fire season are projected to increase in many regions. These changes will be driven by earlier snowmelt, warmer temperatures (particularly summer), drought stress, and changes in soil water content (Keane et al. 2015; Waring and Coops 2016; Young et al. 2016) associated with climate change.

In the continental U.S., the potential for very large fires (>12,355 acres) is strongly linked to meteorological and climatological conditions. Recent research indicates that the potential for very large fires will increase in historically fire-prone regions as a result of climate change, while other regions will experience an earlier start or an overall extension of the fire season as atmospheric conditions become conducive earlier in the year and persist later (Barbero et al. 2015) – see details in The North American Outlook (below).

Drier fuels will also increase fire potential because fuel moisture is highly sensitive to temperature. For example, a recent analysis in Canada found that for each additional degree of warming, a 5 to 15% increase in precipitation is required to maintain fuel moisture, depending on the type of fuel in question (i.e. fine surface fuels, duff layers, or deep organic soils) (Flannigan et al. 2016). In the absence of a sufficient precipitation increase, these fuels begin to dry out and move closer to critical thresholds for fire ignition and spread. In fact, Canada is expected to have more days of extreme fire weather because future precipitation will be insufficient to compensate for the drying associated with warmer temperatures – this is true even for future scenarios with the greatest precipitation increase (i.e. 40%) (Flannigan et al. 2016).

Some of these changes will be non-linear, leading to dramatic increases in fire frequency or severity in some regions once critical thresholds are crossed. A great example can be found in the boreal forest and tundra ecosystems of Alaska where there are distinct temperature and moisture thresholds[1] for fire occurrence that will likely be crossed by the end of this century, significantly increasing the probability of wildfire and potentially leading to novel fire regimes in those areas (Young et al. 2016).


Climate-induced changes in vegetation (including type, density, large scale die-off, etc.) and forest pests will also influence fire risk by affecting fuel loads.

As we discussed in a previous bulletin, climate change will affect the population dynamics and spread of many forest pests and diseases, including mountain pine beetle. Mountain pine beetle outbreaks can, in turn, alter the quantity and characteristics of both live and dead fuels by changing the amount of fuel in the forest canopy, the base height of the canopy, the amount of surface fuel, and other aspects of forest biomass. In this way, they can influence fire probability, severity, and rate of spread, as well as the potential for crown fire (Hicke et al. 2012).

Climate change will also have direct effects on vegetation and forest biomass through long-term shifts in species distribution and forest composition, as well as small- and large-scale mortality events brought on by drought and other extremes. In Part I, we detailed how drought and beetle-induced mortality in western U.S. conifers is already contributing to an increase in fire. These mortality events and longer-term vegetation changes can flip an ecosystem from being fuel- to moisture-limited (or vice-versa), changing what controls fire activity in a given region (as discussed in Part I).

In some cases, the changing fire regime itself will cause vegetation communities to shift or flip from a moisture- to fuel-limited ecosystem. For example, the fire return interval in the greater Yellowstone ecosystem is predicted to decrease (i.e. more frequent fire) to the point that some forested areas will no longer be able to regenerate by mid-century and will instead convert to a new dominant vegetation type that shifts the region into a fuel-limited fire regime (Westerling et al. 2011).

“The bottom line is that we expect more fire in a warmer world.”  (Flannigan et al. 2016)


Modelling Future Wildfire


Globally, fire probability is expected to increase in the mid- to high-latitudes and decrease in the tropics, with these changes becoming more pronounced later in the century. In the near term (i.e. 2010-2039), the most consistent increases will occur in places with an already somewhat warm climate, but there are also major uncertainties in the next few decades. There is more confidence in projections for the end of the century (i.e. 2070-2099) when climate models have a higher level of agreement in their projections because the magnitude of climate change will be even greater, with some locations experiencing an average change in fire probability up to +0.25 (Figure 1; Moritz et al. 2012).

Flannigan et al. (2009) also suggest that a general increase in area burned and fire occurrence is likely, based on their review of close to 50 studies conducted between 1991 and 2009 on future fire activity around the world. Although these studies focused on different fire activity metrics, time frames, and locations, more than three-quarters of the analyses pointed to an increase in fire activity. In particular, they noted that fire seasons are lengthening in temperate and boreal regions and this trend should continue in a warming world.


Most of the research conducted to date in North America points toward a future increase in wildfire, with longer fire seasons and greater fire potential due to more conducive atmospheric conditions in a number of regions (Barbero et al. 2015; Wang et al. 2015; Liu et al. 2013; Young et al. 2016).

For example, in a study mentioned above, researchers from the University of Idaho, the US Forest Service, and the Canadian Forest Service modelled future potential for “very large fires” in different ecoregions due to climate change and they found the potential for very large fires will increase in the continental U.S. The largest absolute increases were predicted for the intermountain West and Northern California, while the largest relative changes were predicted in the northern tier of the country where the potential for very large fires has historically been quite low (e.g., see Barbero et al. 2015, Figure 1). In addition, their analysis suggests the southern U.S. will have an earlier fire season in the future, while the northern regions will experience an overall lengthening of the fire season, with an extension of potential burn days at both ends of the season. These changes are driven by anticipated increases in fire danger and temperature, as well as decreases in precipitation and relative humidity during the fire season (Barbero et al. 2015).

Another study by Liu et al. (2013) used results from a downscaled climate model to evaluate how fire potential will change by mid-century (2041–2070), as measured by the Keetch–Byram Drought Index (a commonly used index designed specifically for fire potential assessment). They predict an increase in fire potential in the Southwest, Rocky Mountains, northern Great Plains, Southeast, and Pacific coast due to warming trends, in addition to longer fire seasons in many regions.

Looking farther north, the research also suggests increases in fire potential across high-latitude regions. Specifically, the annual number of fire spread days in Canada is expected to increase anywhere from 35–400% by 2050, with large absolute increases in the Boreal Plains of Alberta and Saskatchewan and the greatest relative change in coastal and temperate forests (Wang et al. 2015). Similarly dramatic increases in fire activity are predicted for areas of Alaska with historically low flammability in the tundra and tundra-forest boundary areas, with “up to a fourfold increase in the 30-yr probability of fire occurrence by 2100” (Young et al. 2016).

Fire potential is not the only metric we might be concerned about, however. There are also questions about how fire severity may change as a result of climate change. Although fire severity will increase in some cases, as we have seen in the western U.S. with high fuel loads and exceptional drought conditions, future conditions may also decrease fire severity. When some researchers incorporated climate-induced changes in vegetation type, fuel load, and fire frequency, rather than climatic changes alone, they found that a widespread reduction in fire severity was likely for large portions of the western U.S. (Figure 2; Parks et al. 2016). This is because future increases in fire frequency and water deficits will reduce vegetation productivity, the amount of regeneration, and the amount of biomass accumulation on the landscape—all of which contribute to decreased fuel loads that will no longer support high-severity fires (Parks et al. 2016).

Management Considerations  


When considering how to address these wildfire regime shifts, one approach is to “manage for the extremes,” rather than the average fire event or return interval in a given region, because it is the extremes that determine the necessary capacity of fire management organizations and, although these extremes cannot be as easily predicted, they can have serious consequences (Wang et al. 2015; Irland 2013).


In terms of forest management, fuels reduction via pre-commercial or commercial thinning operations and prescribed fire is an obvious strategy for dealing with increased fire potential. That said, fuels reduction is better suited for some forest types than others, namely fuel-limited forest communities (Steel et al. 2015), e.g. yellow pine and mixed conifer forests in California or piñyon-juniper woodland and lower montane forests (dominated by ponderosa pine) in the Rocky Mountain region. In systems where the fire regime is primarily moisture- or climate-limited, a reduction in fuels will not be as effective at reducing fire hazard because fuel is not the limiting factor.


An argument can also be made for taking a more passive approach that “lets nature take its course,” where the fire regime is allowed to change and it ultimately shifts the dominant vegetation type to something new (as discussed above). In this case, the natural disturbance regime eventually transitions plant communities into a state of equilibrium with the new climate (Parks et al. 2016). This approach may ultimately be more appropriate and cost-effective in locations where conditions are expected to become more arid and fire frequency is projected to increase dramatically, compared with resisting change through on-going, active fire suppression efforts. Although not appropriate for most commercial operations, the passive approach may be a consideration for lands with a management focus on maintaining resilient transitional habitat for wildlife in a changing climate.


Of course, it is worth noting that these anticipated changes in wildfire are happening in the larger context of land use change (more development in the wildland urban interface, greater forest fragmentation), fuel accumulation (due to historic fire suppression efforts, landowner reluctance to harvest, and/or insufficient budgets for fuel treatments), and infrastructure/industry changes (lack of “fire wise” development in some regions, loss of institutional firefighting operations with ownership change).

Things to Do

A number of common practices can help land managers prepare for fire risk, which will be important to emphasize (or implement) in the face of increased fire potential. These include:

  1. Put all foresters and other field personnel through the state forestry department’s basic fire training school.
  2. Have the state forest service phone numbers and radio contact on everyone’s cell phone.
  3. Equip everyone’s truck with Indian tanks and fire rakes.
  4. Know who has bulldozers, where they are, and how to reach their owners.
  5. Identify water sources for pumping.
  6. Identify water sources for water bombers.
  7. Identify landing zones for helicopters.
  8. Think about how to communicate with abutting home owners about fire risk. This interface of houses and trees is an increasingly dangerous situation for both the forest owner and home owner.
  9. Utilize the resources below to find up-to-date information on potential fire risk.

Additional Resources

  • A program that produces landscape-scale geospatial products for planning, management, and operations, including maps and databases that describe vegetation, fuel, and fire regimes. Website provides data, reports, tools, maps, etc.
  • Source: USDA Forest Service & US Department of Interior
National Interagency Coordination Center
  • NICC coordinates interagency wildland firefighting resources. They also dispatch Incident Management Teams and resources as necessary when fires exceed the capacity of local or regional firefighting agencies. Website provides Incident Information with daily updates on large fires and Predictive Services, such as weather, fire fuels danger, outlooks, etc., as well as other resources for wildland fire and incident management decision-making.
  • Source: Multi-agency organization, including: BIA, BLM, USFS, USFWS, NASF, & NPS
FRAMES (Fire Research and Management Exchange System)
  • A searchable online portal for fire-related information, including documents, tools, data, online trainings, discussion forums, announcements, and research, as well as links to numerous other fire-related websites and portals for regionally-specific sites and resources.
  • University of Idaho; USFS Rocky Mountain Research Station
Joint Fire Science Program (JFSP)
Geographic Area Coordination Centers
  • Web-portal for incident information, logistics, predictive services (e.g. information about weather, fuels and fire danger), and administrative resources for wildland fire agencies.
  • Source: Geographic Area Coordinating Group (GACG) – interagency; made up of Fire Directors from each of the area Federal and State land management agencies
  • Portal to numerous websites and resources related to wildfire risk and detection, including many listed in this table and others.
  • Source: Multi-agency, including: FEMA, EPA, Dept. of Interior, Dept. of Commerce, NOAA, DOE, USDA, and Army Corps of Engineers
  • Educational resources on wildfire prevention and “firewise” practices for homeowners and professionals, including state-specific information for New England and adjacent Canadian provinces.
  • Source: Northeast Forest Fire Protection Commission’s Prevention and Education Working Team
Wildfire Risk Assessment Portals
  • Several states/regions have these websites, which include web-mapping applications showing fire risk and assessment, as well as information on historic fire occurrence and information for developing community wildfire protection plans.
  • Source(s): State forest service, state forester, universities, etc
Active Fire Mapping Program
  • Provides near real-time, satellite-based detection and characterization of wildland fire conditions in a geospatial context for the continental United States, Alaska, Hawaii and Canada.
  •   *soon moving to new URL:
  • Source: USDA Forest Service Remote Sensing Applications Center
Historic Wildfire Occurrence Data
  • This data product contains a spatial database of wildfires that occurred in the United States from 1992 to 2013, generated for the national Fire Program Analysis (FPA) system. The wildfire records were acquired from the reporting systems of federal, state, and local fire organizations.
  • USDA Forest Service
Global Maps of Fire Risk
  • This on-line map viewer of satellite-derived global vegetation health products, includes one for fire risk that is updated continuously. You can also go back and see archived maps from previous dates.
  • (choose “Fire Risk” under the Data Type dropdown menu)
  • Source: Center for Satellite Applications and Research (STAR) – the science arm of the NOAA Satellite and Information Service
National Fire Protection Association
  • Source for national fire codes and standards, public education, and research on fire risk and prevention.



[1] Young et al (2016) identified two thresholds for fire occurrence, based on historic data: average July temperatures above 13.4⁰C and annual moisture availability (i.e. precipitation minus evapotranspiration) below 150mm.

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Abatzoglou, J.T. and Williams, A.P. 2016. Impact of anthropogenic climate change on wildfire across western US forests. PNAS. 113(42): 11770–11775.

Barbero, R., Abatzoglou, J.T., Larkin, N.K., Kolden, C.A., Stocks, B. 2015. Climate change presents increased potential for very large fires in contiguous United States. International Journal of Wildland Fire. 24(7) 892-899.

Flannigan, M.D., Krawchuk, M.A., de Groot, W.J., Wotton, B.M., Gowman, L.M. 2009. Implications of changing climate for global wildland fire. International Journal of Wildland Fire. 18: 483-507.  

Flannigan, M.D., Wotton, B.M., Marshall, G.A., de Groot, W.J., Johnston, J., Jurko, N., Cantin, A.S. 2016. Fuel moisture sensitivity to temperature and precipitation: climate change implications. Climatic Change. 134:59-71.

Hicke, J.A., Johnson, M.C., Hayes, J.L., Preisler, H.K. 2012. Effects of bark beetle-caused tree mortality on wildfire. Forest Ecology and Management. 271: 81-90.

Irland, L.C. 2013. Extreme value analysis of forest fires from New York to Nova Scotia, 1950-2010. Forest  Ecology and Management. 294: 150-157.

Keane, R.E., Loehman, R., Clark, J., Smithwick, E.A.H., Miller, C. 2015. Chapter 8: Exploring Interactions Among Multiple Disturbance Agents in Forest Landscapes: Simulating Effects of Fire, Beetles, and Disease Under Climate Change in Simulation Modeling of Forest Landscape Disturbances. A.H. Perera et al. (eds.) Springer International Publishing. Switzerland.

Liu, Y., Goodrick, S.L., Stanturf, J.A. 2013. Future U.S. wildfire potential trends projected using a dynamically downscaled climate change scenario. Forest Ecology and Management. 294: 120-135.

Moritz, M.A., Parisien, M-A., Batllori, E., Krawchuk, M.A., Van Dorn, J., Ganz, D.J., Hayhoe, K. 2012. Climate change and disruptions to global fire activity. Ecosphere. 3(6):49.

Parks, S.A., Miller, C., Abatzoglou, J.T., Holsinger, L.M., Parisien, M., Dobrowski, S.Z. 2016. How will climate change affect wildland fire severity in the western US? Environmental Research Letters. 11: 035002.

Steel, Z.L., Safford, H.D., Viers, J.H. 2015. The fire frequency-severity relationship and the legacy of fire suppression in California forests. Ecosphere. 6(1): Article 8, 23pp.

Wang, X., Thompson, D.K., Marshall, G.A., Tynstra, C., Carr, R., Flannigan, M.D. 2015. Increasing frequency of extreme fire weather in Canada with climate change. Climatic Change. 130: 573-586.

Waring, R.H. and Coops, N.C. 2016. Predicting large wildfires across western North America by modeling seasonal variation in soil water balance. Climatic Change. 135: 325-339.

Westerling, A.L., Tumer, M.G., Smithwick, E.A.H., Romme, W.H., Ryan, M.G. 2011. Continued warming could transform Greater Yellowstone fire regimes by mid-21st century. PNAS. 108(32): 13165-13170.

Young, A.M., Higuera, P.E., Duffy, P.A., Hu, F.S. 2016. Climatic thresholds shape northern high-latitude fire regimes and imply vulnerability to future climate change. Ecography. 39: 001-012.

Wildfire in a Warming World: Part 1

(click here to download a pdf of the full article or a one-page synopsis)

By Jennifer Hushaw

Wildfire has been a hot topic in the media lately, with California ablaze due to a combination of dangerous fire weather, on-going drought, and scores of drought- and beetle-killed trees. Most recently, the Blue Cut Fire (pictured above) burned 37,000 acres and the Soberanes Fire, which began in late July, is still uncontained and has already burned over 105,600 acres. Of course, severe wildfires are certainly nothing new—take the Great Fire of 1910 that burned 3 million acres or the Miramichi Fire in New Brunswick that burned over 3.8 million acres in 1825—but climate change is creating new cause for concern. In fact, the 2014 U.S. National Climate Assessment says climate change-related fire is increasing the vulnerability of U.S. forests to ecosystem change and tree mortality.

In Part I of this two-part piece, we examine recent trends in fire activity, untangle the role of climate change, and outline the most important aspects of climate that drive fire patterns on the landscape.

The Global Picture

Climate change is expected  to increased wildfire activity in certain regions (IPCC AR5), but, so far, there is little evidence of an increase in area burned or an increase in fire severity for many parts of world, and even some evidence suggesting that, overall, there is less fire on the landscape now than centuries ago (Doerr & Santín 2016).

A 2013 study by researchers at the University of Maryland, the University of California, and VU University in Amsterdam showed this variability in fire trends across different regions (Giglio et al. 2013). They examined global and regional trends in the monthly area burned from 1996 to mid-2012 using information from the fourth generation of the Global Fire Emissions Database (GFED) and they found different, and sometimes opposite, trends depending on the region, as well as a slight decrease in global area burned over the last 16 years (see Figure 1). The study did not analyze what caused these trends, but researchers did offer some insight into the source of year-to-year variability observed in each region. The level of variability relates to ignition and rainfall—regions with fairly consistent fire activity tend to have widespread and routine human-induced burning (mostly for land maintenance) and similar rainfall from year-to-year (e.g. southern and northern Africa), whereas regions with high variability have more sporadic fire ignition (by humans or otherwise) and more variability in the amount of rain from year-to-year (e.g. Equatorial Asia, where fire activity is closely tied to El Niño events) (Giglio et al. 2013).

Wildfire in the U.S.

In contrast to the global picture, we have seen increasing media reports of large wildfires and more discussion of the ballooning cost of fire control efforts in the United States in recent years. This is especially true when conflagrations break out at the intersection of forests and developed lands, also known as the Wildland Urban Interface (WUI). Given the greater cost of suppression efforts and increased risk of damage to lives and property in these areas, some have suggested that the perceived increase in wildfire may simply be due to increased attention. However, as we will discuss below, there is ample evidence that large portions of the U.S. have indeed experienced more fire.

Most studies of wildfire trends in the U.S. are focused on the western part of the country and they have detected an overall increase in fire activity over recent decades. For example, there is evidence that the number of large fires (> 1,000 acres) and/or total area burned per year increased in many parts of the western U.S. from 1984-2011 (Figure 2; Dennison et al. 2014).

Another research team detected a similar trend since 1970, with a noticeable increase in western forest fire activity beginning around the mid-1980’s (see Figure 1 (a) and (c) in Westerling 2016), including more frequent large fires (> 1,000 acres), fires that burned longer, and longer wildfire seasons (Westerling et al 2006; Westerling 2016). Additionally, there is evidence that the number of very large fires (> ~12,400 acres) has also been increasing over the last 30 years, particularly across parts of the southeastern and southwestern U.S. (Barbero et al. 2014).

These observations square with another recent study (Jolly et al 2015), which showed that conditions have become more conducive to fire in many parts of the world. Researchers used historic climate data from 1979 to 2013 to calculate the length of the fire weather season over the last 35 years, based on several common fire danger indices used in the U.S., Canada, and Australia. They found that, globally, the length of the average fire weather season increased by almost 19% and the amount of land area affected by long fire weather seasons doubled. Their results coincided with evidence of recent trends in the U.S. For example, in 2012, about 47% of the vegetated area of the U.S. experienced a longer-than-normal fire weather season according to their estimates and that resulted in a “near-record setting ~3.8MHa [9.4 million acres] of burned area” (Jolly et al. 2015).

Although, even in the U.S., not all regions are experiencing more wildfire. The Northeast, for example, is known for some historic mega-fires, but now has an annual average area burned of only 0.04% of forest area and fire histories show a decline for the last 60 years in both the area burned and the size of fires (Irland 2013). This decline is likely due to better detection, regulation, and control methods since the 1950’s, but it also means more fuel has accumulated on the landscape (Irland 2013). Importantly, extreme fire behavior can still occur in regions with very infrequent fire and, in fact, almost every part of the Northeast has experienced extreme fire years or unusually large fires since that decline began in the 1950’s (Irland 2013).

A Climate Change Connection?

A number of factors have contributed to the recent increase in U.S. wildfires, including higher fuel loads from historic fire suppression and large-scale forest mortality due to bark beetle outbreaks, but an important question is: To what extent, if any, has climate change contributed to these observed trends? Evidence suggests it has played a role. The Intergovernmental Panel on Climate Change (IPCC) noted that climate change has already had a major impact on wildfire activity in North America, some parts of Europe (specifically Portugal and Greece), and Africa in recent years (IPCC AR5 SPM, Figure SPM.4). In fact, fire is one of the key climate change-related risks they list for North America and Europe and that risk is projected to increase as we move farther into this century (IPCC AR5 SPM, Figure SPM.8). The 2014 U.S. National Climate Assessment also identified a link between climate change and fire, noting that “Climate change is contributing to an increase in wildfires across the U.S. West.” They point specifically to hotter and drier weather and earlier snowmelt, which has increased the length of the fire season and amount of acreage burned (NCA 2014).

Numerous studies suggest changes in climate are largely responsible for observed wildfire trends in the U.S. (Littell et al. 2009; Jolly et al. 2015; Westerling et al. 2006; Westerling 2016; Attiwill & Binkley 2013) and there is evidence for this connection in both modeling and observational studies. For example, Barbero et al. (2014) developed models that successfully replicated historic fire patterns across the continental U.S. using ONLY climate variables, which indicates climate is the major mechanism driving fire activity. Another study found that climate was a significant driver of fire activity in the western U.S. from 1916 to 2003—explaining an average of 39% of the variability in area burned—and there was an even stronger linkage in the most recent decades from 1977 to 2003, where climate explained an average of 64% of the area burned (Littell et al. 2009). A third example, mentioned earlier, is research from Dennison et al. (2014) that found similar wildfire trends across a variety of western U.S. ecoregions with different vegetation types, fire seasons, fuel types, and fire frequencies (see Figure 2), but similar increases in severe drought. This suggests prevailing climatic conditions, rather than site-specific dynamics, are responsible. Evidence suggests climate change will only continue to play a bigger role in shaping fire trends, as we move from global fire regimes that were driven by precipitation during the pre-industrial period and by human ignition and suppression during the 18th century, into a new temperature-driven global fire regime (Pechony & Shindell 2010).

Climate & Fire: Take Homes for Forest Managers

Climate Drives Fire Activity

While there are many factors that determine where, when, and how wildfires burn, climate is well-recognized as one of the primary controls on fire activity (Dennison et al. 2014). At the simplest level, it influences the availability and flammability of fuels, but fire requires three components—biomass to burn, the right atmospheric conditions, and ignition—and climate affects all of these components “in complex ways and over multiple timescales” (Moritz et al. 2012). In the short-term, climate controls ignition and propagation, while in the long term it affects fuels by influencing primary productivity and vegetation (Urbieta et al. 2015).

Time scale Matters

The diagram in Figure 3 (below) outlines how different aspects of climate control fire at different scales. For example, in some North American regions, fire regimes are closely related to natural modes of climate variability, such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) (Whitlock et al. 2010; Fauria & Johnson 2008)—see our previous bulletin on Global Temperature Trends for an explanation of how the ENSO/PDO system influences fire regimes worldwide. We are accustomed to thinking about the drivers of fire at shorter timescales (i.e. at the level of individual fires—seasonal-to-annual—and how they propagate—hourly-to-annual) because fire management and response systems typically operate at those scales, but the influence of decade-to-decade climate fluctuations and the long-term warming trend  are no less important. Changes in average climate, variability, and long-term trends can shift the fire regime in a given region and, since climate conditions are projected to continue changing for the foreseeable future, it is reasonable for forest managers to expect and prepare for deviations from the historic fire regime in their area.

Drought & Fire Risk

More immediate fire-climate impacts will most likely come through drought, which also played a role in the record-breaking 2015 U.S. fire season. Drought is projected to increase in both frequency and severity in many regions and it is known to directly affect fire severity, extent, and frequency (Littell et al. 2016). The uncertainty of future precipitation patterns, as discussed in an earlier bulletin, presents a challenge because there are very few regions of the world where we have confidence in predictions of future drought. That said, warmer temperatures alone can increase the intensity of individual drought events (and associated fire risk) by increasing potential evapotranspiration (as discussed previously)—a factor that has been implicated in the severity of the recent California drought (Diffenbaugh et al. 2015).

What’s Limiting: Fuel or Moisture?

The impact of climate change on fire activity will largely depend on whether an ecosystem tends to be more fuel-limited or climate-limited (Littell et al. 2009; Steel et al. 2015; Whitlock et al. 2010). In the former, fire is limited by the amount of fuel available and these are often drier, more sparsely vegetated areas that experience frequent, lower-severity wildfires. In contrast, fire in climate-limited systems is controlled by fuel flammability and these are generally more moist systems with significant biomass that experience infrequent, more severe wildfires. For example, more precipitation can actually increase fire risk in fuel-limited ecosystems by increasing the quantity of fine fuels (Dennison et al. 2014), compared to climate-limited ecosystems like the boreal forest where moisture rather than fuel accumulation is the primary determinant of fire behavior and more precipitation would typically decrease fire risk (Fauria & Johnson 2008). Factors like human intervention can flip a system from one end of the spectrum to another. For example, researchers studying 130 years of fire history in a Spanish province in the Mediterranean found that rural depopulation and farm abandonment increased fuels and shifted the fire regime in that region from fuel-limited to climate-limited, leading fire activity to become more drought-driven (Pausas & Fernandez-Munoz 2012). Fire activity in these climate-limited ecosystems is sensitive to changes in temperature, so warming may especially increase fire risk in these systems.

Coming up…

In Part II, we outline what the latest research suggests in terms of future fire frequency, extent, and/or severity and we discuss the changing context of more fire in the WUI, rising costs, and changing organizational structures, as well as providing useful resources for more information.

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Attiwill, P., Binkley, D. 2013. Editorial: Exploring the mega-fire reality: A ‘Forest Ecology and Management’ conference. Forest Ecology and Management. 294: 1-3.

Barbero, R., Abatzoglou, J.T., Steel, E.A., Larkin, N.K. 2014. Modeling very large-fire occurrences over the continental United States from weather and climate forcing. Environ. Res. Lett. 9: 11pp.

Dennison, P.E., Brewer, S.C., Arnold, J.D., Moritz, M.A. 2014. Large wildfire trends in the western United States, 1984-2011. Geophys. Res. Lett. 41: 2928-2933.

Diffenbaugh, N.S. Swain, D.L., Touma, D. 2015. Anthropogenic warming has increased drought risk in California. PNAS. 112(13): 3931-3936.

Doerr, S.H. and Santín, C. 2016. Global trends in wildfire and its impacts: perceptions versus realities in a changing world. Phil. Trans. R. Soc. B. 371: 20150345.

Fauria, M.M. and Johnson, E.A. 2008. Climate and wildfires in the North American boreal forest. Phil. Trans. R. Soc. B. 363: 2317-2329.

Giglio, L., Randerson, J.T., van der Werf, G.R. 2013. Analysis of daily, monthly, and annual burned area using the fourth-generation global fire emissions database (GFED4). Journal of Geophysical Research: Biogeosciences. 118: 317-328.

IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

Irland, L.C. 2013. Extreme value analysis of forest fires from New York to Nova Scotia, 1950-2010. Forest  Ecology and Management. 294: 150-157.

Jolly, W.M., Cochrane, M.A., Freeborn, P.H., Holden, Z.A., Brown, T.J., Williamson, G.J., Bowman, D.M.J.S. Climate-induced variations in global wildfire danger from 1979 to 2013. Nature Communications. 6: 7537. DOI: 10.1038/ncomms8537.

Littell, J.S., McKenzie, D., Peterson, D.L., Westerling, A.L. 2009. Climate and wildfire area burned in western U.S. ecoprovinces, 1916-2003. Ecological Applications. 19(4): 1003-1021.

Littell, J.S., Peterson, D.L., Riley, K.L., Liu, Y., Luce, C.H. 2016. A review of the relationships between drought and forest fire in the United States. Global Change Biology. 22: 2353-2369.

Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2.

Moritz, M.A., Parisien, M-A., Batllori, E., Krawchuk, M.A., Van Dorn, J., Ganz, D.J., Hayhoe, K. 2012. Climate change and disruptions to global fire activity. Ecosphere. 3(6):49.

Pausas, J.G., Fernández-Muñoz, S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change. 110: 215-226.

Pechony, O., Shindell, D.T. 2010. Driving forces of global wildfires over the past millennium and the forthcoming century. PNAS. 107(45): 19167-19170.

Steel, Z.L., Safford, H.D., Viers, J.H. 2015. The fire frequency-severity relationship and the legacy of fire suppression in California forests. Ecosphere. 6(1): Article 8, 23pp.

Urbieta, I.R., Zavala, G., Bedia, J., Gutiérrez, J.M., San Miguel-Ayanz, J., Camia, A., Keeley, J.E., Moreno, J.M. 2015. Fire activity as a function of fire-weather seasonal severity and antecedent climate across spatial scales in southern Europe and Pacific western USA. Environ. Res. Lett. 10:114013.

Westerling, A.L. 2016. Increasing western US forest wildfire activity: sensitivity to changes in the timing of spring. Phil. Trans. R. Soc. B. 371: 20150178.

Westerling, A.L., Hidalgo, H.G., Cayan, D.R., Swetnam, T.W. 2006. Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity. Science. 313: 940-943.

Whitlock, C., Higuera, P.E., McWethy, D.B., Briles, C.E. 2010. Paleoecological Perspectives on Fire Ecology: Revisiting the Fire-Regime Concept. The Open Ecology Journal. 3: 6-23.

Resiliency Assessment Framework

(click here to download a pdf of this complete article)

By Eric Walberg


Optimizing forest management to account for the risks and opportunities posed by climate change is a challenge on many fronts. Limitations in the precision of climate models, uncertainties about natural system response, and the need to integrate climate concerns with other aspects of forest management are all complicating factors.

To better position CSLN members to respond to these risks and opportunities Manomet is proposing the development of a monitoring and management system that we are calling the Resiliency Assessment Framework (RAF). The RAF will track changing regional climate and forest conditions and allow CSLN members to evaluate conditions on their lands in light of the changing regional context. This approach is intended to support the inclusion of climate concerns in forest management plans by providing an opportunity to link management decision points to the data inputs from the RAF.

The RAF will be an optional component of the CSLN and will be intentionally structured to minimize additional work load and maximize utility in improving forest management. While many research questions associated with climate change and forest response might be addressed by this type of monitoring framework, our goal is a system that will maximize forest health and productivity.

Proposed Program Structure

The RAF will consist of three major components:

  • climate-science-center-regionsAssessment and Tracking of Changing Regional Context: Manomet will develop a synopsis of climate change and forest response trends and projections for each of the six regions associated with the U.S. Climate Science Centers.
  • Local Monitoring and Evaluation: Participating CSLN members will establish or augment an existing monitoring program to document changing forest conditions on the lands that they manage and support comparison of local and regional forest conditions.
  • Climate Component for Forest Management Plans: Participating CSLN members will develop a climate change component in their forest management plans that links to the monitoring data generated by the RAF.

The majority of the regional data will come from public sources and will be based on research that is already underway or just being started. The frequency and extent of data collection on local forest conditions will be at the discretion of participating CSLN members.

Initial Development, Testing and Evaluation in New England

Initial development of the RAF will take place as a pilot effort in New England over the next year. Manomet will begin the process by developing a regional trends and projections report for the Northeast region and by working with participating CSLN members with land holdings in New England to develop and test local monitoring protocols and draft climate components for forest management plans. Assuming that the pilot phase is successful, the RAF will be expanded region by region in following years.

Questions the RAF Will Address

The two overarching questions that the RAF is intended to address are:

  • How are climate conditions changing by region and site?
  • How are forest conditions changing in response?

Through the pilot phase of the project Manomet will work with CSLN members to develop a set of specific research questions. As previously stated, the intent is to strike a balance between level of effort and utility in improving forest management. Examples of the type of specific questions that might be posed include:

  • Is a warming climate increasing growth rates for particular tree species?
    • Possible regional climate metrics: change in length of growing season, change in average annual temperature, range of seasonal temperatures
    • Possible regional forest metrics: FIA forest growth rates by species
    • Possible local forest metrics: growth rates by species
  • How is the frequency and severity of drought changing and are there discernable forest impacts?
    • Possible regional climate metrics: PDSI (Palmer Drought Severity Index), SPI (Standardized Precipitation Index), soil moisture, VPD (Vapor Pressure Deficit)
    • Possible regional forest metrics: Remote sensing of forest health and mortality, FIA forest health metrics, FIA tree mortality data, canopy water content
    • Possible local forest metrics: tree crown condition, tree mortality
  • How is temperature variability changing and is there a discernable forest response?
    • Possible regional climate metrics: frequency of freeze/thaw events, range of seasonal temperatures, frequency of false springs
    • Possible regional forest metrics: Remote sensing of frost damage following leaf out, FIA forest health metrics, FIA forest growth rates
    • Possible local forest metrics: tree crown condition, growth rates
  • Are storm events changing in frequency and intensity and what is the forest response?
    • Possible regional climate metrics: maximum wind speed, frequency of threshold wind exceedance, persistence of storm systems
    • Possible regional forest metrics: FIA forest health metrics, FIA tree mortality data, wind throw
    • Possible local forest metrics: tree crown condition, wind throw

Obvious challenges exist in differentiating climate impacts from the many other factors that influence forest health and productivity. However, as climate change progresses it is likely that climate-related factors will become more prominent forest stressors and will therefore be more readily identifiable. For example, recent studies of the impact of “hot drought” on forests in the western U.S. have identified a climate change component in the excessive heat that has exacerbated tree mortality. The RAF can potentially serve as an early warning system and hopefully will provide insight on emerging factors in forest health and changing competitiveness of particular tree species.

Measures of Success

Key measures of success for the RAF include the following considerations:

  • Does implementation of the RAF result in forest management insight would not occur in its absence?
  • Is the RAF worth the investment of time and energy?
  • Does the RAF improve understanding of how climate change is impacting forests?
  • Does the RAF provide a mechanism for engagement and education on climate change?
  • Over the long term, does the RAF result in improved forest health and resiliency?

Once the pilot phase is underway it will be possible to identify more specific measures of success associated with specific research questions.

The next step in this process will be a survey of CSLN members to get a better idea of what you are currently doing in the way of forest monitoring and to take a first cut at identifying priority research questions. The survey results will be discussed at the October 26, 2016 CSLN member gathering in Boston.