Archive for category: Bulletins

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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.

POLAR VORTEX
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.

Conclusion

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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References

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 http://www.realclimate.org/index.php/archives/2018/10/. Last accessed Mar. 3 2019.

National Weather Service. “What is the Polar Vortex?” Available online at https://www.weather.gov/safety/cold-polar-vortex. Last accessed mar. 19 2019.

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

Palmer, T.N. 2013. Climate extremes and the role of dynamics. PNAS. 110(14):5281-5282. doi:https://doi.org/10.1073/pnas.1303295110.

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. www.pnas.org/cgi/doi/10.1073/pnas.1222000110.

Petoukhov, V., Rahmstorf, S., Petri, S., and H.J. Schellnhuber. 2013b. Reply to Screen and Simmonds: From means to mechanisms. PNAS. 110(26):E2328. www.pnas.org/cgi/doi/10.1073/pnas.1305595110.

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. http://www.nature.com/doifinder/10.1038/nclimate2268.

Screen, J.A. and I. Simmonds. 2013. Caution needed when linking weather extremes to amplified planetary waves. PNAS. 110(26):E2337. www.pnas.org/cgi/doi/10.1073/pnas.1304867110.

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. https://doi.org/10.1007/s40641-018-0111-4.

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 https://www.worldweatherattribution.org/winter-in-north-america-is-cold-dec-2017-jan-2018/. 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 https://sites.krieger.jhu.edu/waugh/research/polarvortex/. Last accessed Mar. 19 2019.

(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.

SIMILAR RISK FACTORS

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.

NEW HOSTS

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).

LANDSCAPE-LEVEL RISK

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.

 

NEW RESEARCH 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).

 

RESOURCES:

• See the Southern Pine Beetle II General Technical Report from the U.S. Forest Service Southern Research Station for a comprehensive resource on the topic of SPB.
• The Southern Pine Beetle Prevention and Restoration program is a good example of an existing program in the Southeast that managers in the Northeast might use as a model. In particular, the use of similar outreach efforts and management recommendations may become important as SPB moves into the Northeastern region where forest ownership is dominated by non-industrial private owners.
• The Forest Pest Conditions application and the Insect and Disease Detection Survey Data Explorer can provide maps of areas recently affected by SPB.
• The National Insect & Disease Risk Map has mapped potential risk from numerous pests, including southern pine beetle, through 2027.

Conclusion

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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References

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:http://dx.doi.org/10.1016/j.foreco.2017.01.035.

Ayres, M.P. and M.J. Lombardero. 2018. Forest pests and their management in the Anthropocene. Canadian Journal of Forest Research. 48: 292–301 doi:dx.doi.org/10.1139/cjfr-2017-0033.

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: http://dx.doi.org/10.5849/forsci.15-071.

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: http://dx.doi.org/10.1111/j.1365-294X.2011.05086.x.

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:http://dx.doi.org/10.1111/j.1365-2699.2011.02673.x.

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:https://doi.org/10.1139/cjfr-2013-0240.

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:https://doi.org/10.5849/jof.15-002.

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.

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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.

WHAT THEY DID

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.

WHAT THEY FOUND

• 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)

WHAT IT MEANS

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 5^th and 95^th 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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References

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.

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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 climateprediction.net platform) to determine how climate change affects weather on a smaller scale.

HOW IT’S DONE

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).

WHAT’S NEW?

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.

HOW RELIABLE IS IT?

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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References

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: http://wxshift.com/news/blog/linking-extreme-weather-to-climate-change

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: https://public.wmo.int/en/resources/bulletin/unnatural-disasters-communicating-linkages-between-extreme-events-and-climate

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

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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.

Wildfire Trends

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 18^th 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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References

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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.

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