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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Basler, D., and Körner, C. 2014. Photoperiod and temperature responses of bud swelling and bud burst in four temperate forest tree species. Tree Physiology. 34: 377-388.

Carter, J.M., Orive, M.E., Gerhart, L.M., Stern, J.H., Marchin, R.M., Nagel, J., Ward, J.K. 2017. Warmest extreme year in U.S. history alters thermal requirements for tree phenology. Oecologia. 183(4):1197-1210.

Chen, X., Wang, L., Inouye, D. 2017. Delayed response of spring phenology to global warming in subtropics and tropics. Agricultural and Forest Meteorology. 234-235; 222-235.

Delpierre, N., Vitasse, Y., Chuine, I., Guillemot, J., Bazot, S., Rutishauser, T., Rathgeber, C.B.K. 2016. Temperate and boreal forest tree phenology: from organ-scale processes to terrestrial ecosystem models. Annals of Forest Science. 73: 5-25.

Delpierre, N., Guillemot, J., Dufrêne, E., Cecchini, S. 2017. Tree phenological ranks repeat from year to year and correlate withgrowth in temperate deciduous forests. Agricultural and Forest Meteorology. 234-235: 1-10.

Fu, Y.H., Zhao, H., Piao, S., Peaucelle, M., Peng, S., Zhou, G., Ciais, P., Huang, M., Menzel, A., Peñuelas, J., Song, Y., Vitasse, Y., Zeng, Z., Janssens, I.A. 2015. Declining globalwarming effects on the phenology of spring leaf unfolding. Nature. 526: 104-107.

Jackson, R.B., Lechowicz, M.J., Li, X., Mooney, H.A. 2001. Phenology, growth, and allocation in global terrestrial productivity. In: Saugier, B., Roy, J., and Mooney, H.A. (Eds.) Terrestrial Global Productivity: Past, Present, and Future. Academic: San Diego, CA, pp. 61-82.

Jeganathan, C., Dash, J., Atkinson, P.M. 2014. Remotely sensed trends in the phenology of northern high latitude terrestrial vegetation, controlling for land cover change and vegetation type. Remote Sensing of Environment. 143: 154-170.

Körner, C. and Basler, D. 2010. Phenology Under Global Warming. Science. 327: 1461-1462.

Kramer, K., Ducousso, A., Gömöry, D., Hansen, J.K., Ionita, L., Liesebach, M., Lorent, A., Schüler, S., Sulkowska, M., de Vries, S., von Wühlisch, G. 2017. Chilling and forcing requirements for foliage bud burst of Europeanbeech (Fagus sylvatica L.) differ between provenances and arephenotypically plastic. Agricultural and Forest Meteorology. 234-235: 172-181.

Laube, J., Sparks, T.H., Estrella, N., Höfler, J., Ankerst, D.P., Menzel, A. 2014. Chilling outweighs photoperiod in preventing precocious spring development. Global Change Biology. 20: 170-182.

Laube, J., Sparks, T.H., Estrella, N., Menzel, A. 2014b. Does humidity trigger tree phenology? Proposal for an air humidity based framework for bud development in spring. New Phytologist. 202: 350-355.

Morin, X. and Chuine, I. 2014. Will tree species experience increased frost damage due to climate change because of changes in leaf phenology? Can.J. For. Res. 44: 1555-1565.

Myneni, R.B., Keeling, C.D., Tucker, C.J., Asrar, G., Nemani, R.R. 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature. 386: 698-702.

Primack, R.B., Higuchi, H., Miller-Rushing, A.J. 2009. The impact of climate change on cherry trees and other species in Japan. Biological Conservation. 142(9): 1943-1949.

Richardson, A.D., Keenan, T.F., Migliavacca, M., Ryu, Y., Sonnentag, O., Toomey, M. 2013. Climate change, phenology, and phenological control of vegetation feedbacks to the climate system. Agricultural and Forest Meteorology. 169: 156-173.

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 weather@home 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.