KEY MESSAGES Increased temperatures will have positive or negative effects on individual trees and forest-wide processes, depending on local site and stand conditions, but impacts from increased extreme heat will be negative. [high agreement, moderate evidence]   Direct effects of climate change on individual trees will be driven by temperature in energy-limited forests and moisture in water-limited forests. [high agreement, moderate evidence]   The speed and magnitude of climate change may mean that increased forest mortality and contractions in forest distributions will outpace any gains in forest growth and productivity over the long run, leading to a net loss of forested area in Montana. [medium agreement, limited evidence]   Direct effects of climate change on trees and forests, such as warmer, wetter conditions improving forest productivity or warmer, drier conditions increasing tree mortality, will be secondary to the impacts of altered forest disturbance regimes, such as changes in forest fire behavior and area burned. [high agreement, limited evidence]   An increase in fire risk (i.e., probability of occurrence)—including an increase in size and possible frequency and/or severity (i.e., tree mortality)—is expected in the coming century as a result of a) prolonged fire seasons due to increased temperatures, and b) increased fuel loads from past fire suppression. [high agreement, robust evidence]   Rising temperatures are likely to increase bark beetle survival [high agreement, strong evidence], but climate-induced changes to other insects and forest pathogens are more varied and less certain. [medium agreement, moderate evidence]   Forest responses to climate change may be non-linear and complex due to feedbacks. [high agreement, limited evidence]   There may be a reduction in the amount of carbon stored in forests. [low agreement, limited evidence]

 

 

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DIRECT EFFECTS OF CLIMATE CHANGE ON FORESTS

 

Key Messages

• Increased temperatures will have positive or negative effects on individual trees and forest-wide processes, depending on local site and stand conditions. In relatively cool and moist areas, increased temperatures can improve reproduction and establishment, lengthen the growing season, and increase forest growth and productivity. Alternatively, in areas that are already warm or projected to see large temperature increases, warming is likely to decrease growth and increase heat- and drought-related mortality. [high agreement, moderate evidence]
• Direct effects of climate change on individual trees will be driven by temperature in energy-limited forests and moisture in water-limited forests. Increased temperatures and water availability could benefit forest regeneration and growth, particularly in higher-elevation forests in the northern and western parts of the state. Alternatively, decreased water availability, such as in Montana’s southeast, or south-facing slopes, will likely increase tree mortality. [high agreement, moderate evidence]
• The speed and magnitude of climate change may mean that increased forest mortality and contractions in forest distributions will outpace any gains in forest growth and productivity over the long run, leading to a net loss of forested area in Montana. However, range shift responses will be highly dependent on species and region. [medium agreement, limited evidence]

 

The direct effects of climate change on forests include increased temperatures and shifts in precipitation that together can alter humidity, soil moisture, and water stress. These effects result in short-term and long-term impacts to tree establishment, growth and productivity, and mortality. In addition, elevated CO₂ levels may influence forest growth, productivity, and water use. 

Direct effects can be beneficial or detrimental to forest growth and survival. Each tree species will respond differently to climate variation depending on its specific physiological tolerances. Direct effects overlie existing forest conditions arising from past and future human land-use activities (Moritz and Agudo 2013). Thus, net impacts, whether positive or negative, may be difficult to estimate and will vary substantially across Montana.

Forest patterns and conditions depend on the life cycles of individual trees and forest-wide processes. In the remainder of this section, we review how different aspects of climate may influence three primary life-cycle stages or forest processes: seedling establishment and forest regeneration; tree growth and forest productivity; and tree mortality and forest die-off (findings summarized in Table 4-3). Life-cycle stages and forest processes may occur at different temporal and spatial scales, but are related and we discuss each in turn. We close the section with a discussion of changes in species distribution that might be expected from the direct effects of climate change. 

 

Table 4-3. Summary of potential climate-related direct effects to forests.

Direct effect	Possible impacts	Projected net effect
Establishment and regeneration	Positive: Higher CO2 concentrations and temperatures may lead to increased tree fecundity Negative: Higher temperatures and reduced water availability could reduce seedling survival	Possible positive or negative effects are superimposed on climate oscillations, such as the Pacific Decadal Oscillation, which can produce decades of cooler and wetter conditions that may be more favorable for establishment and regeneration
Growth and productivity	Positive: Increased vegetation water use and increased growth and productivity as a result of longer growing season Negative: Reduced growth and productivity in water limited areas	Possible increased growth and productivity concurrent with climate oscillations that increase water availability, particularly at higher elevations and where stand density is low; extreme high temperatures would have net negative impact, regardless of water availability
Mortality	Positive: Few opportunities for reduced direct climate effects on mortality but possibility for reduced mortality from indirect effects Negative: Increased acute and background mortality from increased temperatures and indirectly from increased disturbance	Increased mortality, although may be driven by indirect effects; patterns of mortality will be dependent on initial stand and local site conditions, but more arid regions more susceptible
Range shifts and forest distribution	Positive: Potential range expansion with warmer temperatures and sufficient moisture Negative: Potential range contraction where temperature is too high or in water-limited locations	Possible faster range contraction compared to expansion, with net range reduction particularly in drier areas; no clear direction of elevational shifts; responses will be highly species and location dependent

 

Establishment and regeneration

Although tree recruitment—the process of a seedling becoming established and surviving into adulthood—is affected by many factors, the process is strongly tied to temperature and water availability (Ibáñez et al. 2007). Overall, studies are inconclusive as to net impacts of changing atmospheric chemistry and climate on seedling establishment and growth. Given the climate projections of increased winter and spring precipitation, but drier summers, predicted uncertainties of seedling regeneration are uncertain.

Climate conditions determine the window of time for successful seedling establishment (Ibáñez et al. 2007). Warmer conditions combined with wetter winters and springs may lengthen the window for seedling establishment in high-elevation forests, but may not change significantly in lower elevation forests (though it may shift earlier) (Keane et al. forthcoming). However, even with a lengthened window for establishment, warming temperatures alone may cause seedling mortality and failed regeneration as a result of seasonal mismatches in the timing of flowering and seed production (Cayan et al. 2010; Williams et al. 2013). In the short term, warmer temperatures but drier summers may increase forest regeneration due to a) increased tree flowering and recruitment, and b) drought-related reductions in canopy cover speeding sapling growth (Galiano et al. 2013; Ibáñez and McCarthy-Neumann 2014; Clark et al. 2016). Additionally, higher CO₂ levels have been shown to increase fertility and seed and pollen production in some trees (Ladeau and Clark 2006). However, in the long term, more severe drought is likely to reduce tree establishment by reducing seed germination, as well as increasing mortality of seedlings and saplings (Kolb and Robberecht 1996; Chmura et al. 2011). 

Many responses may be species-specific, for example ponderosa pine seedlings are sensitive to temperature, lodgepole pine seedlings are sensitive to moisture fluctuations (Petrie et al. 2016). Ultimately, in forests not otherwise limited by energy or nutrients variability in moisture availability with natural and climate oscillations may drive establishment success between years (League and Veblen 2006), with indirect disturbance effects (e.g., fires, landslides, insect outbreaks, and pathogen attacks) greatly affecting long-term recruitment success (Clark et al. 2016). 

 

Growth and productivity

Warming temperatures, increased atmospheric CO₂, and longer growing seasons provide opportunities for increased photosynthesis, thereby improving forest growth and productivity (Ehleringer and Cerling 1995; Joyce and Birdsey 1995; Waring and Running 2007; NPS 2010). However, these same changes can also reduce forest productivity, particularly in water-limited systems. Thus, net forest response is uncertain, but likely negative under extreme temperature increases. 

Forest productivity will increase up to some optimal temperature and then begin to decline if temperatures continue to rise. This decrease results because plant respiration also increases with temperature, and some of the photosynthetic gains (that lead to increased productivity) are lost through a) growth and maintenance respiration (Ryan et al. 1995), or b) seasonal differences between photosynthetic gains in the spring and increased respiration in the fall. These tradeoffs can result in no net increase in productivity (Piao et al. 2008). Additionally, extremely high temperatures can lead directly to increased water stress because of drier soils. The temperature threshold at which declines would occur is complicated by the fact that elevated CO₂ levels may increase water-use efficiency in plants (Waring and Running 2007) and thereby lower plant water stress (Franks et al. 2013). 

Although CO₂ fertilization has likely increased forest growth at a global scale (McMahon et al. 2010), this increase may be evident in only about 20% of forests, with the remaining 80% unable to capitalize on benefits of higher atmospheric CO₂ because of water or nutrient limitations (Gedalof and Berg 2010). A recent study suggests that Montana forests will likely show substantially lower productivity overall given only small projected increases in precipitation (Charney et al. 2016). Reduced snowpack and earlier snowmelt (see Water chapter) may further limit any potential gains in productivity. In general, changes in temperature, precipitation, and snow could alter forest productivity in Montana as described below. 

• Forest productivity may increase in montane, subalpine, and alpine areas.—These areas may not exceed optimal temperatures, even under end-of-century projections, and these high-elevation areas should have sufficient moisture with increased winter and spring precipitation and longer snowpack duration, allowing growing seasons to lengthen and forests to benefit from higher atmospheric CO₂ (Keane et al. forthcoming). Gains in forest productivity have already been observed in relatively cooler/wetter sites at higher elevations and northern range limits in other regions (Littell et al. 2008; Bhuta et al. 2009; Salzer et al. 2009; D’Orangeville et al. 2016).
• Forest productivity may decrease in lower elevation, warmer, and drier sites.—Conversely, lower elevation areas are likely to see more extreme high temperatures combined with low soil water availability later in the year, resulting in reduced productivity. 

Ultimately, shifts in productivity will be site- and species-specific and vary across years. Above- and below-normal temperature and precipitation years associated with natural climate oscillations may determine whether growing seasons lengthen, contract, or shift in time. But even under ideal temperature and moisture conditions, productivity gains will be dependent on local site conditions, such as where there are sufficient soil nutrients or where stand density is low and little competition exists for available resources (Ford et al. 2016). Further, if extreme heat events increase substantially, impacts will be negative regardless of water availability.

 

Mortality and die-off

The expected increase in drought severity will increase tree mortality in forests. Already, widespread, catastrophic forest die-off events throughout the western US have been directly or indirectly related to drought (Breshears et al. 2005; Allen et al. 2010; Ganey and Vojta 2011; Worrall et al. 2013). Multiple researchers have shown that extended drought correlates with declining tree growth and increased risk of mortality (Allen et al. 2010; O’Connor 2013; Williams et al. 2013). Similarly, the combination of increased warming and drought conditions is the likely cause of recent rapid increases in background (non-catastrophic) forest mortality rates in Montana and the interior West (van Mantgem et al. 2009). For trees beyond the seedling stage, increased temperatures may be responsible for tree mortality more so than water stress (Luo and Chen 2013), although water stress may have been more important historically (Rapacciuolo et al. 2014). 

Initial forest conditions affect levels of tree mortality from direct climate effects. For example, soil type and depth, elevation, and aspect all influence water availability for forests. Stand condition may also increase tree mortality by increasing the likelihood of indirect effects (discussed below). Stand condition is particularly important on state and federal forests where a policy of fire suppression for the last 100 yr has increased tree density and the risk of mortality from defoliating and boring insects, and from wildfire. 

 

Species range shifts and forest distribution

Climate conditions and disturbance regimes largely control plant distributions (ranges). Over the millennia, the main responses of species to climate change has been to adapt to changing conditions, move to a new site (range shift), or go extinct (Davis and Shaw 2001). Although many tree species in Montana are relatively plastic—meaning they can adapt in the short term to a wide range of climate conditions—this plasticity could be challenged by severe or prolonged drought or substantially modified disturbance regimes (Allen et al. 2010). Long-term adaptability is determined by the genetic diversity of forests and individual species (Davis et al. 2005), both of which may have been reduced by lack of understanding in the past of how forest management activities affect forest genetics (see Genetics sidebar).

	The Importance of Genetic Diversity  Forest genetics—the genetic variation and inheritance of various genes of forest trees—will primarily determine a forest’s ability to adapt to climate change over the long term. Genetic diversity largely determines a species’ ability to survive extreme events and adapt to changing conditions (Ledig and Kitzmiller 1992). Historically, the high genetic diversity of many tree species allowed forests to tolerate a wide range of environmental conditions and adapt to shifts in climate (Westfall and Millar 2004; Nicotra et al. 2015).  However, it is unclear whether species with even high levels of genetic and physical diversity can adapt fast enough to the rapid and extreme shifts in climate that are projected over the next century (Vose et al. 2012).  Human actions have undoubtedly altered forest genetics, at least in part through silvicultural practices. Because physical characteristics (e.g., tree height or basal diameter) are used to select trees for harvest, silvicultural practices can alter forest genetics, which are the bases for these physical differences. Selective harvesting may have substantially altered the presence of rare genetic characteristics (Cheliak et al. 1988; Schaberg et al. 2008), which are often those needed by a species to adapt to climate change.  Even though data regarding trends in forest genetic diversity are scarce, a general consensus exists that natural genetic diversity may be at risk globally as a result of human activities (Schueler et al. 2012; Alfaro et al. 2014; FAO 2014). Managing forests to retain or increase this diversity is one of the best options for conservation.

 

Some evidence exists that tree ranges are already shifting to colder locations in the Pacific Northwest (Monleon and Lintz 2015). However, climatically suitable places are often geographically limited, and alpine vegetation may be running out of mountain as it seeks colder climes (Gottfried et al. 2012). To complicate matters, the optimum elevations for some plant species are shifting downhill tracking changes in water availability, as opposed to simply moving uphill, tracking changes in temperature (Crimmins et al. 2011). Divergence in the direction between optimal temperature and moisture conditions may make it difficult for trees to stay in equilibrium with favorable climate conditions (Dobrowski et al. 2013). Regardless of direction of range shift, there is concern whether tree species can disperse and regenerate quickly enough to keep pace with the magnitude and rate of projected climate changes (Zhu et al. 2012). Although dynamic vegetation models tend to predict an overall expansion of cool forests and woodlands (Shafer et al. 2015), some tree species may actually experience reduced ranges due to geographical obstacles to range expansion in response to climate (Coops and Waring 2001). Current best global estimates suggest that forest mortality is outpacing benefits from increased tree productivity due to increased atmospheric CO₂ (Allen et al. 2010), signifying an overarching contraction of forest range (Dobrowski et al. 2015).

 

	Feedbacks Forest responses to climate change may be complex due to feedbacks, which are the interplay between different climate change-related effects. One effect may amplify or diminish another effect, and that interaction has the potential to result in changes that are non-linear, unpredictable, and even dramatic.      For forests, a notable challenge will be understanding and preparing for interactions in disturbances (Buma 2015). For example, if a fire burns a forest, it will release stored carbon. If many fires burn globally, there will be an increase in atmospheric CO2, the main driver of climate change. Further, more fires would mean fewer trees and that could mean that less water vapor transpired into the air, resulting in drier conditions. Drier and warmer conditions would lead to more fires—a positive feedback loop where changes are amplified. In a negative feedback loop, changes are diminished or reach a steady state. For example, increased atmospheric CO2 may increase forest productivity. More trees would sequester more atmospheric CO2, thereby reducing atmospheric CO2 and dampening the initial CO2 fertilization effect.           A simplified example of the feedbacks between wildfire and climate (adapted from Vose et al. 2016).    Feedbacks may occur between resources as well, for example, between forests and water quantity. The feedbacks are not always as assumed. For example, forest die-off caused by drought has, in some areas, been shown to reduce streamflow as opposed to increasing it, as might seem more intuitive (Guardiola-Claramonte et al. 2011).      Even where ecologists recognize a feedback, they often do not understand all the connections or even if the interplay between components leads to positive or negative outcomes. It is also likely that many feedbacks exist that are currently unknown. Regardless, scientists believe that feedbacks are likely to increase the ecosystem impact of individual disturbances, which may have substantial implications for changes in distribution and heterogeneity of forests in the future (Bonan 2008; Vose et al. 2012; Richardson et al. 2013).

 

 

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ADAPTATION STRATEGIES FOR A CHANGING CLIMATE

 

Managers should consider multiple scenarios of potential climate shifts and contemplate a suite of adaptation strategies. Vose et al. (2012) suggest four general types of adaptation options in managing forests for the potential impacts of climate change: 

1. Promote resistance.—Enhance ability of species of system to resist forces of climate change;
2. Increase resilience.—Enhance capacity of system to absorb impact without substantial changes to processes and functionality;
3. Enable ecosystems to respond.—Assist a system’s transition to an altered state that is adaptive to a changed climate while minimizing disruptive outcomes; and
4. Re-align highly altered ecosystems.—Use restoration techniques to allow a system’s function to continue through changing climate conditions.

We recommend a bet-hedging approach, understanding the range of potential options and their possible consequences, and selecting among those that provide the most likely benefit given future uncertainty. We list many potential specific adaptation strategies in Table 4-6. Several guidebooks exist for developing adaptation options on forested lands; these include Responding to Climate Change on National Forests (Peterson et al. 2011), Climate Change in Forests of the Future (Millar et al. 2007), and Forest Adaptation Resources (Swanston et al. 2016). 

Table 2-3. Decadal rate of change for annual average temperatures in °F (°C) for the seven Montana climate divisions (Figure 2-3), statewide, and US from 1950-2015. A value of 0 indicates no statistically significant change between decadal averages.

Plant from multiple species, seed sources, and climate zones, particularly from locally-adapted sources Manage to maintain genetic diversity and phenotypic plasticity Create opportunities for rapid natural selection for species with high predicted potential for adverse impacts from climate change (Sturrock et al. 2011; Erickson et al. 2012; Alfaro et al. 2014; FAO 2014) Plant drought tolerant and native species Retain diversity of species and promote legacy trees Manage or restore mosaic (variable pattern of species and ages) and maintain or improve landscape connectivity Plant in asynchronous rotations and manage for diverse age classes Thin, plant, and use prescribed fire to favor species adapted to disturbance (Millar et al. 2007; Vose et al. 2016; Keane et al. forthcoming)

Table 4-6. General adaptation strategies to increase resilience of forests to climate change and variability.

Adaptation option	Time period	Examples (further reading)
Increase genetic and phenotypic diversity	Mid to long term	Breed for climate resilience and disease resistance
Improve forest structure, diversity, and resilience	Long term	Plant various species in microsites (small areas with locally variable climate, topographical, and soil conditions) with existing species mix as guide
Improve establishment	Short to long term	Plant drought-tolerant species in years with strong El Niño forecasts, particularly during Pacific Decadal Oscillation warm phase; plant trees that require sufficient water during establishment in La Niña years and during Pacific Decadal Oscillation cool phases Focus planting more in spring as fall planting becomes more difficult with reduced soil moisture and test different planting timings as springs shift earlier
Improve water availability	Long term	In snow-dominant locations, reduce canopy cover on north slopes (reduce interception of moisture by canopy), retain canopy cover on south slopes (increase shading), in all locations maintain sufficient shading on south slopes to retain soil moisture
Improve soil quality	Mid to long term	Alter timing of logging to reduce soil compaction Retain woody debris to retain soil moisture and promote nutrient cycling
Reduce fire risk and fire severity	Mid to long term	Apply additional efforts for fire prevention in drier, warmer years; allow spot burning in cooler, wetter years Use prescribed fire and thinning to minimize fuel loading and favor fire-resistant species (Vose et al. 2016; Keane et al. forthcoming)
Manage forest diseases	Mid to long term	Monitor, forecast, and plan regarding forest diseases and use an adaptive management framework Ensure management activities do not spread disease Breed for increased disease resistance (Sturrock et al. 2011)
Consider assisted migration and adaptation	Mid to long term	Assisted adaptation—often defined as management to assist gene flow or selection for specific genetic traits—may be a more useful tool than assisted migration whereby a species is deliberately moved to a different habitat; carefully consider implications of either action Identify potential climate refugia to focus restoration efforts Plant a mix of seeds genetically selected and adapted to likely future and current conditions (McLachlan et al. 2007; McKenney et al. 2009; Aitken and Whitlock 2013; Alfaro et al. 2014)
Manage forest carbon (mitigation)	Mid to long term	Planting new trees to increase forested area Increase carbon storage in existing forests (e.g., replace dying stands, manage for maximum productivity and reduced fire risk via pre-commercial thinning23) Use of wood as biomass energy (McKinley et al. 2011; Bellassen and Luyssaert 2014)

 

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CONCLUSIONS

 

Much is known regarding how forest ecosystems will respond to climate change, even amid the uncertainties. Two conclusions can be made with high confidence: 

Rising temperatures and shifts in precipitation and moisture balance of forests are likely to increase negative direct effects on forests, particularly in water-limited systems and in years with low precipitation. 

In some regions, indirect effects of climate, due primarily to increased frequency and severity of wildfire and beetle outbreaks, will have a greater impact than direct climate effects. 

By instituting adaptation and mitigation programs, forest managers can act now to lessen the likelihood and magnitude of climate change impacts on Montana’s forests. Such programs, best undertaken in an adaptive management framework, include, but are not limited to, reducing fire risk; managing forest diseases; improving forest establishment; increasing forest carbon storage; improving water availability and soil quality; improving forest diversity, structure, and resilience; and increasing genetic and phenotypic diversity of forests.

 

	Socio-cultural Concerns Sacred sites and traditional first foods.—Foods play a vital role in physical, mental, and spiritual health of indigenous communities. Many tribal communities rely on first foods for sustenance, and these foods are equally important to the sustenance of tribal culture. In Montana, examples of first foods and plants that are found in forests and open woodlands include camas (Camassia quamash), purple coneflower (Echinacea angustifolia), chokecherry (Prunus virginiana), red raspberry (Rubus idaeus L.), huckleberries (Vaccinium spp.), kinnick-kinnick (Arcostaphylos uva-ursi), wood’s rose (Rosa woodsii), wild strawberry (Fragaria virginiana), arrowleaf balsamroot (Balsamorhiza sagittata), and fireweed (Epilobium angustifolium), in addition to deer, elk, and other game.    Climate change may reduce availability of these foods and plants, as well as shift gathering sites to locations not under tribal control (Voggesser et al. 2013). For example, Holden et al. (2012) showed huckleberry and serviceberry (Amelanchier alnifolia) productivity is sensitive to climate variation. Forest changes may also impact sacred sites. For example, some sacred sites are named after the long-standing vegetation communities grown in that area; yet many of the species may no longer be present under future climates. Native people have harvested camas (Camassia quamash) for millennia as a food source in Montana. Here, native youth dig for camas roots as part of the 2011 Northwest Montana Native Youth Conservation Corps. Courtesy of US Fish and Wildlife Service.      Timber production and wood industry.—Potential climate change-induced impacts to commercial forestry have been reviewed in several places (e.g., Kirilenko and Sedjo 2007; Vose et al. 2016; Keane et al. forthcoming), as has the status of Montana’s forest products industry (e.g., McIver et al. 2013). In summary, potential increases in forest productivity from climate shifts (Lin et al. 2010; NPS 2010) could result in increased timber production in Montana (Garcia-Gonzalo et al. 2007). However, decreased vigor and increased mortality might offset gains in productivity from climate alone (Kirilenko and Sedjo 2007; Vose et al. 2016). Increased diseases, insects, and fires could also reduce quality of timber, thereby reducing the value (Spittlehouse and Stewart 2004; Kirilenko and Sedjo 2007; Gillette et al. 2014).

 

 

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²² This report focuses on climate change impacts, not mitigation. Still it is important to recognize that forests are a key carbon sink, and have the potential to reduce atmospheric CO₂ through carbon sequestration (i.e., the removal of carbon from the atmosphere to be stored in an alternative form) (Ingerson 2007; Pan et al. 2011). Pan et al. (2011) estimated that forests offset 13% of carbon emissions in North America. Birdsey (1992) estimated Rocky Mountain forests make up about 15% of total US forest carbon storage. 

²³ New research indicates that there is not a trade-off between managing for productivity and carbon storage; stands managed with early (prior to onset of canopy closure and intense competition), pre-commercial thinning had lower densities, larger trees, greater structural complexity, and stored as much aboveground carbon as un-thinned stands (Schaedel et al. 2017).

 

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