Climate Change & Forest Productivity

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

By Jennifer Hushaw

Considerable attention has been paid to understanding how climate change may alter the abundance and distribution of tree species, as discussed in our previous bulletin on modeling future forests, but an equally important consideration is how climate change may alter productivity. Research suggests climate change has the potential to affect not just where, but how well our forests grow. This bulletin highlights recent work by researchers at Virginia Tech, University of Maine, the US Forest Service, and others that estimates how productivity may shift as a result of changing climate.

Productivity & Site Index

One measure of potential forest productivity is site index—defined as the average total height (in meters) that dominant and co-dominant trees attain at a specified base age in even-aged stands. Of course, this is based on the assumption that forests are even-aged and consist of a single species, which can be problematic in regions dominated by uneven-aged, mixed species stands, such as the eastern US. However, it is a commonly used metric that provides some utility. Researchers have responded to these known limitations by attempting to predict observed site index using site attributes like climate, topography, and soil characteristics.

Study Overview

In a recent study published in the Canadian Journal of Forest Research (Jiang et al. 2015), researchers created models to predict site index (base age 50) for 20 eastern tree species (as well as hardwoods and conifers generally*) based on 15 soil and 37 climate variables. The models successfully predicted current site index values observed through USFS Forest Inventory and Analysis (FIA) data, which provides confidence in their ability to accurately represent on-the-ground conditions. Then they were re-run with future, rather than contemporary, climate data to estimate how site index would change under projected future conditions.

They utilized vegetation and site index data from the FIA program, contemporary (1961 to 1990) and future (2030, 2060, and 2090) climate data from the USFS Rocky Mountain Research Station representing four different future emissions scenarios, and soils data from the USDA Soil Survey Geographic (SSURGO) database.

The models were developed using a statistical technique (Random Forests—a classification and regression tree analysis) that has been shown to be very adept at analyzing these types of research questions and datasets. With the analysis they employed, they were also able to calculate confidence intervals to determine if the predicted changes in site index were significant or not.


Researchers used all climate and soil variables (to ensure maximum predictive accuracy) to generate maps of site index change. They found that site index may increase or decrease in the future, depending on the species and the geographic region in question. Site index models were generated for individual species, as well as two broad species groups (hardwood and conifer), but only the species-group results were reported.

Key Findings

Current site index:
  • There was more variation in observed site index for conifers than hardwoods across the eastern US.
  • The models that performed best at predicting site index included BOTH climate and soils data.
  • Models that used only climate data to predict site index performed better than those using soils alone.
  • Variables that were useful for predicting site index included:
    • Soil pH
    • Effective soil depth (especially for conifers)
    • Total available soil water capacity
    • Ratio of summer precipitation to total precipitation
    • Summer-winter temperature differential
    • Growing-season precipitation (April—September)
  • For conifers, current site index showed a pattern of increasing from north to south.
  • For hardwoods, current site index showed a pattern of increasing from north and west to southeast.
Future Site Index:
  • For conifers, there was a significant increase in average site index (+0.5 – +2.4m) over the 21st
  • For hardwoods, there was a significant decrease in average site index (up to -1.7m) over the 21st
  • Several regions showed contrasting results depending on the climate change scenario.
  • Variables that were important for determining future changes in site index were related to:
    • The ratio of growing-degree-days to summer precipitation
    • The start and length of the frost-free season
    • Average and accumulated growing-season temperatures
    • Changes in moisture index or summer temperatures in combination with changes in midwinter ambient temperatures
  • Under the lowest emissions scenario, more FIA plots showed a significant increase in site index and fewer showed a significant decrease, whereas the higher emissions scenarios consistently showed the opposite result—suggesting that there may be some overall benefit for forest productivity under moderate warming that disappears under higher levels of warming.


This study is an example of the research being done to better understand how changing climate conditions will alter forest productivity. A key take-home is how different the forest response can be depending on the rate and level of warming, which is also a key area of uncertainty. This highlights the important ramifications of different climate trajectories and suggests that forest managers may want to keep their eye on those trends over time. In some cases, it may be beneficial to shift the species mix toward those projected to experience an increase in site index. However, these results should also be considered in conjunction with other research related to potential productivity changes, such as growth increases due to CO2 fertilization or decreases due to extreme heat events. We will continue to monitor new and emerging research on this topic going forward.


Note: For those interested in a similar analysis for western US tree species, please see a paper by Weiskittel et al (2011) entitled Linking climate, gross primary productivity, and site index across forests of the western United States.

* Species comprising the hardwood and conifer species groups in this analysis:

Hardwood Conifer
White oak Loblolly Pine
Yellow poplar Shortleaf Pine
Quaking aspen Eastern white pine
Northern red oak Balsam fir
Sugar maple Red pine
Red maple Slash pine
Black oak Black spruce
White ash Tamarack (native)
Green ash N. white cedar
Sweetgum Virginia pine


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Jiang, H., Radtke, P.J., Weiskittel, A.R., Coulston, J.W., Guertin, P.J. 2015. Climate- and soil-based models of site productivity in eastern US tree species. Can. J. For. Res. 45: 325-342.

Weiskittel, A.R., Crookston, N.L., Radtke, P.J. 2011. Linking climate, gross primary productivity, and site index across forests of the western United States. Can. J. For. Res. 41: 1710-1721.



Carbon Markets and Forests: What Does the Future Hold?

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

By Eric Walberg


Global efforts to reduce carbon emissions are ramping up and carbon markets will play an increasingly important role in limiting warming to the 2 degree C ceiling established by the Paris Climate Agreement.  I participated in the largest annual North American carbon market conference in San Diego, CA, May 4-6, 2016. The California Cap and Trade system formed the backdrop for many of the sessions, but the conference drew international participants and presentations covered a range of global topics. Recurrent themes included the impact of the Paris Climate Accord, the future of the California carbon market, linkages between the California Cap and Trade system and other sub-national programs, the future of the Clean Power Plan, and possible inclusion of Reducing Emissions from Deforestation and Forest Degradation (REDD) projects in the California market.

What are the opportunities and ramifications for the forestry sector? What role will forests play in meeting the national commitments associated with the Paris agreement? This month’s bulletin is an exploration of those questions as informed by the presentations and discussions at the conference.

Key Themes from the Conference


The ramifications of the December 2015 Paris Climate Agreement were discussed in several of the sessions. The general consensus is that the Agreement sends a positive signal for global expansion of carbon pricing, regulations, compliance markets, and voluntary markets in efforts to limit warming to the 2 degree C goal. The Paris Agreement establishes a bare bones framework for the creation of new international carbon markets and cooperative approaches to reducing greenhouse gas emissions.  The high-level framing in Article 6 of the Agreement (see hyperlink pg. 23) must be fleshed out before the nature of the resulting markets and cooperative agreements becomes clear.  It is too early to know what opportunities for the forestry sector might result.

A related topic in these discussions was the tremendous need for additional capital to finance the conversion to a green economy. Additional regulation and/or establishment of a price on carbon were two methods discussed to drive capital towards needed solutions. Green bonds are also seen as a growth area with some early indications of investor preference for green bonds driving higher prices in the secondary market.  China is poised to have a major impact as they establish a national cap and trade system in 2017 and become a major issuer of green bonds. Market commentators estimate that China’s green bond market could be worth $230 billion in the next five years.


The California Cap and Trade system appears to have addressed many of the fundamental flaws of earlier systems. A price floor in the offset market has created demand and stability. Rigorous project review on the part of the California Air Resources Board has created investor confidence, but has also created significant hurdles for offset project developers. Thus far, the bulk of the emission reductions in California have been driven by actions within capped sectors (to reduce emissions), rather than through purchase of offset credits from non-capped sectors such as forestry. The California system continues to function as a laboratory of innovation and will influence international efforts to fulfill the Paris Agreement.

The initial enabling legislation (AB32) established the program through 2020 and efforts to extend the cap and trade framework are underway in the California legislature. The general sense from the conference discussions is that that program will be extended, but will likely be modified in the process. The program also faces a lawsuit by the California Chamber of Commerce (CCC). A state judge rejected the assertion by the CCC that the program is an illegal tax, but an appeal is pending.


Forest offset projects account for the majority of offset projects approved in the California Cap and Trade system so far. Three categories of forest offset projects are possible: (1) afforestation, (2) avoided conversion, and (3) improved forest management. Thus far, the majority of the approved projects are in the improved forest management category. Several sessions touched on the notion of “charismatic” carbon credits—projects that bring co-benefits such as flood control and water quality protection. It is not clear if this is a factor in the prevalence of forest offset projects in the compliance market.

One of the sessions included an in-depth discussion of areas of risk associated with forest offset projects and how parties attempt to allocate that risk in contract negotiations. The five areas of risk were: (1) the possibility of credit invalidation, (2) failure to deliver a successful project, (3) reversal of a project, (4) land restrictions associated with projects, and (5) price*.

  • Invalidation of a project is a risk during the first eight years of a project and could result from a material misstatement (greater than 5%) of the carbon sequestration value of a project, failure to comply with all relevant environmental, health and safety laws, or double dipping (selling the same offsets into multiple programs).
  • Project delivery failure could be associated with multiple causes such as insolvency, failure to meet statutory deadlines, negligence, etc.
  • Reversal could be due to unintentional causes such as fire or wind storm, or intentional causes such as negligence or willful intent. In the event of an unintentional reversal the credits are covered by a buffer pool maintained by the California Air Resources Board. In the case of intentional reversal the owner is responsible for replacing the credits.
  • Land restrictions are primarily to meet the carbon sequestration goals associated with the contract. Harvesting is permitted as long as carbon sequestration goals are met. The land can be transferred but subsequent owners must assume all carbon responsibilities. A second category of land restrictions are those associated with compliance with the previously mentioned environmental, health and safety laws.
  • Price concerns include both the market value of the credits and the contractual allocation of risk and financial reward among all of the parties involved in assembling an offset project.

*Source: From a presentation entitled: “The Art of the Deal: Legal Considerations with Carbon Projects” by Erika Anderson of Anderson Law on May 4, 2016 at the Navigating the American Carbon World conference in San Diego, California.


The California Cap and Trade system is a hub for a growing network of sub-national entities that are linking their markets. The Western Climate Initiative (WCI) was launched in 2007 to evaluate and implement policies to tackle climate change. Membership of the WCI includes California, Washington, Oregon, Arizona, New Mexico, Montana, Utah, British Columbia, Manitoba, Ontario, and Québec. Building on the WCI, California and Québec completed an agreement in 2013 linking their programs. Ontario has indicated that they will be next to link their program.

In a parallel effort, California began engaging on REDD through the creation of the Governors’ Climate and Forest Task Force in 2008. This subnational government initiative is a network and forum focused on forest conservation, climate mitigation, and exploring linkage between REDD programs and the California Cap and Trade system. This process resulted in development of a 2010 Memorandum of Understanding between California, Acre, Brazil, and Chiapas, Mexico to assess the details associated with program linkage. Technical and policy recommendations were developed by a team of experts and delivered to all three of the partners in 2013. It is not yet clear when this process might result in formal linkage of REDD projects with the California market or exactly what form that linkage would take.

Challenges and Opportunities for the Forestry Sector

As signatory nations strive to meet the emission reduction goals associated with the Paris Agreement it is certain that afforestation, forest protection, and enhanced forest management will be key features of national programs. Given the prominent role that forest offset projects play in the California Cap and Trade system it is reasonable to assume that forest offsets will be included in other markets. Regulatory change and/or establishment of a price on carbon will likely increase the value of the climate regulation services provided by forests. The following are my thoughts on the challenges and opportunities for the forestry sector based on the conference discussions.

  • Participation in the California offset market: Barring a successful legal challenge to the program or significant regulatory change, the opportunity for forest offset projects in the California market will continue to be the most tangible and immediate opportunity for the forestry sector in the compliance market. Opportunities for offset projects will continue to exist in the voluntary market, but it is not likely that prices will rival those offered in the compliance market.
  • REDD offset market: Early indications are that, if and when REDD projects enter the California market, the only project categories that will be approved are afforestation and reforestation. It is possible that enhanced forest management projects will eventually be included pending a successful pilot period with afforestation and reforestation projects.
  • Possible inclusion of biomass energy and/or forest offsets in programs that develop in response to the Clean Power Plan: Given the legal challenges to the CPP it is difficult to know if the draft regulatory language will survive intact. If the existing framework does move forward it appears that the use of biomass energy will be available to states as one element of transition to renewable energy portfolios. The details of the carbon accounting and economics associated with biomass under the CPP are not known at this point. It does not appear likely that forest offset projects will be accepted by EPA as a substitute for reducing emissions from existing power generation facilities, but it is possible that carbon markets that develop in response to the CPP could include forest offsets in a broader context.
  • Regional Greenhouse Gas Initiative (RGGI) forest offsets: A 2014 reduction in the RGGI emissions cap by 45% resulted in renewed interest in the market and allowance prices have increased from $2.80 in March of 2013 to $7.05 in December of 2015, with a drop to $5.25 in March of 2016. These prices have not been high enough to foster demand for offset projects but that could change in the future if allowance prices rise.
  • Competitive advantage for wood building materials: Regulatory change and carbon pricing will likely increase the expense of competing materials with a larger carbon footprint, such as concrete and steel.
  • Increased efforts to reduce deforestation in the tropics: These efforts may reduce opportunities for commercial forestry operations in the tropics and could result in increased demand for forest products from temperate zones.
  • REDD and related projects: New commercial opportunities may emerge for sustainable forestry and agroforestry as a component of REDD projects and other efforts to comply with the Paris Agreement.

Manomet will continue to monitor developments in the policy realm and carbon markets and will report back to Climate Smart Land Network members on an as needed basis.

Modeling Future Forests

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

By Jennifer Hushaw

Understanding how forests responded to past changes in the Earth’s climate has been a long-standing area of research, but in recent years there has also been growing interest in anticipating how modern climate change may lead to shifts in tree species abundance and distribution. Of course, climate is just one of many factors that determine why trees grow where they grow, including soils, competition, land-use history, and synergistic relationships with other species, e.g. seed dispersers. Partly as a result of this complexity, there is a lot of uncertainty in the results of vegetation models that estimate future habitat suitability or distribution of tree species.

Two things we can say for sure are that (1) tree species will respond independently to changing conditions, so we may see novel species associations and forest types in some locations in the future and (2) there will be significant time lags in forest response (see box, right).

In this bulletin, we briefly review several modeling efforts and how they compare, as well as highlighting potential limitations and best practices for utilizing the results.

Vegetation Models: An Overview

The vegetation models used to assess potential tree species shifts can be broadly sorted into two categories on either end of a spectrum, from empirical (i.e. statistical) to process-based (i.e. mechanistic) models.

Empirical models quantify statistical relationships between species occurrence data, such as plot data from the US Forest Service Forest Inventory and Analysis (FIA), and relevant environmental variables, such as soils and climate, then use those correlations to project into the future. These are often referred to as species distribution models, niche models or bioclimatic envelope models.

Example: DISTRIB, the core model used in development of the Climate Change Tree Atlas (see table below), uses a statistical approach known as regression tree analysis to define the ecological niche of a species based on (1) a series of climate, soil, elevation, and landuse predictor variables and (2) data from FIA about the relative abundance of a species in the overstory. More specifically, it utilizes a relatively new ensemble data-mining technique called Random Forests, which has some improvements that are designed to avoid “overfitting” the data. For more detail on the technique read this paper by Prasad et al 2006. Based on these results, the model has been used to map where the habitat (in terms of climate conditions, soil characteristics, etc.) is suitable for a particular species now and in the year 2100. This tells us something about how likely a species is to persist in particular area.

Note: In the future, the Climate Change Tree Atlas will use DISTRIB in conjunction with a simulation model called SHIFT to go beyond predictions of future suitable habitat and estimate actual species movement in terms of the likelihood of colonization.

Process-based models are generally more complex because they simulate the actual underlying processes, such as disturbance, growth, and regeneration. Forest gap models, ecosystem models, forest landscape models, and dynamic global vegetation models (DGVMs) fall under this category.

Example: LANDIS PRO is a spatially explicit forest landscape model that simulates processes at the species- (e.g. growth, seedling establishment, mortality), stand- (e.g. competition, stand development), and landscape-scale (e.g. disturbance from fire, insects, harvest, etc.). By “growing” the forests in this way, LANDIS can be used to compare species in the future under a climate change and no climate change scenario. This tells us something about how likely a species is to become established in a particular area.

These categories are not mutually exclusive and there are an increasing number of hybrid approaches used in research. Nor is one approach necessarily better than another—each has its strengths and weaknesses depending on scale, data availability, and the particular research question. A helpful summary of key differences is below:

For more detailed information on this topic, we recommend visiting the Landscape Analysis section of the US Forest Service Climate Change Resource Center website.

Table 1 in this paper by Littell et al (2011) also has a useful comparison of the strengths and weaknesses of different types of empirical and process models, for reference.

Model Comparison

The table below compares several modeling efforts that estimate changes in habitat suitability or distribution for U.S. tree species under future climate change. Model names are hyperlinks that take you to the project website where you can view results, including maps (in some cases), for different species. This table is intended to help forest managers quickly navigate to existing projections of species shift and weigh the merits and characteristics of each approach.

Comparing the results from different models reveals whether they generally agree (lending greater confidence) or disagree on the outlook for particular species. Some of this work is being carried out by the US Forest Service through their on-going series of Vulnerability Assessments (see final row in the table below) and the CSLN will alert Network members to similar comparative efforts as they arise.

Best Practices

Models incorporate imperfect information and are a simplified version of reality, but by understanding these imperfections, we can use models to decrease the uncertainty associated with the future.” ~ Littell et al 2011

  • Remember there will be significant time lags.
  • Consider projections for individual species, rather than forest types.
  • Use models to help reduce uncertainty about the future by identifying potential surprises and vulnerabilities1, potential magnitude of effects, and insight into mechanisms.2
  • Use more than one type of model (wherever possible) to assess likely vegetation shifts1—we can have greater confidence where different models agree.
  • Understand the assumptions in a given model and the implications of those assumptions.1
  • Use MODFACs, a decision support framework that scores adaptability for different tree species, in conjunction with models to determine whether a species is likely to fare better or worse than modeled projections.
  • Mistake maps of habitat suitability for depictions of where a tree species will actually be growing at that point in the future.
  • Use model projections as exact predictions of what will happen with future forest shifts.

1 Littell et al 2011    2 Kerns & Peterson 2014

Click the image to open a pdf of the model comparison table (with live hyperlinks):



Underestimating Adaptability

As we noted in a previous bulletin, there are some limitations associated with modeling efforts that rely on statistical relationships between environmental variables and current species distributions derived from FIA data (i.e. the realized niche), since that represents only a portion of the possible conditions under which a species could grow (i.e. the fundamental niche). Revisit part of our July 2015 bulletin on uncertainty and forest response for a brief explanation of how the absence of data on the fundamental niche can lead to underestimating the potential adaptability of some tree species. This is not to say that forests aren’t vulnerable in other ways, such as increasing damage from exotic pests and extreme weather, but they may be more adaptable in terms of temperature tolerance than some results suggest.

Take-Home Message

As an initial step, we recommend CSLN members spend a little time perusing the results of the modeling efforts listed above, to get a sense for the general outlook for species that dominate their economic or management concerns. Noting where (and if) the models agree can highlight potential areas of vulnerability (or opportunity) to be explored further. Members who have an interest in digging-in on projections for a particular species, can contact the CSLN staff for additional assistance.

All the modeling efforts agree on at least one thing—conditions are going to change. Most tree species will begin to experience novel climate conditions in some portion of their range and, in some cases, that may lead to local extirpation. Ultimately, the uncertainty is in knowing exactly where and when these species distribution shifts will happen. Generally, we expect species range expansion at the leading edge, in northern and higher elevations, and range contraction at the trailing edge, in southern and low-altitudinal limits. In particular, look for initial forest composition changes at range margins because it is regeneration success or failure there that will determine whether a species persists or migrates.


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Adams, H.D., Williams, A.P., Chonggang, X., Rauscher, S.A., Jiang, X., McDowell, N.G. 2013. Empirical and process-based approaches to climate-induced forest mortality models. Frontiers in Plant Science. 4 (438):5pp.

Iverson, L.; McKenzie, D. (February, 2014). Climate Change and Species Distribution. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center.

Kerns, B.; Peterson, D.W. (May, 2014). An Overview of Vegetation Models for Climate Change Impacts. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center.

Littell, J.S., McKenzie, D., Kerns, B.K., Cushman, S., Shaw, C.G. 2011. Managing uncertainty in climate-driven ecological models to inform adaptation to climate change. Ecosphere. 2(9): 102.

Loarie, S.R., Duffy, P.B., Hamilton, H., Asner, G.P., Field, C.B., Ackerly, D.D. 2009. The velocity of climate change. Nature. 462:1052-1055.

Pearson, R.G. 2006. Climate change and the migration capacity of species. Trends in Ecology and Evolution. 21(3):111-113.


Climate Change & Wildlife Impacts: Part 2

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

By Jennifer Hushaw, Si Balch, and Eric Walberg

In Part I of this bulletin, we described how climate change may soon rival human influence as the biggest driver of biodiversity change, and in this piece we look more closely at the links between climate and the habitat requirements of some specific groups of wildlife and game species in North America.

We don’t have much knowledge about exactly how climate will affect wildlife, even when compared to the uncertainties of forest response to climate change (as discussed previously). This is because wildlife species are higher up the trophic chain, with complicated interactions that determine their health and geographic limits (e.g. predator-prey relationships). In contrast, the distribution of vegetation communities is linked directly to climatic drivers and a short list of other factors, such as soils. The key to anticipating potential impacts is to understand the habitat characteristics that allow a species to survive in a particular place and determine how climate change might influence those conditions. The most robust predictions will be cases where a species has life history traits that are known to be particularly climate sensitive, such as the snowshoes hare’s reliance on snow cover for camouflage.

Wildlife species are far more mobile than plants and will therefore be able to respond quickly to changing climate conditions—changes in the behavior, distribution, or population of wildlife species are early indicators of climate change in the field. Quick response times will also make it easier for managers to adjust their management strategies and adapt based on results that are observable within a decade or so, rather than the multiple decades, or longer, needed to observe shifts in vegetation.

The impacts of climate change are not always direct—climate can and will affect species in less obvious ways through shifting habitat suitability, changes in prey availability or abundance, altered patterns of herbivory, and others. These indirect impacts can pose a major risk to wildlife when they exacerbate existing stressors. Any effort on the part of forest owners or managers to maintain, improve, or increase habitat for climatically-vulnerable species will help buffer against shifts in desired wildlife.

Species Highlights

Studies examining the impacts of climate change on specific wildlife species are still a relatively new realm of science and, while there is an incredible amount of research being done, the information available varies greatly depending on the species—as shown in the relative length of the sections below. The management recommendations included here are also general in nature, reflecting the fact that many of the standard wildlife management techniques we already employ are suitable for responding to the impacts of climate change for number of species.

Deer, Moose, Elk

Climate change will affect the population dynamics, range limits, habitat selection, browsing/foraging behavior, and disease outbreaks of these ungulate species.

As conditions change, moose and deer may alter their habitat selection, shifting where and when they utilize certain types of habitat. For example, decreases in lake ice in Michigan have led to more lake effect snow that creates harsh winter conditions for deer and increases their reliance on shelter in conifer swamps (although this increased precipitation is expected to shift more toward rain over the next century) (Hoving & Notaro 2015). White-tailed deer are not expected to decline as a direct result of climate change, but these types of changes in migration patterns and seasonal habitat are likely (Hoving et al 2013). Similarly, it has been documented that moose will change their behavior to alleviate heat stress, by moving to areas of higher and denser forest canopy when they reach a daytime temperature threshold of around 68⁰F (Melin et al 2014; Street et al 2015; NWF 2013a).

Range limits will also shift, and in some cases they already have, e.g. white-tailed deer have expanded into western boreal forests and climate has been shown to be an important factor determining their presence in that region (Dawe et al 2014). At the northern edge of their range, white-tailed deer are controlled by low winter temperatures and snow depth, so conifer stand deer yards are important for their survival because they provide thermal cover and reduced snow depth. As the climate changes, this cold/snow limiting line will move and two things are likely to result: (1) the more southerly deer yards will become less critical to survival and (2) deer populations will increase. Similarly, research on moose in China has revealed that climate is an important factor influencing population dynamics there; increases in late spring temperatures, in particular, have the potential to shift the southern limits of moose distribution northward (Dou et al 2013).

Changes in moose and deer population dynamics have been linked to large-scale climate patterns, particularly the North Atlantic Oscillation (NAO), which determines much of the snowfall and winter temperatures in northern latitudes (Post & Stenseth 1998). Likewise, recent research suggests that warmer temperatures and a shorter period of high quality forage in spring have led to nutritional deficiencies in maternal moose that decreased recruitment in the southern part of their range (Monteith et al 2015). As cold-adapted species, moose are generally considered to be highly vulnerable to climate change and decreases in abundance are likely by the middle of the century (Hoving et al 2013).

Climate change-induced decreases in snowpack have also led to shifts in browsing or foraging behavior in both moose and elk. In the case of moose, low snow conditions can increase browse on balsam fir compared to sugar maple or Viburnum (Christensen et al 2014). For elk, less snowpack means easier access to aspen shoots, which has caused substantial declines in aspen recruitment, particularly in the Rocky Mountains (Brodie et al 2012). In fact, climate change may actually provide some positive benefits for elk in the form of milder winters and better forage (NWF 2013a). Importantly, these kinds of climate-driven changes in plant-herbivore interactions can have wide reaching affects within the larger ecological community (Auer & Martin 2012).

Lastly, as discussed in Part I, climate change is altering pest and disease dynamics, including the transmission of wildlife diseases. White-tailed deer are vulnerable to hemorrhagic disease (HD), including epizootic hemorrhagic disease and bluetongue viruses, which are transmitted by biting midges. The first fall frost usually kills the insects, but longer summers will mean longer exposure times and hot, dry weather (which is likely to increase) has been strongly associated with past outbreaks, which suggests that the risk of widespread deer mortality from these diseases will increase (Hoving et al 2013; NWF 2013a). In recent years, warmer, shorter winters have also spelled trouble for moose populations, as winter ticks have proliferated enough to cause a significant increase in moose mortality (heavy infestations leave moose weak, vulnerable to disease, and at risk of cold exposure and death in cases where they rub off their insulating hair in an attempt to rid themselves of the ticks) (NWF 2013a).

Management Considerations:

  • Monitor for changing browsing patterns.
  • Provide areas of high, dense forest canopy for moose, particularly in southern parts of their range.
  • Factor increased deer browsing into regeneration planning.


Canada Lynx

Climate change will affect the population dynamics, distribution and abundance of prey species, hunting success, connectivity with peripheral populations, and range margins of lynx populations.

Canada Lynx is a charismatic animal that has drawn a great deal of conservation interest since its listing as a threatened species under the Endangered Species Act in March of 2000. It is considered highly vulnerable to climate change because it is a cold-adapted species that is particularly well-suited to hunting in deep snow, which gives it a competitive advantage over other predators (an advantage that will be lost with decreasing snow cover).

The decrease in snow cover will not only affect hunting success, but will also affect the distribution and abundance of the primary prey species, the snowshoe hare, whose populations are expected to shift northward due to climate change (Murray et al 2008). This is partly because hares in southern locations (with decreasing snow cover) often find themselves mismatched with their surroundings when they molt into their white winter coat in the absence of snow, which makes them far more visible to predators, with weekly survival decreases up to 7% (Zimova et al 2016). In contrast, the range of snowshoe hares has expanded in some northern locations, particularly Arctic Alaska, where warming temperatures and expanded shrub habitat have created more suitable conditions (Tape et al 2015).

Along with prey species abundance, climate itself is an important determinant of lynx population dynamics. Large-scale climate patterns, including the North Atlantic Oscillation index (NAO), the Southern Oscillation Index (SOI), and northern hemispheric temperature, play a role in producing and modifying the classic 10-year population cycles associated with lynx and snowshoe hare in the boreal parts of their range, by influencing rain and snowfall patterns (Yan et al 2013).

Climate change is also affecting connectivity between core and peripheral lynx populations, especially island populations that are sustained by immigration of individuals from other areas. Individuals from the core of the lynx range migrate over frozen rivers to reach island habitats, so warming conditions and less frequent formation of ice bridges will leave these populations even more isolated (Koen et al 2015; Licht et al 2015). As a result, range margin shifts are expected (and in some cases already observed) that include contraction of these smaller, peripheral groups, as well as northward contraction of the southern range boundary and the core population areas (Carroll 2007; Koen et al 2014).

Management Considerations:

  • Provide large, contiguous tracts of landscape, especially where there is connectivity with more stable Canadian populations of lynx.
  • Maintain patches of young, dense conifers for hare habitat.



Climate change may affect bat population distributions, reproductive success, hibernation behavior, and access to food.

Climate is known to influence the biogeography of bats, as well as their access to food, timing of hibernation, development, and other factors, so it is likely that changing climate conditions may adversely impact some bat species—some specific life history characteristics that may put bats at risk from climate change include (Sherwin et al 2012):

  • Small range size,
  • high latitude or high altitude range,
  • range that is likely to become water stressed,
  • fruit-based diet,
  • restricted to aerial hawking (prey pursued and caught in flight),
  • and restricted dispersal behavior.

Throughout the globe, there have been a number of documented cases of shifting bat populations linked to climate change, including range expansion of at least one Mediterranean species (Ancillotto et al 2016) and mostly northward shifts in a number of species in China (Wu 2016). In the Czech Republic, evidence suggests that a temperate, insectivorous bat is benefiting from rising spring temperatures, but the effect may be buffered by excessive summer rain that decreases reproductive success (Lučan et al 2013)—an example of the complicated nature of predicting exactly how climate change will impact a given wildlife species.

Of course, climate change is not the most immediate concern in the United States, where the introduction of white-nose syndrome to the eastern U.S. in the early 2000’s led to a massive decline in bat populations. However, changing climate conditions do have the potential to further stress these decimated populations, which is a cause for concern. This also highlights the need to protect the genetic diversity within refugial populations, especially on the leading edge for northward migration (Razgour et al 2013).

One particularly hard hit species, the Northern Long-eared Bat (NLB), was listed as threatened under the Endangered Species Act (ESA) and a final rule was released in January 2016 detailing the protections for this species under the ESA. Use these links to access a range map for the NLB and up-to-date maps of documented cases of white-nose syndrome, as well as details about the Final 4(d) Rule for the NLB under the ESA—there are some considerations for forest managers.

Management Considerations:

  • Leave a ¼ mile buffer around known hibernaculum*.
  • Leave a 150ft buffer around documented or potential maternal roosting trees*, especially during the pupping season in June & July.

* Contact your state agency or US Fish & Wildlife Service for more information about hibernaculum and maternal roost tree locations.


Forest Song Birds

Climate change will alter migration patterns, population dynamics, and the quality and availability of habitat for forest song birds.

Song bird species have exhibited a variety of responses to recent climate change. In particular, shifts in timing have been observed for some migratory species, including spring arrival shifted several days to more than a week early (depending on the species), such as Baltimore Oriole, Eastern Towhee, Red-eyed Vireo, Ruby-throated Hummingbird, and Mountain Bluebirds (NWF 2013b; Manomet). There is mixed evidence regarding changes in fall migration, with both early and late shifts observed in migrants passing through Massachusetts (Ellwood et al 2015).

Birds have the advantage of being able to respond rapidly to warming temperatures, but their ability to adapt depends on where they overwinter, how they receive their migration cues, and the level of mismatch between migration timing and the availability of associated food sources. In fact, evidence from 33 years of bird capture data collected by Manomet’s land bird conservation program suggests that short-distance migrants respond to temperature changes, while some mid-distance migrants respond to temperature and/or changes in the Southern Oscillation Index, and long-distance migrants tend not to change over time (Miller-Rushing et al 2008).

The vulnerability of individual species is also related to their specific habitat requirements and whether climate change may alter the availability of quality breeding or foraging areas. For example, a study of over 160 bird species in the Sierra Nevada mountains of California found that those associated with alpine/subalpine and aquatic habitats ranked as the most vulnerable, while those associated with drier habitats (i.e. foothill, sagebrush, and chaparral associated species) may experience range expansion in the future (Siegel et al 2014). Challenges may also arise for bird species that rely on temperature-sensitive prey species for food, such as aerial insectivores (e.g. Common Nighthawk, Chimney Swift, and Bank Swallow) that eat flying insects.

Lastly, as we have seen for other groups of species, rapid shifts in the distribution of wild birds will have implications for the spread and abundance of wildlife diseases (Van Hemert et al 2014).

Management Considerations:

Note: Visit the Climate Change Bird Atlas from the U.S. Forest Service for maps of projected change in species distribution for 147 birds in the eastern U.S.


Game Birds (Grouse, Turkey, Quail)

Climate change may affect habitat suitability and availability for important game bird species, as well as their breeding success and population dynamics—positive and negative projections vary from species to species.

Climate plays a role in the distribution of game bird species, as it does with many others. In fact, the population dynamics of several gamebirds seem to be influenced by large-scale climatic patterns (Kozma et al 2016; Williford et al 2016; Lusk et al 2001), but the effects of climate change are expected to vary significantly from one species to the next. For example, Black Grouse in Finland have experienced population declines for four decades related to seasonally asymmetric climate change. In particular, springs have warmed faster than the early summer period, so grouse lay their eggs earlier and then experience higher chick mortality when they hatch before temperatures are sufficiently warm (Ludwig et al 2006). Similarly, Spruce Grouse is considered moderately vulnerable because its montane spruce-fir habitat is rare (and likely to decline) in the southern edges of its boreal range. On the other hand, Ruffed Grouse is a resident species in the northeast U.S. whose range is projected to decrease and shift further north, even as overall populations remain relatively stable (Rodenhouse et al 2008; Hoving et al 2013).

In contrast, some gamebirds are likely to fare even better under climate change, including Wild Turkey (which has expanded northward (Niedzielski & Bowman 2015) and will benefit from less severe winters (Hoving et al 2013)), Northern bobwhite (which is likely to increase (Hoving et al 2013)), and Sage Grouse (which studies suggest may enjoy an increase in suitable habitat in some regions, such as southeastern Oregon, by the end of the century (Creutzburg et al 2015)).

Management Considerations:



Climate change has already led to increased temperatures in freshwater systems, putting cold-water fish species at risk of physiological stress or extirpation in certain waterways, while some warm-water species may experience increased growth rates and northward expansion.

Climate change has the potential for significant adverse impacts on cold-water fish species such as brook and rainbow trout. These species depend on access to cold water for reproduction and may also suffer from an increase in summer low flow stream conditions. As discussed in the August 2014 Bulletin, designing stream crossings to accommodate floods associated with the increase in heavy precipitation also has the benefit of minimizing fragmentation of aquatic habitat. Intact stream systems allow fish and other aquatic species to move in search of appropriate temperature and flow regimes.

For warm-water fish, evidence suggests that some species, such as small mouth bass, may experience increased growth rates as temperatures rise (although this growth effect may taper off if conditions become too warm later in the century) (Pease & Paukert n.d.). Some warm-water fishes have also moved northwards and are likely to continue expanding into freshwater systems traditionally dominated by cold-water species (Groffman et al 2014).

Management Considerations:

  • Upsize culverts, transition to arched structures, or use removable crossings to provide the win/win of reduced infrastructure damage from floods and enhanced connectivity of aquatic habitat.



Climate change will drive changes in habitat availability and suitability for amphibian species, which are highly sensitive to changes in temperature and precipitation.

There is weak evidence that climate change is driving observed declines in amphibian populations in various locations worldwide (Li et al 2013), but a number of studies suggest that future climate change is likely to lead to declines and/or range contractions (Barrett et al 2014; Loyola et al 2014; Wright et al 2015). These changes will be driven by a reduction in climatically suitable habitat, reduced soil moisture (which will reduce prey abundance and lead to loss of habitat), reduced snowfall and increased summer evaporation (which will change the duration and occurrence of seasonal wetlands) (Corn 2005).

Amphibians are particularly vulnerable to changing climate because their ectothermic physiology makes them very sensitive to temperature and precipitation changes, they have low dispersal capability, and often have strong associations with temporary wetlands that are likely to be threatened by climate change (Tuberville et al 2015).

Management Considerations:

  • Maintain appropriate buffer areas around water bodies, vernal pools, ephemeral and intermittent streams that act as amphibian habitat.


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Hoving, C.L. and Notaro, M. 2015. Ice, Snow, and Swamp: Managing Deer in Michigan’s Changing Climate. Michigan Journal of Sustainability. Volume 3, Spring: pp 101-110.

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Koen, E.L., Bowman, J., Wilson, P.J. 2015. Isolation of peripheral populations of Canada lynx (Lynx canadensis). Canadian Journal of Zoology. 93(7): 521-530.

Kozma, R., Melsted, P., Magnusson, K.P., Hoglund, J. 2016. Looking into the past – the reaction of three grouse species to climate change over the last million years using whole genome sequences. Molecular Ecology. 25(2): 570-580.

Li, Y., Cohen, J.M., Rohr, J.R. 2013. Review and synthesis of the effects of climate change on amphibians. Integrative Zoology. 8: 145-161.

Licht, D.S., Moen, R.A., Brown, D.P., Romanski, M.C., Gitzen, R.A. 2015. The Canada Lynx (Lynx canadensis) of Isle Royale: Over-harvest, Climate Change, and the Extirpation of an Island Population. The Canadian Field-Naturalist. 129: 139-151.

Loyola, R.D., Lemes, P., Brum, F.T., Provete, D.B., Duarte, L.D.S. 2014. Clade-specific consequences of climate change to amphibians in Atlantic Forest protected areas. Ecography. 37: 65-72.

Lučan, R.K., Weiser, M., Hanák, V. 2013. Contrasting effects of climate change on the timing of reproduction and reproductive success of a temperate insectivorous bat. Journal of Zoology. 290(2): 151-159.

Lusk, J.J., Guthery, F.S., DeMaso, S.J. 2001. Northern bobwhite (Colinus virginianus) abundance in relation to yearly weather and long-term climate patterns. Ecological Modelling. 146(1-3): 3-15.

Melin, M., Matala, J., Mehtatalo, L., Tiilikainen, R., Tikkanen, O., Maltamo, M., Pusenius, J., Packalen, P. Moose (Alces alces) reacts to high summer temperatures by utilizing thermal shelters in boreal forests – an analysis based on airborne laser scanning of the canopy structure at moose locations. Global Change Biology 20: 1115-1125.

Miller-Rushing, A.J., Lloyd-Evans, T.L., Primack, R.B., Satzinger, P. 2008. Bird migration times, climate change, and changing population sizes. Global Change Biology. 14: 1959-1972.

Monteith, K.L., Klaver, R.W., Herset, K.R., Holland, A.A., Thomas, T.P., Kauffman, M.J. 2015. Effects of climate and plant phenology on recruitment of moose at the southern extent of their range. Oecologia. 178: 1137-1148.

Murray, D.L., Steury, T.D., Roth, J.D. 2008. Assessment of Canada Lynx Research and Conservation Needs in the Southern Range: Another Kick at the Cat. The Journal of Wildlife Management. 72(7): 1463-1472.

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Niedzielski, B. and Bowman, J. 2015. Survival and cause-specific mortality of the female eastern wild turkey at its northern range edge. Wildlife Research. 41(7): 545-551.

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Climate Change & Wildlife Impacts: Part 1

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

By Jennifer Hushaw

Climate change may soon outmatch traditional human influence as the biggest driver of change in biological diversity (i.e. biodiversity) over the coming century (Bellard et al. 2012; Jones et al. 2016). We will see dynamic changes as some species expand their ranges to take advantage of new suitable habitat, others shift into new regions and form novel species associations, and many others alter their physiology, behavior, or preferred range in an attempt to adapt to the changing conditions. Given that forests are home to 80% of the world’s terrestrial biodiversity (WWF 2015), the conservation and management of forestland has been, and will continue to be, a key part of efforts to maintain habitat for these species as they readjust. In this way, forest managers can help guard against the potential for biodiversity loss by enhancing forest health and ecosystem function through management.

This bulletin is the first in a two-part piece—it outlines the observed and expected climate change impacts for wildlife globally, as well as the forest management approaches that dovetail best with supporting at-risk species. Part two will delve more specifically into the anticipated climate impacts and wildlife management considerations for a number of key species in North America.

Climate Change & Wildlife Impacts

Climate change will impact wildlife directly through changes in temperature and water availability, and indirectly through effects on food sources, associated species, and habitat conditions. Some organisms will certainly benefit from the coming changes, while others may decline or even go extinct—depending (in large part) on the unique responses of individual species and populations (Staudinger et al. 2012). We will examine some specific examples in Part 2 of this bulletin, but the major ways climate change will affect wildlife include:

Transforming habitat

Changes in temperature, precipitation, and underlying vegetation will transform the habitat for many species. Extreme events may also induce shifts of entire ecosystems. Organisms with specific habitat requirements will not adapt as easily, while generalists will fare better.

Shifts in timing/seasonal changes

Changes in seasonality (e.g. shorter winters, earlier springs) may cause important species relationships to be out of sync, such as pollinators and their plants, predator-prey relationships, host-parasite relationships, etc. These changes can also affect the availability of food sources for migrating species and the optimal timing of reproduction for certain organisms.

Range shifts/migration

More mobile species will ‘follow’ their optimal climate conditions into new regions. As species ranges shift over time, we will see natural communities, species associations, and interactions that are entirely novel.

Spreading pests and disease

Climate-induced species shifts are expected to increase the frequency of new host-parasite associations and emerging infectious diseases, as new species come in contact with each other (Hoberg & Brooks 2015)—with implications for the health of human and wildlife populations. Warmer temperatures may also increase pathogens within intermediate hosts and vectors, or increase survival of animals that harbor disease (USGS 2012).

What makes wildlife species resilient to climate change?

There are particular characteristics that make species and populations more or less at risk from climate-related disturbance. The least at risk will have “tolerance to broad range of factors, such as temperature, water availability and fire,” a “high degree of phenotypic plasticity,” a “high degree of genetic variability,” “short generation times (rapid life cycles) and short time to sexual maturity,” high fertility, “‘generalist’ requirements for food, nesting sites, etc.,” “good dispersal ability,” and “broad geographic ranges.” In contrast, the most at risk species will have nearly opposite characteristics in each of these areas, e.g. poor dispersal and low genetic variability (Staudinger et al. 2012).


As the climate changes, many species will find that their current habitat is outside their climate niche—the full range of temperature and precipitation conditions in which they normally occur—and in order to adapt they must change where, when, or how they operate.

Where – Species can respond spatially by shifting their range and following their optimal climate conditions.

When – Species can respond temporally by changing their phenology, e.g. shifting the timing of key life cycle events.

How – Species can respond physiologically or behaviorally by developing tolerance to warmer/drier conditions or changing their diet, activity, or energy budget.

(Bellard et al. 2012)

In the short-term, these changes can happen through phenotypic plasticity, while in the long-term they will happen through evolution, but the rate of climate change may still be faster than the pace at which many species can effectively adapt.

Observed Changes

Changes in the phenology, range boundaries, and abundances of wildlife species have been documented across the globe. Over the past decade, synthesis studies have pulled this literature together to see whether current climate change has left a discernable fingerprint on the Earth’s biodiversity. The challenge for biologists is separating (relatively) small, systematic trends that might be caused by climate from short-term local changes that might be caused by land-use or natural species shifts (Parmesan & Yohe 2003). Taken together, the evidence suggests that climate change impacts are already being felt. In fact, the IPCC concluded (with high confidence) that: “Many terrestrial, freshwater, and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances, and species interactions in response to ongoing climate change.” (IPCC 2014)

Observed changes and conclusions from these meta-analyses include:

  • Earlier spring events (bud burst, flowering, breaking hibernation, migrating, breeding)—documented on all but one continent and in all major oceans.
  • Changes in phenology that put an increasing number of predator-prey and insect-plant systems out-of-sync (with mostly negative consequences).
  • Documented examples on all continents (and in most of the major oceans) of warm-adapted communities expanding and individual species shifting their ranges poleward.
  • Range-restricted species (i.e. polar and mountaintop organisms) experience the biggest range contractions, with evidence of some actual extinctions. Tropical coral reefs and amphibians are the most negatively impacted.
  • These observed changes have been linked to climate change through long-term correlations between climate and biological variation, experiments in the field and laboratory, and basic physiological research.

(Parmesan 2006)

More recent work supports these results and suggests that species are migrating even faster than previously thought. In their meta-analysis, Chen et al. (2011) found that species moved away from the equator at a median rate of 16.9 km per decade and upslope at a median rate of 11.0m per decade. Overall, they found a significant shift toward higher latitudes and elevations (for 3/4 of species) and the shifts were largest where climate has warmed the most. Nevertheless, about one quarter of species actually shifted in the opposite direction, highlighting the diversity of species response and the fact that there are sometimes other competing drivers at work.

Model Projections  

The most recent report from the IPCC examined all the scientific evidence regarding impacts on the world’s biodiversity and concluded (with high confidence) that:

”A large fraction of both terrestrial and freshwater species faces increased extinction risk under projected climate change during and beyond the 21st century, especially as climate change interacts with other stressors, such as habitat modification, over-exploitation, pollution, and invasive species.”

They also noted that this risk will increase with the magnitude and rate of climate change (IPCC 2014).

There is limited evidence of extinctions caused by warming so far, but we know that slower, natural climate changes in the Earth’s past led to major ecosystem shifts and species extinctions, so there is reason to believe we will see similar (or even more severe) impacts under the rapid warming we’re experiencing now (IPCC 2014). As such, there has been a lot of research aimed at projecting species loss under climate change. Researchers use a wide variety of modeling techniques—from as simple as calculating a species’ current climate niche and seeing where it will be in the future, to capturing ecological processes or incorporating detailed physiological data. Each approach has its strengths and weaknesses, but most indicate “alarming consequences for biodiversity” (Bellard et al. 2012).

Importantly, we expect significant time-lags in species response (e.g. decades to centuries for vegetation), which can accumulate in ecosystems because of the way species interact with each other. This means that it is easy to underestimate the amount of biodiversity change at any given time and suggests that we should watch for non-linear changes in these ecological systems (Essl et al. 2015).

Forest Management Considerations  

Managing Wildlife Under Climate Change

It is now widely acknowledged that climate change should be an explicit consideration when setting conservation and wildlife management priorities because it will be such a major driver of change in the coming century. However, most of the approaches currently used to incorporate climate change into spatial conservation prioritizations are focused on the continuous and direct impacts of climate change, without accounting for either the discrete (e.g. extreme events) or indirect impacts (Jones et al. 2016)—a critical area for improvement.

More generally, there are a number of management strategies that can help promote the resilience of wildlife populations in a changing climate, which include:

  • Conserve a diversity of landscapes, in terms of topography and soils2
  • Protect/represent refugial habitats1,2
  • Prioritize ‘future habitat’1
  • Increase or enhance regional connectivity1,2
  • Increase heterogeneity1
  • Sustain ecosystem process and function2
  • Increase amount and connectivity of wildlands3
  • Aim for representation, resiliency, and redundancy—networks of intact habitat that represent full range of species and ecosystems in a given landscape3
  • Increase management for species in areas where they are expected to advance, e.g. northern range limits3
  • Reduce other, non-climatic stressors4
  • Establish habitat buffer zones and wildlife corridors4
  • Consider translocation of species to new areas and replanting disturbed areas with less climate-sensitive species4
  • Expand monitoring to consider longer-term changes4

(1Jones et al. 2016; 2Groves et al. 2012; 3USFS 2013; 4Glick et al. 2009)

Managing for Carbon vs. Wildlife

Forests are increasingly recognized as a critical component of climate change mitigation efforts because of their ability to sequester carbon, as evidenced by the inclusion of forests in the recent Paris Climate Accord, but there can sometimes be a tension between managing for carbon benefits and managing for biodiversity/wildlife.

For example, researchers examined the land-use implications of the emissions trajectories used by the IPCC, to see how our climate mitigation efforts might affect global biodiversity ‘hot spots’ and they found that these efforts were generally well-aligned with biodiversity protection because they reduced loss of vegetation cover. However, the most ambitious target (RCP 2.6) actually led to a loss of natural cover because it involves widespread conversion to bioenergy crops, in order to achieve net negative emissions—showing that climate and wildlife goals are not necessarily linked (Jantz et al. 2015).

Another recent analysis in Sweden (Felton et al. 2016) examined the tradeoffs between forest management strategies for climate change adaptation and mitigation and biodiversity goals for tree species composition, forest structure, and spatial/temporal forest patterns. They found that some strategies were definite win-wins, but there were also some notable conflicts, summarized below:

Felton et al 2016 Table

While the relative tradeoffs may differ slightly from region-to-region, this same framework can be used anywhere to assess the relative benefits of these strategies when there is a simultaneous goal of enhancing wildlife habitat. A useful approach is to focus on the win-wins and prioritize climate-focused management actions that are most consistent with biodiversity goals.


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IPCC, 2014: Summary for policymakers. 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. 1-32

Jantz, S.M., Barker, B., Brooks, T.M., Chini, L.P., Huang, Q., Moore, R.M., Noel, J. Hurtt, G.C. 2015. Future habitat loss and extinctions driven by land-use change in biodiversity hotspots under four scenarios of climate-change mitigation. Conservation Biology. 29(4):1122-31.

Jones, K.R., Watson, J.E.M., Possingham, H.P., Klein, C.J. 2016. Incorporating climate change into spatial conservation prioritisation: A review. Biological Conservation. 194: 121-130.

Parmesan, C. 2006. Ecological and Evolutionary Responses to Recent Climate Change. The Annual Review of Ecology, Evolution, Systematics. 37:637-69.

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

Staudinger, M.D., Grimm, N.B., Staudt, A., Carter, S.L., Chapin III, F.S., Kareiva, P., Ruckelshaus, M., Stein, B.A. 2012. Impacts of Climate Change on Biodiversity, Ecosystems, and Ecosystem Services: Technical Input to the 2013 National Climate Assessment. Cooperative Report to the 2013 National Climate Assessment. 296 p. Available at:

US Forest Service. 2013. Climate Change Adaptation and Mitigation Management Options: A Guide for Natural Resource Managers in Southern Forest Ecosystems. [Vose, J.M., Klepzig, K.D (eds.)]. CRC Press.

US Geological Survey. 2012. Climate Change and Wildlife Health: Direct and Indirect Effects. Fact Sheet 2010–3017, revised 2012.

World Wildlife Fund. 2015. “Forest Habitat: Overview.” Accessed January 15, 2015.

Climate Change and Extreme Weather, Part 2: Forest Impacts

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

By Jennifer Hushaw

In Part I of this bulletin, we described why and how we expect to see an increase in the frequency and intensity of some extreme weather and climate events as the planet warms. We examined the observed and projected trends in extreme heat, heavy precipitation, drought, and flooding. These extremes are likely to have a more immediate and drastic impact on our forests than long-term change in average conditions because they can trigger disturbance events that affect the health and composition of natural communities.

Extremes Shape Plant Communities

Long-term, gradual changes in average conditions are important for driving vegetation change, as we have discussed, but research also suggests that extremes play an equal or greater role in shaping the distribution, survival, productivity, and diversity of plant communities (Reyer et al. 2012). For example, a 2012 study found that growth in European forests for the past 500 years (based on extensive tree-ring data) responded synchronously to extreme climatic events during that period (Babst et al. 2012). This linkage between forest growth and extremes is likely to become even more important going forward, as we experience the increase in extremes discussed in Part I.

On a smaller scale, individual plant processes are also affected differently by extremes than by changing average conditions. For example, an increase in average temperature can affect phenology by lengthening the growing season and causing earlier leaf unfolding, but a change in variability in the form of early and late frosts can increase the risk of (possibly lethal) frost damage. Likewise, plant water relations can be disproportionately affected by extremes—an increase in night-time warming and average temperature can cause a slight increase in stomatal conductance, whereas an extreme event like drought can actually lead to mortality via stomatal closure and carbon starvation or hydraulic failure (Reyer et al. 2012). In this way, extremes are likely to play an outsized role in inducing the vegetation shifts researchers have been predicting.

Impact of Extremes on Forests


Of all the climate and weather extremes discussed here, drought receives the overwhelming amount of concern, attention, and research focus. There have been many studies conducted in southern Europe and southwestern United States in particular—both regions where climate models consistently project an increase in the frequency and intensity of future drought. In Europe, for example, researchers have found evidence of a widespread increase in crown defoliation between 1987 and 2007 due to drought, with long-term chronic effects that were most pronounced on drier sites (Carnicer et al. 2011).

U.S. Drought Impacts

In the US, much of the research on drought-related mortality has taken place in the semi-arid forests of the southwest, which provide a useful natural experiment because they are “highly vulnerable to climate-warming-induced drought” (Kolb 2015). These forests have already experienced drought-induced mortality in the form of large-scale die-off of piñyon-juniper woodlands, with an estimated 2.5 million acres affected (Peterman et al. 2013). A number of studies in this region have attempted to identify moisture thresholds (in terms of precipitation-evapotranspiration index, precipitation, vapor pressure deficit, tree-ring-based index of forest drought stress, pre-dawn leaf water potential, and others) that successfully predict tree mortality due to drought (Huang et al. 2015; Clifford et al. 2013; Williams et al. 2012; McDowell et al. 2015). When these thresholds are applied to future climate projections, the results generally point to massive forest mortality later in the century, e.g. McDowell et al. (2015) averaged a number of different modelling approaches and found that 72% of the needleleaf evergreen tree forests in the southwest US will experience mortality by 2050, with nearly 100% forest mortality by 2100.

CA-drought_Asner-et-al-2015California has also been in the spotlight for drought impacts in recent years, including a new study that found 10.6 million hectares of forest experienced measurable loss in canopy water content during the recent drought, with 1 million of those hectares experiencing severe water losses of greater than 30% (Asner et al. 2015). The California drought is illustrative of the types of droughts (and related impacts) we are likely to see more of in the future, as recent research has shown that anthropogenic warming contributed to the severity of the current drought and is increasing the probability of these co-occurring warm-dry conditions (Diffenbaugh et al. 2015). The drought impacts in California are also a great example of the worrisome ‘layering of stressors’ that can occur under these conditions, i.e. drought stresses trees, making them more vulnerable to insect attack, which ultimately increases mortality and the fuel loads to support massive wildfires.

The Real Risk of Climate Change-Type Drought

Despite the obvious risk from hotter drought, there are some natural processes that help stabilize plant populations after these types of disturbance events, such as self-thinning that reduces competition from neighbors, competition release through better adult reproductive performance, or phenotypic variability that helps mitigate mortality. These promote resilience by helping to balance the mortality caused by the event with better survival or increased recruitment (Lloret et al. 2012). These kinds of processes can be harnessed for adaptation purposes or used to help identify forests that are inherently more resilient. Given that not every extreme event leads to a shift in vegetation and these counterbalancing processes are often at work, there is some reason to be optimistic.

However, a recent comprehensive review of the literature suggests that the global outlook is generally negative—Allen et al. (2015) examined all the evidence suggesting that forests are more vulnerable to drought, e.g. trees die faster under warmer drought conditions, evolution is too slow relative to projected change, models are over-optimistic because mortality processes are not sufficiently represented, as well as all the evidence that forests are less vulnerable, e.g. physiological acclimation and adaptation capacities are large, species diversity and microsite variation can buffer mortality, CO2 fertilization and water-use efficiency can compensate for drought and heat stress. When all the pertinent issues were examined, they concluded we are most likely underestimating global vulnerability to hotter drought. Their conclusion was based, in part, on the fact that a number of drivers that are known with high confidence all point toward greater vulnerability, in particular:

  • Droughts eventually occur everywhere.
  • Warming produces hotter droughts.
  • Atmospheric moisture demand increases nonlinearly with temperature during drought.
  • Mortality can occur faster in hotter drought, consistent with fundamental physiology.
  • Shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly.
  • Mortality happens rapidly relative to growth intervals needed for forest recovery.

As we have discussed previously, drought risk actually varies with location, site characteristics, and forest type—see our previous bulletin for more detail on evaluating drought risk.

Extreme Heat

In an earlier bulletin, we explored how extreme heat affects plants at the cellular, leaf, and whole plant level, showing that trees have a variety of physiological and morphological responses that help them cope with extreme heat stress. While many species can behave plastically or increase their heat-tolerance by acclimating to warmer temperatures, there are, of course, limits to how adaptable trees can be in the face of these extremes.

Drought-induced forest die-off receives more attention in the scientific literature, but it is recognized that the trend of increasing hot temperature extremes will also pose problems for forests worldwide. As stated by Teskey et al. (2014): “Mortality from drought is far more likely than mortality from heat stress, but the severity of drought stress, and the speed of its onset, is greatly increased under high temperatures.” This is primarily because of the effect of temperature on vapour pressure deficit (VPD)—a key variable in plant water stress. VPD is essentially a combination of temperature and relative humidity that represents the ‘drying power’ of the air (technically, it is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated, or the vapour pressure in the air compared to the vapour pressure in the leaves). Generally, the higher the VPD the more water plants lose through transpiration. A number of studies have highlighted the critical role of VPD in determining the level of forest drought stress (Breshears et al. 2013; McDowell et al. 2015; Williams et al. 2012) and they suggest that rising VPD is “potentially the largest threat to survival” (McDowell et al. 2015) because climate models consistently predict that VPD and temperature will increase and we have confidence in those projections.

Floods/Heavy Preciptiation

While excess water (due to flooding or water-logged soils) is on the other end of the spectrum from severe drought, it can have similar negative consequences, especially for species that are not well-adapted to those conditions. This includes (paradoxically) decreased water absorption, stomatal closure that reduces CO2 uptake and growth, and low oxygen conditions in the soil that inhibit root respiration (Reyer et al. 2013). This kind of forest stress is likely to increase in many places during the next century, as we experience large flood events, especially in the Northeast and Midwest regions highlighted in Part I of this bulletin, and more heavy precipitation. Although, in the U.S., the effects are likely to be greatest in those regions with the least flood tolerant species mix.

Note: We recommend referencing Russell et al. 2014 for maps of average flood tolerance and diversity of flood tolerance among tree species in the eastern U.S.

Extremes Can Have Big Implications for the Global Carbon Budget

Most coupled carbon-climate models (e.g. CMIP5 used in the most recent IPCC Assessment Report) show vegetation productivity and carbon sinks increasing in temperate and boreal regions. Although, new research is suggesting that extreme events and associated disturbances can offset or reverse that trend—“even a small shift in the frequency or severity of climate extremes could substantially reduce carbon sinks and may result in sizeable positive feedbacks to climate warming” (Reichstein et al. 2013).

Extremes can increase the risk of accelerated climate change if large-scale changes in terrestrial ecosystems, such as forest die-off or extensive wildfires, create feedbacks with the climate system.  For a sense of the scale, view this map from Reichstein et al. 2013, which shows areas where extreme events caused a large decrease in gross primary productivity between 1982 and 2011 (color indicates the cause: water scarcity = blue, extreme high temperatures = red, both = pink, neither = grey, and darker colors indicate greater losses).



Extremes are likely to have an outsized influence on vegetation communities and are likely to be the source of the most immediate climate change impacts in our forests. In particular, the risk of more impactful drought due to the combination of warmer temperatures and reduced precipitation (or changes in seasonality) will make certain regions, such as the southwest U.S., particularly vulnerable to large-scale forest die-off events. In other regions, extreme events may contribute to an increase in forest stress that can reduce productivity or increase vulnerability to other stressors, including insect outbreaks. Critically, the risk of these extreme heat and precipitation events will increase rapidly as average temperatures continue to rise.

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Allen, C.D., Breshears, D.D., McDowell, N.G. 2015. On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. Ecosphere. 6 (8): 129.

Asner, G.P, Brodrick, P.G., Anderson, C.B., Vaughn, N., Knapp, D.E., Martin, R.E. 2015. Progressive forest  canopy water loss during the 2012-2015 California drought. Proc. Natl. Acad. Sci. 113 (2): 7pp.

Babst, F., Carrer, M., Poulter, B., Urbinati, C., Neuwirth, B., Frank, D. 2012. 500 years of regional forest growth variability and links to climatic extreme events in Europe. Environ. Res. Lett. 7: 045705 (11pp).

Breshears, D.D., Adams, H.D., Eamus, D., McDowell, N.G., Law, D.J., Will, R.E., Williams, A.P., Zou, C.B. The critical amplifying role of increasing atmospheric moisture demand on tree mortality and associated regional die-off. Frontiers in Plant Science. 4 (266): 1-4.

Carnicer, J., Coll, M., Ninyerola, M., Pons, X., Sánchez, G., Peñuelas, J. 2011. Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought. Proc. Natl. Acad. Sci. 108 (4): 1474-1478.

Clifford, M.J., Royer, P.D., Cobb, N.S., Breshears, D.D., Ford, P.L. 2013. Precipitation thresholds and drought-induced tree die-off: insights from patterns of Pinus edulis mortality along an environmental gradient. New Phytologist. 200: 413-421.

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

Huang, K., Yi, C., Wu, D., Zhou, T., Zhao, X., Blanford, W.J., Wie, S., Wu, H., Ling, D., Li, Z. 2015 Tipping point of a conifer forest ecosystem under severe drought. Environ. Res. Lett. 10 (2), Article ID: 024011.

Kolb, Thomas, E. 2015. A new drought tipping point for conifer mortality. Environ. Res. Lett. 10: 031002. DOI: 10.1088/1748-9326/10/3/031002

Lloret, F., Escudero, A., Iriondo, J.M., Martínez-Vilalta, J., Valladares, F. 2012. Extreme climatic events and vegetation: the role of stabilizing processes. Global Change Biology. 18: 797-805. DOI: 10.1111/j.1365-2486.2011.02624.x

McDowell, N.G., Williams, A.P., Xu, C., Pockman, W.T., Dickman, L.T., Sevanto, S., Pangle, R., Limousin, J., Plaut, J., Mackay, D.S., Ogee, J., Domec, J.C., Allen, C.D., Fisher, R.A., Jiang, X., Muss, J.D., Breshears, D.D., Rauscher, S.A., Koven, C. 2015. Multi-scale predictions of massive conifer mortality due to chronic temperature rise. Nature Climate Change. Advance Online Publication. DOI: 10.1038/NCLIMATE2873.

Peterman, W., Waring, R.H., Seager, T., Pollock, W.L. 2013. Soil properties affect pinyon pine – juniper response to drought. Ecohydrol. 6: 455-463.

Reichstein, M., Bahn, M., Ciais, P., Frank, D., Mahecha, M.D., Seneviratne, S.I., Zscheischler, J., Beer, C., Buchmann, N., Frank, D.C., Papale, D., Rammig, A., Smith, P., Thonicke, K., van der Velde, M., Vicca, S., Walz, A. Wattenbach, M. 2013. Climate extremes and the carbon cycle. Nature. 500: 287-295. DOI: 10.1038/nature12350.

Reyer, C.P.O., Leuzinger, S., Rammig, A., Wolf, A., Bartholomeus, R.P., Bonfante, A., de Lorenzi, F., Dury, M., Gloning, P., Aboujaoudé, R., Klein, T., Kuster, T.M., Martins, M., Niedrist, G., Riccardi, M., Wohlfahrt, G., de Angelis, P., de Dato, G., François, L., Menzel, A., Pereira, M. 2013. A plant’s perspective of extremes: terrestrial plant responses to changing climatic variability. Global Change Biology. 19: 75-89. DOI: 10.1111/gcb.12023

Russell, M.B., Woodall, C.W., D’Amato, A.W., Domke, G.M., Saatchi, S.S. 2014. Beyond mean functional traits: Influence of functional trait profiles on forest structure, prodiction, and mortality across the eastern US. Forest Ecology and Management. 328: 1-9.

Teskey, R., Wertin, T., Bauweraerts, I., Ameye, M., McGuire, M.A., Steppe, K. 2014. Responses of tree species to heat waves and extreme heat events. Plant, Cell and Environment. DOI: 10.1111/pce/12417.

Williams, A.P., Allen, C.D., Macalady, A.K., Griffin, D., Woodhouse, C.A., Meko, D.M., Swetnam, T.W., Rauscher, S.A., Seager, R., Grissino-Mayer, H.D., Dean, J.S., Cook, E.R., Gangodagamage, C., Cai, M., McDowell, N.G. 2013. Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change. 3: 292-297.

The Paris Climate Conference and Forests: Ramifications of the Agreement

(click here to download a pdf of this article)

By Eric Walberg

The Paris Agreement was unanimously approved by delegates to COP21 and the role of forests in climate regulation is highlighted in several sections of the document. The Agreement is precedent setting in that it signals a global transition to a low carbon economy and establishes a working relationship among participating nations to move towards that goal. What are the implications for forests? What are the ramifications for forest owners and managers? Given the aspirational nature of the Agreement and the significant latitude provided to participating nations in how they meet their commitments, it is impossible to know in detail how things will play out. The following commentary outlines the impacts and opportunities for the forestry sector given what we know of the direction that has been established and the timing of the process.

Explicit References to Forests in the Agreement

Two sections of the Paris Agreement deal explicitly with forests.  Section 55 in the finance section acknowledges the importance of “adequate and predictable financial resources, including for results-based payments” … “for reducing emissions from deforestation and forest degradation”. This section also highlights “the role of conservation, sustainable management of forests and enhancement of forest carbon stocks, as well as alternative policy approaches, such as joint mitigation and adaptation approaches”. Finally, this section identifies “public and private, bilateral and multilateral sources such as the Green Climate Fund and alternative sources” as methods of funding forest protection.

Article 5 of the Agreement opens with a statement that “Parties should take action to conserve and enhance, as appropriate, sinks and reservoirs of greenhouse gases” … “including forests”. The following section of Article 5 acknowledges two approaches to forest preservation. The first reference is to pay-for-performance approaches such as REDD that monetize the carbon stored in forests. The second reference is to joint mitigation and adaptation approaches that attempt to link carbon and non-carbon benefits of forests in identifying preservation goals.

Likely Ramifications:
  • Increased global emphasis on protection and restoration of forests,
  • Increased funding to support forest protection and restoration in developing nations.
Possible Ramifications:
  • Establishment of new carbon credit markets and trading opportunities, possible increase in demand for forest products from geographic areas outside of the areas targeted for protection.

Sections of the Agreement That Will Also Impact the Role of Forests

Beyond the sections of the Agreement that explicitly reference forests, several sections of the document deal with issues that will influence the value and management of forests, such as land use tradeoffs, accounting and compliance with Intended Nationally Determined Contributions (INDCs), ratcheting of commitments over time, financial support of developing nations and opportunities for international credit trading.

Countervailing Land Use Goals

The Agreement frames forest protection in the context of the need to deal with a range of potentially conflicting efforts, including alleviation of poverty, food production, and adaptation to climate change. Article 2 sets the goal of “Increasing the ability to adapt to the adverse impacts of climate change and foster climate resilience and low greenhouse gas emissions development, in a manner that does not threaten food production”. Article 4 sites the need to “achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty.”

Likely Ramifications:
  • Articles 2 and 4 of the Agreement highlight the need for efficient use of a finite land base to meet multiple objectives,
  • Opportunities will exist for:
    • Innovative low-carbon approaches to integration of forest and agricultural systems,
    • Economic development projects that maximize carbon sequestration and storage,
    • Forest revitalization and afforestation in geographic areas that bolster climate change adaptation, and,
    • Biomass energy production that contributes to meeting INDCs.

Regular Evaluation of Progress on Intended Nationally Determined Contributions

The Agreement establishes a schedule for participating nations to report progress towards achievement of the goals identified in their INDCs. Article 4, paragraph 9 states that “Each Party shall communicate a nationally determined contribution every five years

Likely Ramifications:
  • Development of a more complete and detailed global inventory of forest change over time.
Possible Ramifications:
  • Reduction in flow of illegal timber,
  • Reduction in conversion of forests to other uses.

Ratcheting of Intended Nationally Determined Contributions Over Time

The INDCs that were submitted prior to the Paris Conference are not sufficient to achieve the goal of limiting warming to 2 degrees C. The Agreement establishes a process for updating commitments over time to achieve the needed reductions in greenhouse gas emissions and/or increases in sequestration and storage. Article 15, paragraph 3 of the Agreement states that “The outcome of the global stocktake shall inform Parties in updating and enhancing, in a nationally determined manner, their actions and support in accordance with the relevant provisions of this Agreement”.

Likely Ramifications:
  • An increase in value of the carbon sequestration and storage services provided by forests as emission targets become more stringent,
  • Increased reliance on the services supplied by forests as an element of meeting INDCs.

International Offsets

Article 6 of the Agreement sets up a framework for cooperation among participating nations in meeting emission targets and allows for “internationally transferred mitigation outcomes”.

Possible Ramifications:
  • Another potential funding mechanism and/or carbon credit trading opportunity that could monetize forest services,
  • International agreements that seek to implement least cost solutions to meeting INDC targets.

Price on Carbon

Section 137 of the Agreement “recognizes the important role of providing incentives for emission reduction activities, including tools such as domestic policies and carbon pricing”.

Possible Ramifications:
  • If carbon pricing becomes a reality it would dramatically increase the value of the carbon sequestration and storage services provided by forests and could be a game changer for carbon credit markets.

Implications of the Gap in the U.S. INDC

Several recent studies indicate that the measures outlined in the INDC submitted by the United States are insufficient to meet the identified 2025 target of a 26 – 28% reduction in greenhouse gas emissions below the 2005 level. Enhanced efforts to meet the 2025 goal could take many forms.

Possible Ramifications:
  • Additional regulation of industrial sectors such as energy production, transportation, and manufacturing,
  • Additional regulation of non-industrial sectors including forestry and agriculture.

Closing Thoughts

The Paris Agreement lays the groundwork for a major transformation in the global economy. Achieving the goal of balance between greenhouse gas emissions and sequestration and storage by the second half of this century will require both a technological revolution and a transition to land use and development patterns that efficiently meet multiple objectives. Are we up to the challenge? Stay tuned, it’s going to be a wild ride!

Climate Change and Extreme Weather, Part 1: Trends & Projections

By Jennifer Hushaw

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

As the climate changes, we will not only experience a gradual change in average conditions, but also an increase in the frequency and intensity of some types of extreme weather and climate events. A number of these observed or anticipated changes have been highlighted in past bulletins, including extreme heat, heavy precipitation, and more intense drought. These extremes may pose the biggest climate change risk for forest ecosystems in the short-term. Extremes create conditions that are beyond what species have historically experienced and they can push natural systems past their threshold for resilience and recovery because there is generally little time or capacity for natural adaptation in those situations. This potential for irreversible impacts elevates extremes on the list of climate-related forest management concerns.

Climate is the statistics of weather. More specifically, the climate is the statistical distribution of any given weather variable (clouds, rainfall, temperature, wind speed, etc.), which generally resembles a bell curve (see figures, below). For instance, the average January temperature in New York City is 35⁰F, but it has been as warm as 70⁰F and as cold as -4⁰F. Climate change is, by definition, a change in that distribution.

There are a number of ways the distribution can change. As the climate warms and the average temperature increases, the entire curve will shift toward the right (see figure, below). This means more warm extremes, new record heat events, and conditions that used to be ‘extreme’ becoming more normal.shifted_mean

In some regions, the variability may also change (a.k.a. the width of the bell curve) due to changing dynamics that are also linked to climate change, such as changing sea ice conditions, shifts in the jet stream, vegetation change, etc. (see example in figure, below). The curve may become wider (more variability) or narrower (less variability), depending on the region.


In fact, a recent analysis of land temperatures over the northern hemisphere indicated that the bell curve is already shifting and widening as the planet warms—see this figure from Hansen et al. 2012 in New Scientist.


Importantly, the risk of extreme heat and heavy precipitation events goes up exponentially as average temperatures increase. A recent study quantified how the probability of extreme heat and heavy precipitation events changes in response to increasing global temperatures (see figure, below) and their results for the present-day agree well with what we have actually observed. They  found that the “probability of a hot extreme at 2⁰C [global] warming is almost double that at 1.5⁰C and more than five times higher than for present-day,” so the difference is “small in terms of global temperatures but large in terms of the probability of extremes” (Fischer and Knutti 2015).

Note: See Fischer and Knutti 2015, Figure 2 (a) and (b) for a graph showing how the probability of moderate extremes (blue; events that exceed the 99th percentile) and the probability of the largest extremes (red; events that exceed the 99.9th percentile) change relative to pre-industrial daily precipitation and temperature conditions, at a given warming level. Probability ratio (PR) compares the probability of an extreme in present-day or future climate to the probability of that extreme before industrialization—it is the factor by which the probability of an event has changed, i.e. a PR of 1.5 means an event is 1.5 times more likely to occur.


Observed Changes in Extremes

Note: Observations and projections summarized from IPCC Special Report on Extremes (2012) Table 3-1 and the Third U.S. National Climate Assessment (2014).

Extreme Heat

Since 1950, there have been more record hot days/nights than record cold days/nights, globally. This has also been true in the U.S.—in the 1950’s the ratio of record highs to record lows was roughly 1 to 1 and by the 2000’s the ratio was 2 to 1 (see figure, below). The data also suggest that many regions have experienced more or longer heat waves over this time period.



Heavy Precipitation

There have been large regional variations in heavy precipitation trends, but overall there are more regions with significant increases in heavy precipitation events than decreases.


There has been significant regional variation in drought trends—some regions (e.g. southern Europe and West Africa) have experienced more intense and longer droughts, but there are opposite trends in other places.


There is not much evidence for a change in flooding trends globally, but there have been significant trends in certain regions. For example, the southwestern United States has seen a decrease in flood magnitude, while the Midwest and New England have seen an increase (see Figure 3 (a) in Peterson et al. 2013 for a map of trends in flood magnitude). Globally, there has also been an observed trend toward earlier occurrence of spring peak river flows in snowmelt- and glacier-fed rivers.


Future Extremes

Extreme Heat

As discussed previously, the science indicates that we will experience more unusually warm days and nights as the climate warms. Similarly, we also expect an increase in the length, frequency, and/or intensity of heat waves over most land areas.

Heavy Precipitation

We will see an increase in the frequency of heavy precipitation events or the proportion of total rainfall that comes in the form of these heavy events. This is expected because a warmer atmosphere holds more water vapor when it is saturated, so when it rains in a warmer world more moisture can rain out. This trend is expected to be especially true in the high latitudes and tropics, as well as winter in the northern mid-latitudes.


It is expected that in the future we will experience more impactful ‘hotter droughts’ as a result of warmer temperatures. For example, a recent study in the Southwest and Central Plains of Western North America used 17 different climate models to forecast future soil moisture in those regions and found consistent predictions of severe drying by the end of this century, with an 80% likelihood of a decades-long megadrought between 2050 and 2100 (Cook et al. 2015). For a more in-depth discussion of future drought risk, you may want to re-visit our previous bulletin on global precipitation.

Model projections tend to agree on a future increase in the duration and intensity of drought in certain regions, including southern Europe and the Mediterranean, central Europe, central North America, Central America and Mexico, northeast Brazil, and southern Africa. Currently, poor model agreement makes it difficult to say how drought risk may change in other regions.


A lack of sufficient evidence and model agreement makes it difficult to say what may happen globally in terms of flood magnitude and frequency, but we at least expect that heavy precipitation events will contribute to more rain-generated local flooding, particularly in watersheds with topography that exacerbates the effects of heavy rainfall events (e.g., steep slopes, canyons that rapidly concentration flow).


How do we know an extreme event is related to climate change?

In recent years, there has been a lot of interest in understanding whether specific extreme events, e.g. the 2003 heat wave in Europe, Hurricane Katrina, or the 2012 Midwest heat wave, were caused by climate change. As with any phenomenon that arises from a number of factors, we can’t pin down the cause to a single factor. However, we can begin to understand how a particular factor contributed to the likelihood or probability of the event.

A helpful analogy is a car accident on a wet road. Was the car accident caused by the rainy conditions and wet road? Not exactly—there were other factors at play, including the curviness of the road, the speed of the car, visibility, the driver’s level of fatigue, and others. However, we can look at the statistics of road accidents and determine how much a wet, rainy road increases the chances of an auto accident. In fact, there is an emerging area of climate research that is doing similar work to isolate how much anthropogenic climate change has increased or decreased the likelihood of certain climate extremes, compared to natural background conditions.

This type of research has revealed that climate change very likely contributed to a number of heat waves and heavy precipitation events, such as the 2010 Russian heat wave (Otto et al. 2012), the 2003 European heat wave (Stott et al. 2004), record summer temperatures in Australia in 2013 (Lewis and Karoly 2013), and flooding in England and Wales in 2000 (Pall et al. 2011). Another recent analysis looked at heavy precipitation and hot extremes globally and found a discernable human influence (Fischer and Knutti 2015).


Climate change is changing the odds of certain extreme events—it is loading the dice, so we are more likely to experience certain extremes, such as heat waves, heavy precipitation event, or severe drought.  As more of these events take place, researchers can begin to assess how the probabilities are changing, potentially improving our ability to estimate future risk from climate extremes.

A useful way to think about climate change adaptation in forestry is through the lens of risk management. The same is true for assessing the risk posed by climate extremes. Breaking risk into its component parts (see figure, below) reveals the best strategies for risk reduction.


The first two components of this equation are related to the climate variable, e.g. drought, and the last two are related to the forest, e.g. location, stocking, species mix. Taken together, these four variables determine the overall risk posed by a particular climate extreme.

By leveraging the right management approach, you can reduce your exposure and/or vulnerability to a given climate stressor. Although, you can’t do much to change the first two variables, beyond getting more (and better) information. This is where new data about changing probabilities can play an important role in improving our assessment of risk with regard to climate extremes. Manomet will be closely following emerging research in this area and updating members as the projections become clearer.



Cook, B.I., T.R. Ault, and J.E. Smerdon, 2015: Unprecedented 21st-century drought risk in the American Southwest and Central Plains. Sci. Adv., 1, no. 1, e1400082, doi:10.1126/sciadv.1400082.

Fischer, E.M. and Knutti, R. 2015. Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nature Climate Change. 5(6): 560-564. DOI: 10.1038/NCLIMATE2617

Hansen, J., M. Sato, and R. Ruedy, 2012: Perception of climate change. Proc. Natl. Acad. Sci., 109, 14726-14727, E2415-E2423, doi:10.1073/pnas.1205276109.

IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.

Lewis, S. C. & Karoly, D. J. 2013. Anthropogenic contributions to Australia’s record summer temperatures of 2013. Geophys. Res. Lett. 40, 3705-3709.

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

Otto, F. E. L., Massey, N., van Oldenborgh, G. J., Jones, R. G. & Allen, M. R. 2012. Reconciling two approaches to attribution of the 2010 Russian heat wave. Geophys. Res. Lett. 39, L04702.

Pall, P. et al. 2011. Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000. Nature 470, 382-385.

Stott, P. A., Stone, D. A. & Allen, M. R. 2004. Human contribution to the European heatwave of 2003. Nature 432, 610-614.


Forest Pests and Climate Change

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

By Jennifer Hushaw

Part 1: Overview of Climate-Pest Interactions

Among the many potential impacts of climate change, changes in insect and disease populations rise to the top as the most immediate and possibly significant impact on our forests. This is because of the destructive potential of forest pests and the direct link between climate and pest survival or spread. In particular, climate influences:

  • Frequency and intensity of outbreaks
  • Spatial patterns, size, and geographical range of outbreaks
  • Life cycles, range shifts, range expansions or contractions

Being ectothermic, insects are particularly sensitive to temperature, as it directly influences their metabolic rate, consumption, development, and the timing of life history stages. Water availability is also an important factor determining the interaction between plants and insects. Forest pathogens are similarly sensitive to temperature and moisture conditions. As a result of this sensitivity, these organisms will be directly affected by changing climate, in addition to being indirectly influenced via climate change impacts on other organisms, such as their host species.

Forest insects and pathogens have a number of characteristics that will allow them to rapidly respond to climate change, including: (1) physiological sensitivity to temperature, (2) high mobility, (3) short generation times, and (4) high reproductive potential.

Direct Climate Impacts

A number of climate change-related variables will have direct impacts on the population dynamics of forest insects and pathogens:

  • High overall temperatures, especially milder winters
    • Greater over-winter survival
    • Increased spore production and infectiousness
    • Decreases in insect populations at a certain level of warming, as most insects are susceptible to heat stress between 82 and 90⁰F
  • Longer and warmer growing season
    • Lengthening of reproductive season
    • Accelerated life cycles; increase in number of generations per year
    • Earlier appearance in spring
  • Changing snow pack
    • Affecting overwinter survival
  • Climate variability
    • Affecting performance and survival
Indirect Climate Impacts

Pests will experience the indirect effects of climate change through the following avenues:

  • Host plants
    • Distribution of primary (or alternate) host plants
      • Reduced/shifting habitat suitability may result in the loss of suitable host plants for some pest species within their preferred climate niche
      • Changing habitat suitability may also cause tree stress and increased susceptibility to attack
    • Nutritional quality
      • Elevated CO2 and temperature typically increases the concentration of leaf carbohydrates and decreases nitrogen content, lowering nutritional value
        • This can lead to increased herbivory from “compensatory feeding” – herbivores consume more low-quality food to meet their nutrient needs
      • Plant resistance
        • Little is known about the mechanisms by which increased CO2 and temperature affect plant production of secondary metabolites (defense chemicals), which deter feeding – at this time, we only have observational data that indicates climate-induced changes vary by species and context
        • Conditions that promote increased plant growth may be a double-edged sword because they are often associated with declines in plant production of defense compounds, which is a benefit for certain insects
      • Phenology
        • Accelerating phenology due to warming temperatures can lead to a mismatch between plants and associated organisms, which can be positive or negative for plant life – negative when it leads to a misalignment between plants and their pollinators (this is especially an issue for specialist plant-pollinator interactions) and positive when it reduces the frequency or severity of insect/pathogen attacks
      • Rate of plant development
        • When the rate of plant development shifts relative to insect development, it can amplify or minimize consequences of herbivory
  • Community interactions with:
    • Natural enemies (predators, parasitoids, pathogens), e.g. shifting bird populations may increase predation on arthropods in some areas, increasing the strength of top-down control on those pest populations
    • Competitors
    • Mutualists
  • Abiotic damage that increases tree stress and makes hosts more susceptible to attack*, such as: (1) Storm Damage, (2) Drought, and (3) Extreme heat events.

*Note: Pests can generally be divided into two categories: primary pests that can typically attack healthy trees and secondary pests that tend to attack trees that are already weakened by a predisposing factor like drought, water-logging, or injury (e.g. ambrosia or ips engraver beetles). It is likely that the biggest impacts from abiotic damage will come from these secondary pests.

Part 2: Summary of Anticipated Impacts

As with projections of tree response to climate change, we expect the response of insects and pathogens to be species-specific, or at least to vary depending on host type and feeding guild (e.g. defoliators, gall makers, wood borers, etc.). However, there are some general predictions that can be made:

Asynchrony/Ecological Mismatches

It is likely that projected changes in seasonality will lead to instances where the life cycle or developmental stages of host species and pests are no longer aligned, which could exacerbate or alleviate pest impacts, depending on the species involved. For example, insects that typically feed on young, nutrient rich foliage may be negatively impacted if the growing season begins earlier and causes faster leaf maturation after budbreak, especially if the timing of their spring emergence does not change. Likewise, the timing of spore release by pathogens can be an important determinant of disease incidence and severity, but changing climate conditions are likely to change those interactions. These issues of synchronicity are at the heart of many complex species interactions that may be disrupted by climate change because pests, hosts, and predators may all have different sensitivities to changing climate and varying levels of tolerance and/or plasticity to deal with those changes. Therefore, it is likely that in the future the rate or timing of development may no longer be in alignment among species that have historically been tightly linked.

Range Shifts/Redistribution

In addition to being misaligned in time (as described above), hosts and pests may become misaligned geographically due to climatically-driven changes in species ranges. Generalist pest species tolerate a wide range of climate conditions, are highly mobile, and use a variety of host species, so they will likely fare better as their ranges expand into northern regions. Whereas, more specialized species, such as those commonly found in tropical areas, are likely to see their ranges shift entirely, or contract in instances where migrating populations encounter hard (e.g. continental edges) or soft (e.g. soil moisture) range boundaries. These shifts may be good or bad for forest ecosystems, depending on the species and region in question.

In a general sense, we expect to see the following trends in pest distribution:

  • Expanded northern ranges
  • Invasion of new habitat and forest types
  • Range shifts toward poles and higher elevations (most shifts in temperate regions)
  • Better survival and increased impact from poleward populations

Note: In some cases, poleward populations may be locally adapted to colder temperatures, so there may not be as much of an advantage conferred by warmer temperatures in those regions.

Changing Pest Populations and Outbreak Frequency

Some research suggests that a warmer world doesn’t necessarily mean more forest pest impacts because we may mostly see distribution shifts toward higher latitudes and altitudes, rather than an increase in the overall incidence of pest outbreaks. However, climate change is also likely to amplify abiotic stressors, such as drought, extreme heat, and increased storm strength, which creates conditions that are favorable to more frequent and intense outbreaks.

Herbivorous insects will generally fare well because warmer temperatures will increase winter survival, promote faster development rates, and sometimes allow their populations to grow faster than normal because they can complete more life cycles in a season. That last characteristic (increased voltinism, i.e. number of generations per year) is particularly troubling because it will lead to more herbivory. Multiple generations of herbivores can do significant damage in a single season, especially for long-lived plants (increased likelihood of mortality and impacts on future growth and reproduction) and conifers (they don’t typically releaf and their resin defenses can be over-ridden by large numbers of attackers). In fact, there is evidence that a similar increase in herbivory happened in North America during a past period of global warming known as the Paleocene-Eocene Thermal Maximum approximately 55.8 million years ago.

Increased Pathogen Infectiousness

Evidence suggests that pathogens, including those that affect tree species, are likely to increase their infection rate under increased humidity and temperature conditions associated with climate change. These organisms are also generally able to evolve, adapt, and migrate more quickly than their long-lived hosts, so their role in forest disturbance regimes will probably increase. An increase in pathogen development and survival rates, disease transmission, and host susceptibility will have deleterious effects on forest ecosystems, but there will also be some subset of pathogens that actually decline with warming and lead to improved conditions for their host species.

Increased Vulnerability in Water-Stressed Regions

Some of the most obvious and immediate pest-related impacts will occur in regions with reduced precipitation and more frequent or severe drought conditions. This is particularly true for insect groups and pathogens that typically affect water-stressed hosts – the increased bark beetle activity coincident with drought in the western U.S. is a prime example. Drought conditions create physiological stress for trees that increases their susceptibility to attack and reduces their ability to survive and/or recover. A recent meta-analysis (Jactel et al 2012) of drought effects on damage by forest insects and pathogens found that primary damaging agents (i.e. insect or fungal species that can develop on healthy trees) living in wood caused lower damage than those living on foliage, indicating that the type of feeding substrate was very important for the level of pest damage.

Part 3: Regional Pest Highlights

The following section contains several regional examples of forest pests and pathogens that are likely to be influenced by climate change, including a brief, high-level overview of how climate is anticipated to impact each species.


            Spruce Budworm

Current research on the effects of climate change on eastern spruce budworm (SBW) suggests that there will be an increase in its range at higher latitudes and higher altitudes. This is because SBW is limited primarily by cool summer temperatures, which prevents the eggs from hatching before winter and does not give the larvae sufficient time to find winter shelter. Warmer summer temperatures will allow the pest to move into new territory and that has already been seen at unusually high latitudes on the north shore of the Saint Lawrence River in Quebec (c. 2009).

            Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is vulnerable to low temperatures and its continued spread is primarily limited by minimum winter temperatures, rather than hemlock abundance. In particular, studies have found that an average winter temperature of 23⁰F (-5⁰C) is limiting, which historically meant that about half of the Northeast region was too cold for HWA (including upstate New York and most of Vermont, New Hampshire, and Maine). However, warming temperatures will allow for HWA range expansion – by mid-century half of the area that is currently temperature-limited will become suitable and by the end of the century (under the higher emissions scenario) the entire Northeast will have average winter temperatures above 23⁰F. HWA is an example of a pest that will be directly influenced by climate change, with temperature conditions becoming progressively more suitable for its migration northward and into higher elevation areas. 

Gypsy Moth

Gypsy moth is another example of an insect that is limited by cold in the northern portions of its range. A number of studies indicate that increasing temperatures will lead to an increase in defoliated area and an expansion of gypsy moth populations into new regions. However, precipitation changes are also important, as evidence suggests that a warmer and drier climate will actually decrease defoliated area. Drier conditions can also reduce the buildup of Entomophaga maimaiga, a lethal fungus that thrives during wet spring weather and was introduced from the gypsy moth’s native range in Japan in the early 1900’s as a way to control its population in the U.S.


            Southern Pine Beetle

Model projections and observed changes of Southern Pine Beetle (SPB) populations generally indicate that rising temperatures lead to more outbreaks. Winter minimum temperatures below 6.8⁰F (-14⁰C) cause population declines, but in recent years warming temperatures have allowed these insects to move north into areas where cold was once limiting, including the New Jersey pinelands, Long Island, and, most recently, Connecticut. Although, seasonal changes in temperature are also relevant because increases in winter and spring temperature are projected to increase outbreaks, while increases in fall temperatures will tend to ease outbreaks. The intensity and area of outbreak is also related to precipitation levels, with more precipitation being beneficial for the insects, although it is a less important factor than temperature.

On a positive note, there is evidence that extremely hot summer temperatures are lethal for SPB, leading to increased mortality, reduced activity, and hindered flight. Therefore, it is possible that future increases in extreme heat events in the southeastern U.S. will provide a benefit in terms of SPB population control. Whereas, pine stands in the northern stretches of the beetles’ range will likely see the greatest increase in beetle activity and impact.

            Ips Engraver Beetles

Ips engraver species can be cold-limited in the sense that low temperatures disrupt egg development and synchronized flight activities. These species can also see a reduction in the number of generations per year when cold temperatures persist over a long winter. In this way, climate warming may directly benefit ips by increasing their reproductive rates. In fact, several studies of Ips species in Scandinavia have indicated that higher temperatures will lead to an increase in the frequency and length of late summer swarming events in those regions, as well as an increase in the number of generations per year.

Ips rarely attack healthy trees and instead tend to target trees that are under stress, a condition that is likely to increase in prevalence as various climate stressors interact to increase physiological stress on trees. Drought stress is one example of a condition that increases the risk of ips attack and there is some evidence that decreased precipitation, and the consequent reduction in host tree resistance, contributed to ips outbreaks in the southwest U.S. in the early 2000’s. Projections of increased drought stress under future precipitation patterns may contribute to an increased risk of ips attack.

            Fusiform Rust

A number of climate-related factors influence the extent and severity of fusiform rust infection, including temperature, humidity, and late winter/early spring weather. However, the disease is already widely distributed throughout the host range, so it is likely that climate change will not directly cause an expansion of the affected area. Instead, experts expect this disease to experience indirect climate impacts via changes in the distribution of its host species as a result of rising temperatures, e.g. increased planting of loblolly pine in northern regions or migration of pine (and alternatively, oak) hosts from coastal areas into the Appalachian Mountains.


            Mountain Pine Beetle

As with many other North American forest pests, the latitudinal and elevational limits of the Mountain Pine Beetle (MPB) range are delineated by climatic conditions related to average annual minimum temperature. The beetles generally cannot survive to complete successful brood development in places where this average minimum is less than -40⁰F (-40⁰C). Given that the range of potential hosts is far more extensive, there is significant potential for MPB to expand under the right climate conditions. Warmer winters in British Colombia with an absence of cold snaps sufficient to kill MPB (a week or more of temperatures at or below -31⁰F) have already allowed the insect to have outbreaks in more northerly areas. Studies conducted to date generally predict MPB will continue to expand northward, eastward, and toward higher elevations, with the potential for a reduction at lower elevations in the northwestern region of the U.S. due to future climate-related losses of suitable host species in those areas.

Sudden Oak Death

Sudden Oak Death (SOD) is caused by a fungus-like water mold called Phytophthora ramorum, which produces spores that spread easily in warm, wet conditions, e.g. it is often transmitted when rainwater splashes the spores onto susceptible plants. Extreme weather events contribute to mortality from SOD – heavy rains and extended wet weather create optimal conditions for infection and mortality results when this is followed by extended dry periods, because infected trees are not able to manage water as effectively. Unfortunately, researchers expect climate change, particularly increases in temperature and coastal fog, to exacerbate the effects of this pathogen and shift the at-risk area northward. Alternate hosts include a variety of woody species, especially bay laurel in California. Notably, several studies have highlighted the potential for this pathogen to colonize the southeastern U.S., given the climatic conditions and distribution of potential host species in that region.


            Emerald Ash Borer

Emerald Ash Borer (EAB) have very low supercooling points (the temperature far below freezing that insects can survive through physical and chemical changes in their bodies), but exposure to temperatures at or below 32⁰F (-30⁰C) can cause overwintering mortality and help keep their populations in-check. In this sense, warming temperatures are likely to increase the rate of overwinter survival and potentially allow EAB to colonize previously unsuitable areas.

However, these insect do express some phenotypic plasticity in terms of their cold tolerance – while they can successfully acclimate after being exposed to colder temperatures over several months, they will lose that cold tolerance (i.e. deacclimate) if they experience warm mid-winter temperature fluctuations, and it is not reversible. This means that a mid-winter warm spell may cause EAB to deacclimate and then suffer mortality during the next cold snap because they have lost their cold tolerance. A potential opportunity, in terms of EAB population control, is the projected increase in the likelihood of extreme warm winter events associated with climate change.


There are hundreds of pests and pathogens, both native and introduced, which interact with the forest ecosystems we manage. A challenge is that there are widely varying levels of knowledge about the physiology, life cycle, and climate niche from one organism to the next. The list of species for which researchers have specifically addressed the question of climate impacts is fewer still. The pests and pathogens highlighted in Part 3 are some examples of higher-profile biotic threats for which we have some of this information. Although, there are many others where there is a weak climate link (especially in cases where climate is not the dominant limiting factor) or there is a lack of literature discussing the pest specifically in the context of climate change. Examples include: Asian Longhorned Beetle, Oak Wilt Disease, Dogwood Anthracnose, Pear Thrip, and many others. There is also the practically inevitable reality that new species will continue to be introduced from abroad for which we will have very little initial information.

Given the lack of complete information about climate change impacts on the catalogue of forest pests and diseases, it is useful to take a general, high-level view of pest-climate interactions, such as that presented in Part 1 and 2 of this bulletin. The best strategy is often to identify the life cycle characteristics or physiological limits of a particular pest that are most likely to be impacted by changing climate; for example, a need for synchronicity with budbreak of a particular species, level of cold tolerance, vulnerability to mortality from climate variability and temperature extremes, or a high degree of host specificity. This is where local knowledge and personal experience with a particular pest and forest type becomes really valuable for anticipating how climate, pest, and host may interact in novel ways in the future. Additionally, the importance of monitoring for detecting early changes in pest behavior or abundance cannot be overstated, so it is beneficial to proactively have those monitoring systems in place on your land. However, staying alert to new information is also key, especially in terms of looking beyond your ownership and being aware of pests that may potentially move into your area from other regions as a result of climatic shifts. This bulletin will act as a foundational document on the subject of forest pest-climate interactions and, going forward, the Climate Smart Land Network will continue to monitor and highlight newly documented links between changes in regional climate and important forest pests and pathogens.

So what can land managers do now?

Insects and disease have always been recognized as serious threats to forests and as a result they have received significant research and communication funding. This includes town tree wardens, university, state, and federal funding and support. The whole wood product transport quarantine system is designed to address these issues.

But things are changing rapidly and we need all the eyes and ears we can get.  Foresters and arborists, are ideal data gathers, question askers and teachers in this situation. Stay connected with state insect and disease departments as well as arborist information sources. Watch for new conditions in the woods and report them to these same organizations. If there are any research or education efforts existing, explore becoming part of those efforts. Spread your knowledge to others.



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Certainty and Uncertainty in Climate Change and Forest Response Part 2: The Forest Response

By Jennifer Hushaw & Si Balch

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

Even if we could perfectly predict the details of future climate change, there would still be uncertainty about how ecological systems will respond. Most of the information we have about how forests respond to long-term climatic changes is related to the northward migration of species following the last deglaciation, but this is not necessarily a good proxy for the change we are experiencing today because the current rate of warming is much faster and the landscape is not the barren frontier it was after the glacial retreat – now the movement of species is complicated by other factors, including inter-species competition.

A great deal of work has been done to model how habitat suitability is likely to change in the future for individual tree species and, despite the inherent uncertainty in these models, the results provide some sense for how the species mix may change in various regions – a topic that will be explored in a future bulletin.

The following list includes some key topics in forest response to climate change and describes the aspects that are well-understood, as well as the major areas of uncertainty in each arena:

Response of Insects & Disease

The interaction of changing climate with both native and invasive insects and diseases is likely to be one of the most immediate and drastic climate-related impacts. While these pests are better understood than many of the following topics, there is still considerable uncertainty about life-cycles and population dynamics in new conditions – some pests may benefit from these changing conditions, while others may be held in check.

Warmer Temperatures

Increasing average temperatures and a longer growing season will provide an opportunity for increased forest productivity, especially in northern latitudes. Although, regions with increasing seasonal water stress may see fewer productivity gains than wetter areas.

CO2 Fertilization

Higher ambient CO2 levels can increase plant growth rates. The magnitude of additional carbon storage and whether the growth response will be offset by losses in productivity due to heat stress or other climate-related factors remains uncertain. There is likely to be little if any increase on poor sites where other nutrients are limiting.

CO2 & Water-use Efficiency

Higher ambient CO2 levels can also increase plant water-use efficiency (the ratio of water loss to carbon gain). This effect varies from one species to the next because there is variability in how plants regulate water loss during dry conditions, so the net gains (globally) are still uncertain.


The latest genetics research has revealed that some of an organisms genes can be turned “on” or “off” by external environmental factors and those changes can sometimes be inherited by offspring (a.k.a. transgenerational epigenetics). This is how plants with the same genome can flower at different times in response to different local temperatures. It is also the mechanism whereby environmental stressors experienced by the parent plant affect the next generation.

This has important implications for the adaptability and potential plasticity of different tree species in a novel climate. It suggests that some species may be more resilient to change than we might expect based on their current response to stress, since new environmental conditions can stimulate changes in gene expression that create a different response. For example, some research has shown that a parent plant exposed to repeated drought conditions can produce seedlings that express more drought tolerance. The role of epigenetics in shaping plant population response to climate change is very uncertain and is just beginning to be researched.

Fundamental, Realized, and Tolerance Niche

Each species has a fundamental niche – the full range of conditions and resources under which it can persist indefinitely – but species don’t live there, they live within a smaller realized niche – the places where competition doesn’t preclude them. Some researchers have also introduced the idea of a tolerance niche – this is a marginal zone beyond the fundamental niche in which species can survive, even if they can’t maintain self-sustaining populations (Figure 1, below). However, many of our assumptions about future changes in climate suitability are based only on a species’ current range (its realized niche) rather than the complete range of conditions (its fundamental or tolerance niches), lending uncertainty to habitat shift projections. Considering these broader niches suggests that species may be capable of persisting under a wider set of climate conditions than where they currently grow.


Limits to Knowledge about Plant Physiology

How an individual tree species responds to climate change is likely to be linked to its underlying physiological characteristics, such isohydric vs. anisohydric behavior (plant strategies for stomatal regulation), seedling vs. mature tree response to environmental stressors, heat tolerance, etc., but we only have detailed knowledge about these characteristics in a limited number of model species used in research and some of the more valuable timber species.


These unknowns all contribute to the ‘cascade of uncertainty’ that accumulates as researchers take the outputs of future emissions scenarios, use those in simulations of global climate change, downscale those outputs in a particular region, and then use those projections in forest models. At each step in this chain, there may be:

  • Statistical uncertainty
    • Sampling error, inaccuracy, etc.
  • Scenario uncertainty
    • Plausible changes, which are based on assumptions
  • Uncertainty due to ignorance
    • Known unknowns (uncertainty due to recognized ignorance)
    • Unknown unknowns (uncertainty due total ignorance)

Coping with Uncertainty

Forestry has always required the consideration of future conditions – whether that is anticipating future market demand, timber prices, insect threats, or new invasive species of concern. In the past, however, we could assume that, aside from a few wet or dry years, the average climate conditions would be the same throughout the life of an individual tree from seedling establishment to harvest. We now understand that that assumption may not hold, particularly in regions with a long rotation age. The exact degree and rate of climate change in any given location is uncertain and this may change our ability to achieve desired stand conditions in the future, by hindering our ability to choose the species best-suited for a particular site, for example.

Forest management decisions are influenced by our assumptions about how well we can predict future conditions and our ability to “design” forests well-suited for those conditions. In the context of a changing climate, these assumptions are more relevant than ever, as managers try to determine the most appropriate strategy for developing “resilient” forests that will continue to provide desired products and services. On one end of the spectrum, we have completely deterministic strategies that aim to develop “forests of the future” (where the species mix and stand structure are well-adapted to future conditions) and, at the other end, we have purely indeterministic approaches that aim for more diverse forests (in terms of species mix, structure, age-class, etc) based on the idea that they have more natural capacity to buffer change (Figure 2, below). While the deterministic approach has proven to be successful in the 10- to 15-year planning horizon that is often of most concern for foresters, an indeterministic approach often has the advantage if reality doesn’t meet expectations (in this case, climate predictions). All forest management decisions fall somewhere along this continuum between two valid strategies that are not mutually exclusive, and forest managers will likely use some of each approach.


While these strategies can be employed at any scale, stands are likely best managed from a deterministic basis (the shorter the rotation the more appropriate the deterministic approach), while forests (made up of multiple stands and biologically diverse micro sites) are likely best managed on an indeterministic basis. The latter approach is aimed at developing a maximum flexible forest that has the ability to adapt to any change in the ecological or economic environments, by identifying the most important ecosystem features for allowing high resilience and self-regulating capacity, such as: diverse structural elements, increased species diversity and natural regeneration, secured vitality and fitness of highest possible number of species (including genetic families across all organisms). Ultimately, the ability and desire to implement one or the other of these strategies will depend on management objectives, risk tolerance, and planning horizon.



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