Forests

Christopher J. Fettig; Grant M. Domke; Anne S. Marsh; Michelle Baumflek; William A. Gould; Jessica E. Halofsky; Linda A. Joyce; Stephen D. LeDuc; David H. Levinson; Jeremy S. Littell; Chelcy F. Miniat; Miranda H. Mockrin; David L. Peterson; Jeffrey Prestemon; Benjamin M. Sleeter; Chris Swanston

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Forests in the US are increasingly affected by continued warming, changing precipitation patterns, and disturbances associated with climate change, such as wildfires and diseases. These changes are limiting the capacity of forest ecosystems to store carbon, provide clean water and air, produce timber and other products, and offer opportunities for recreation. Improved adaptation efforts can help maintain resilient forest ecosystems.

Forest ecosystems provide ecological, economic, and social goods and services (hereafter ecosystem services) to natural systems and humankind. These include air purification; regulating water quantity and quality; provisioning fish and wildlife habitat, food, medicine, shelter, wood, and other forest products; provisioning aesthetics, outdoor recreation, and spiritual renewal; and regulating climate through carbon transfers and other processes.¹ The livelihoods, health, nutrition, and cultural practices and traditions of many Indigenous and Tribal Peoples depend on forest ecosystems (Ch. 16). Social and economic drivers influence how and when forests are managed to maintain or restore ecosystem services critical to human health and welfare.

Authors

Federal Coordinating Lead Author: Christopher J. Fettig, USDA Forest Service, Pacific Southwest Research Station
Chapter Lead Author: Grant M. Domke, USDA Forest Service, Northern Research Station
Agency Chapter Lead Author: Anne S. Marsh, USDA Forest Service, Washington Office
Chapter Authors: Michelle Baumflek, USDA Forest Service, Southern Research Station; William A. Gould, USDA Forest Service, International Institute of Tropical Forestry; Jessica E. Halofsky, USDA Forest Service, Pacific Northwest Research Station; Linda A. Joyce, USDA Forest Service, Rocky Mountain Research Station; Stephen D. LeDuc, US Environmental Protection Agency, Office of Research and Development; David H. Levinson, USDA Forest Service, National Stream and Aquatic Ecology Center; Jeremy S. Littell, US Geological Survey, Alaska Climate Adaptation Science Center; Chelcy F. Miniat, USDA Forest Service, Rocky Mountain Research Station; Miranda H. Mockrin, USDA Forest Service, Northern Research Station; David L. Peterson, USDA Forest Service, Pacific Northwest Research Station; Jeffrey Prestemon, USDA Forest Service, Southern Research Station; Benjamin M. Sleeter, US Geological Survey, Western Geographic Science Center; Chris Swanston, USDA Forest Service, Washington Office

Contributors

Technical Contributors: Ning Liu, USDA Forest Service, Southern Research Station; D. Max Smith, USDA Forest Service, Rocky Mountain Research Station; Brian F. Walters, USDA Forest Service, Northern Research Station
Review Editor: Brooke Eastman, West Virginia University, Department of Biology
USGCRP Coordinators: Samantha Basile, US Global Change Research Program / ICF; Aaron M. Grade, US Global Change Research Program / ICF; Joshua Hernandez, US Global Change Research Program / ICF

Recommended Citation

Domke, G.M., C.J. Fettig, A.S. Marsh, M. Baumflek, W.A. Gould, J.E. Halofsky, L.A. Joyce, S.D. LeDuc, D.H. Levinson, J.S. Littell, C.F. Miniat, M.H. Mockrin, D.L. Peterson, J. Prestemon, B.M. Sleeter, and C. Swanston, 2023: Ch. 7. Forests. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH7

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Forests represent more than one-third (766 million acres) of the land base in the US, with an additional 125 million acres of trees outside of forests in woodlands and developed areas. The amount of forest and tree cover has remained relatively stable over the last 100 years despite substantial land-use change into and out of forest and tree cover, especially in recent decades (Figures 7.1, 6.2).² Forest land area and tree cover have declined slightly in the contiguous US in the last two decades due mostly to cropland expansion and urbanization (Figure 6.4),³^,⁴ including expansion of the wildland–urban interface (WUI).⁵ Forests contributed more than 4% of total US manufacturing gross domestic product in 2020 (nearly $336 billion in 2022 dollars), and the forest products industry is among the top 10 manufacturing sector employers in the US.⁶

Map of the contiguous United States showing areas designated as having tree cover, with insets showing detailed views of 9 locations in the contiguous US, as explained in text and caption. Legend shows a color ramp indicating the number of years classified as land cover, ranging from 1 (pale yellow) to 36 (dark green). Outlines of Alaska and Hawai‘i are also shown, but are gray because no data was available for those states. On the map of the contiguous US, regions showing the longest-persisting tree cover are the Northeast, Southeast, northern and southern parts of the Midwest, and mountainous and coastal portions of the Northwest, Southwest, and Northern Great Plains. Insets show tree cover variation in more granular detail, as follows: A. Map of Cascades, Washington, shows a central treeless area in gray and white surrounded by patches of paler greens and yellow, described on map as “forest regeneration near Mount St. Helens, Washington, along with harvest on private forested lands.” B. Map of Coast Range, Oregon, shows a checkerboard pattern of dark green and paler greens, described as “forest harvest and regrowth in central Oregon.” C. Map of Sierra Nevada Mountains, California, shows largely dark green areas, with one large patch of paler greens and yellows, described as “forest harvest and wildfire in the central Sierra Nevada Mountains of California.” D. Map of the Middle Rockies in Idaho and Wyoming shows large patches of dark green beside a patchwork of paler greens and yellows with white areas at the upper right, lower right, and left edge, described on map as “forest harvest in Targhee National Forest, Idaho, and wildfire in and around Yellowstone National Park, Wyoming.” E. Map of Blackland Prairies, Texas, shows a large white area at center, right, and bottom, with smaller dark and pale green areas at top and left, described as “urbanization in and around San Antonio, Texas.” F. Map of the Southern Rockies in Colorado shows a large pale green area at top center, surrounded by dark green, with white areas at lower right and across the top, described as “Wildfire in and around Mesa Verde National Park.” G. Map of Greenville, Maine, shows many small, interspersed patches of dark green, paler greens, yellows, and some white patches, described as “forest harvest in western Maine.” H. Map of the Central Appalachians in West Virginia shows mostly dark green, with patches of gray and white and small patches of paler greens, described as “coal mining and forest reclamation in the Appalachian Mountains of West Virginia.” I. Map of the South Central Plains in Louisiana shows a grid pattern of mostly dark and pale greens, with one patch of gray surrounded by some pale yellow at center left, described as “forest harvest and land-use change near Winnfield, Louisiana."

The persistence of tree cover in the United States varies due to many driving forces.

Figure 7.1. This figure shows the number of years each 30 m by 30 m pixel was classified as forest cover during 1985–2020. Insets (A–I) show that patterns of forest cover vary considerably across regions, often due to differences in the factors causing forest change. Patterns in tree cover change in the US are driven, in large part, by climate-related disturbances, land use, and land-use change. Data were unavailable for Alaska, Hawaiʻi and the Affiliated US Pacific Islands, and the US Caribbean. Figure credit: USGS.

The vulnerability of US forests to climate change and climate-related disturbances varies (relative to natural variability) due to differences in biophysical conditions and local and regional variations in climate (Chs. 2, 3). For example, although 21st-century temperatures (2001–2020) have increased almost everywhere in the US (relative to 1951–1970), this warming has not occurred uniformly across the US (KM 3.4; Figure 3.11). Due to these differences, the capacity of some US forests to provide ecosystem services is increasingly affected by climate change and climate-related disturbances (KMs 7.1, 7.2).⁷ For example, the amount of forest burned and greenhouse gas (GHG) emissions from fires have increased substantially since 1990, mostly in the West, with three of the five worst wildfire years (based on area burned and GHG emissions) occurring since 2015 (Figure 7.2).³ Proactive adaptation will assist the provisioning of ecosystem services from forests. Examples of adaptation in US forests have proliferated since 2017 (KM 7.3; Ch. 31) on federal, state, local, Tribal, and private lands (e.g., Moser et al. 2019; USDA 2022, 2021⁸^,⁹^,¹⁰). The effects of climate change on forests in specific regions of the US are discussed in several of the regional chapters (e.g., Chs. 21, 22, 23, 24, 27, 28, 29).

Line and bar chart shows estimated annual greenhouse gas emissions (abbreviated G H G) from wildfires and prescribed fires from 1990 to 2021, as explained in the text and caption. Left y-axis shows area burned from 0 to 2 million acres. Right y-axis shows GHG emissions from 0 to 300 million metric tons CO2 equivalent. Between 1990 and 1999, area burned—shown in gray bars—ranged from about 0.1 million to 0.5 million acres. Since 2000 area burned has been much higher in many years, reaching above 1.5 million acres in 2004, 2015, and 2021. GHG emissions, indicated by solid black line, correlated with area burned, ranging from close to zero in 1995 and 1997 to a peak above 250 million metric tons CO2 equivalent in 2004, with values at or above 100 million metric tons from 2017 through 2021. Non-GHG emissions, shown in a dotted black line, are also somewhat correlated with area burned and range from close to zero million metric tons CO2 equivalent in years when the fewest acres burned to about 25 million metric tons in 2004, 2015, and 2021.

The amount of forest area burned and associated greenhouse gas emissions have increased in recent decades in the United States.

Figure 7.2. Estimated forest area burned and greenhouse gas emissions (carbon dioxide, methane, and nitrous oxide) from wildfires and prescribed fires in the contiguous US and Alaska have increased since the late 1990s. Climate change is affecting the likelihood and scale of wildfires in US forests. In some cases, wildfires (particularly in the western US) have slowed or stopped recovery of forests from previous disturbances, reducing their capacity to sequester and store carbon. Adapted from Domke et al. 2023.³

Forests Are Increasingly Affected by Climate Change and Disturbances

Climate change is increasing the frequency, scale, and severity of some disturbances that drive forest change and affect ecosystem services . Continued warming and regional changes in precipitation are expected to amplify interactions among disturbance agents and further alter forest ecosystem structure and function .

Climate change affects disturbances such as wildfires, insects, diseases, and land uses, as well as the interactions among these disturbances, all of which shape forest ecosystems through changes in growth, mortality, regeneration, and recruitment of vegetation over space and time (Figure 6.1 in Vose et al. 2018¹¹). Disturbances altered by climate change pose risks to current and future forest health (i.e., the extent to which ecosystem processes are functioning within their natural range of historical variation) and will affect forest conditions across landscapes for years to centuries. Weather events such as droughts, hurricanes, windstorms, and floods may exacerbate disturbance effects, especially in extreme cases (Figure 7.3; Chs. 8, 9). The exposure and sensitivity of forests to climate change and climate-related disturbances vary with disturbance, forest condition, management history, and the rate and magnitude of change.

Coastal ghost forests result when trees are killed by sea level rise and saltwater intrusion.

Figure 7.3. The photo shows a coastal ghost forest (foreground) near Blackwater National Wildlife Refuge, Maryland. As sea levels rise in response to climate change, the replacement of coastal forests with tidal wetlands will affect many ecosystem services, including storm surge buffering capacity. Photo credit: ©Matthew L. Kirwan, Virginia Institute of Marine Science.

Climate change is affecting the likelihood and scale of wildfires in US forests. For example, the amount of forest burned by wildfires in the West has increased relative to the mid- to late 20th century¹²^,¹³ due, in part, to warming increasing vapor pressure deficits and rates of evapotranspiration (KM 4.1),¹²^,¹⁴ as well as decreases in precipitation (KM 4.1).¹⁵ Fire activity is projected to increase with further warming and reductions in precipitation,¹⁴^,¹⁶ although increases depend on regional fuel types and may eventually decrease in some forests due to reductions in fuel loads.¹⁷ The area burned by high-severity wildfires (e.g., stand-replacing fires) has increased in the West since 1985 by about eightfold,¹⁸ partly due to warmer, drier conditions (Figure 7.4; KM 2.1). Where abundant fuels are available, western US forests have experienced an increase in the proportion of area burned at high severity, especially in the Southwest (KM 28.5).¹⁸ Increased fire severity is expected to become more widespread in US forests in the future.¹⁹ Atypical re-burns and levels of fuel flammability that were historically rare are expected to become more common (Focus on Western Wildfires).

Art × Climate

Michael Krondl
Portrait of the Apocalypse 1
(2022, charcoal on paper)

Artist’s statement: The subject of my work is our relationship to the natural world, as a society and as individuals, and has focused on rising temperatures that come with climate change. In the 'portraits' series I have turned to making drawings of burned forests, specifically in the form of life-sized 'portraits' of individual fire-ravaged trees that almost seem to return the viewer's stare. The drawings are based on detailed photographs taken in Mesa Verde National Park. Using burnt wood (charcoal) seems the only logical medium here.

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Determining the effects of climate change on wildfires is more difficult in regions outside the West, for example, in areas where prescribed fire use has changed substantially over time (Southeast), where wildfire was historically rare (Northeast), and where forests represent a small portion of the landscape (agricultural regions in the central US). Furthermore, fire intensity (energy released during wildfire) and severity depend on fuel availability and flammability, which are directly affected by management (including wildfire suppression) and land use,²⁰ as well as climate-driven changes in weather. However, altered meteorological conditions (e.g., relative humidity and wind speed), especially extreme conditions promoting wildfire spread, have become more prevalent in recent decades²¹ and are attributed, in part, to climate change (KM 2.2).²² Changes in human demography in the WUI and increases in the ratio of human to natural fire ignitions²³ have combined with climate change to alter historical expectations of fire initiation and spread. One study found that a long-term trend in nighttime vapor pressure deficit, not simulated in climate models, explained recent fire managers’ observations that the rates of spread of fires in the West slowed less at night.²⁴

At left, a map of the contiguous US shows the model simulated average number of days per year from May through October with extreme weather conditions that are associated with very large fires for 1971 through 2000, with the land area divided into Bailey ecosections as explained in the text and caption. A legend titled “average number of days” shows six colors indicating values ranging from less than 0.05 days (light yellow) to 0.75 to 1.0 (dark red); gray indicates areas not modeled. Areas along much of the Atlantic and Gulf coasts show values of up to 0.10. The Appalachian Mountains show values of less than 0.05. Portions of the Northern and Southern Great Plains show values up to 0.10. Much of Northwest, Southwest, and western portions of the Northern Great Plains show larger values, ranging from 0.1 up to 1.0. Other areas of the Northeast, Southeast, Midwest and eastern portions of the Northern and Southern Great Plains are not modeled. The map at right shows projected changes in the number of days for 2040–2069 compared to the 1971–2000 average for the RCP8.5 scenario. The legend indicates percent changes in number of days, with light blue indicating decreases of up to 25% and 15 shades of yellow to dark red showing increases ranging from 0–25% (light yellow) up to greater than 1000% (dark red); gray indicates areas not modeled. The same areas are shown as modeled as in the map at the left. Nearly all such areas show increases, with one exception being small decreases in southern Florida. Most areas show increases of 25%, with generally larger increases at more northern latitudes, with changes of more than 450 percent in portions of the Northern Great Plains and Northwest, and changes of more than 1000 percent in portions of the upper Midwest.

Conditions conducive to very large fires are projected to increase.

Figure 7.4. The left panel shows historical (1971–2000) values for the annual number of days in May through October with extreme weather conditions conducive to very large fires (VLFs; more than 12,000 acres). The right panel shows the percent change in the number of days for a projected future (2040–2069) climate under a very high scenario (RCP8.5). Changes are summarized by Bailey ecosections, which are areas of similar vegetation and climate defined by Bailey (2016).²⁵ The number of days with conditions associated with VLFs more than doubles in many ecosections, with more than a fourfold increase for parts of the Northwest, fivefold for the northern Rockies, and over sevenfold for the Upper Midwest. Projected conditions are an average of a 17-GCM (global climate model) ensemble selected for data availability. Areas with no color indicate lack of data (sufficient data are unavailable or where wildfires were historically rare). Data were unavailable for Alaska, Hawaiʻi and the Affiliated US Pacific Islands, and the US Caribbean. Figure credit: USGS.

In addition to wildfire, native and invasive (non-native) insects, diseases, and plants are important forest disturbances with climatic and non-climatic factors influencing their extent and effects. Tree mortality from bark beetles in the West has increased in the late 20th and early 21st centuries due, in part, to climate change (Box 7.1).²⁶^,²⁷^,²⁸^,²⁹ The effects of climate change on other forest insects and diseases are less certain. For example, white pine blister rust (a disease caused by the non-native fungus Cronartium ribicola) is expected to decrease where conditions become warmer and drier (Southwest) but increase where conditions become warmer and wetter (high-elevation subalpine forests).³⁰ Warming, altered precipitation patterns, extreme events, and disturbances all influence invasion pathways and may facilitate the establishment and spread of invasive species (KM 8.2).³¹ In Hawaiʻi, climate change and the spread of invasive grasses have increased the frequency and extent of wildfires.³² In the Southeast, warming temperatures have allowed cold-sensitive invasive species like kudzu to move farther north,³³ affecting forest structure and composition. Kudzu and other woody vines are stimulated by increased carbon dioxide (CO₂)³⁴ and in some cases outcompete trees and other plants.³⁵ Even in the absence of other disturbances, warming and drought are important drivers of tree mortality in the US and globally.³⁶

Sea level rise, another climate-related disturbance, affects the distribution, structure, and composition of forests (KM 9.2). Saltwater intrusion has reduced the health, diversity, and productivity of some coastal forests in the East (Figure 7.3).³⁷ Sea level is projected to result in the loss of existing mangrove forests in many places in future decades.³⁸^,³⁹^,⁴⁰ However, warming during winter has facilitated northward migration of mangroves in the Southeast (Figure 7.7).⁴¹

Forest structure, function, and diversity are affected across a broad range of spatial scales (Figure 7.5). Variation in environmental conditions, historical and contemporary disturbances, management history, and land use have modified many forests, making them more vulnerable to droughts, wildfires, and other disturbances.⁴² These disturbances accelerate tree mortality; alter tree and other plant species distributions, age and size distributions, and regeneration success; and can lead to conversion to non-forest ecosystems (e.g., Falk et al. 2022; Stanke et al. 2021⁴³^,⁴⁴).

Ecological transformations and shifts in forest habitats are occurring because of climate change (KM 8.1).⁴⁵ In some low-elevation forests in the West, tree regeneration over the last 20 years has been limited by unfavorable climate. High wildfire severity and low seed availability has further reduced postfire regeneration in some locations.⁴⁶^,⁴⁷ Eastern tree species migration is associated with increased seed production but is limited to some extent by the occurrence and distribution of large urban areas in the East.

A matrix shows observed effects of climate change for four types of forests on the vertical axis and five categories of change on the horizontal axis, as explained in text and caption. Colors (dark purple, light purple, gray, and white) indicate the level of confidence in the attribution of these changes to climate change; where possible and appropriate, symbols indicate the direction of change as an increase, decrease, or multidirectional change. For boreal forests, tree mortality has increased, with high or very high confidence in the attribution; forest structure has changed, with medium confidence; carbon storage has decreased, with medium confidence; tree species have undergone multidirectional range shifts, with medium confidence; and understory species have undergone multidirectional range shifts, with low confidence. For temperate broadleaf forests, tree mortality has increased, with high or very high confidence; forest structure has changed, with medium confidence; carbon storage has changed, with medium confidence; and tree species have undergone multidirectional range shifts, with high confidence. For temperate coniferous forests, tree mortality has increased, with high or very high confidence; forest structure has changed, with high or very high confidence; carbon storage has declined, with low confidence; and tree species have undergone multidirectional range shifts, with high confidence. For coastal temperate forests, tree mortality has increased, with high or very high confidence; forest structure has changed, with low confidence; and carbon storage has declined, with high or very high confidence. There was limited or insufficient evidence regarding both tree and understory species range shifts for coastal temperate forests, as well as for understory species range shifts for temperate broadleaf and temperate coniferous forests.

Climate change and climate-related disturbances are affecting forests in the United States.

Figure 7.5. The figure shows recently documented effects, specific to individual forest types, that have been attributed to climate change and climate-related disturbances. Effects include increased tree mortality across all types with high confidence, changes in forest structure with variable confidence, less carbon storage across three of the four forest types, and variable shifts in plant species composition. Confidence levels reflect the uncertainty in attributions based on available literature. Arrows indicate the direction of change where suitable data exist. In the case of temperate forests, structure is changing but not in a unidirectional way. Boreal forest reflects changes only in Alaska. Assessments in the figure are based on recent relevant literature, and citations can be found in the metadata. Adapted with permission from Figure SPM.2 in IPCC 2022.⁴⁸

Although the effects of climate change on tree species have been well studied, effects on understory plants are poorly understood.⁴⁹ In Wisconsin, shifts in understory plant species lag regional climate changes, but less so for species with broader site occupancies and larger seed masses.⁵⁰ In Oregon’s Siskiyou Mountains, average temperature increases of about 3.6°F since 1948 have caused differing effects on plant communities. Low-elevation herb communities are now consistent with a hotter, drier climate and resemble plant communities in more southerly topographic positions. At higher elevations, herbs of northern biogeographic affinity have increased.⁵¹

Some management activities and land-use changes, especially rapid expansion of the WUI,⁵ have reduced the adaptive capacity of forests to variations in climate and climate-related disturbances.⁵²^,⁵³^,⁵⁴ The WUI is more prevalent in the East but is expanding at a faster rate in the West.⁵ In the East, forests in the WUI retain larger trees and aboveground biomass than less developed forests, but with less structural diversity (i.e., WUI forests have fewer saplings, seedlings, and dead trees). This raises concerns about diminished ecological function, reduced diversity of wildlife habitat, and vulnerability to warming.⁵⁵ In the West, wildfire exclusion in dry forests historically adapted to frequent wildfire has altered forest structure and composition, resulting in higher surface and canopy fuel loads and increased vulnerability to high-severity wildfire.⁵⁶^,⁵⁷

Box 7.1. Bark Beetles and Climate Change

Bark beetles spend most of their lives within a host tree, feeding and reproducing beneath the bark. Climate change has increased the impacts of some bark beetles due to 1) warming, which in some cases has increased life cycles and decreased overwintering mortality of beetles within the host tree; and 2) drought, as drought-stressed hosts have compromised defenses and offer little resistance to colonization by bark beetles.²⁶^,²⁷^,⁵⁸ Warming during summer increases the probability of a spruce beetle completing its life cycle in one year compared to two years,⁵⁹ which has increased spruce beetle populations in some areas (Figure 7.6). In California, warming and drought incited mortality of more than 100 million trees during 2014–2017, most attributed to western pine beetles (Figure 7.6) colonizing ponderosa pines.⁶⁰^,⁶¹ About 30% of the tree mortality in California was attributed to warming accelerating the life cycle of western pine beetle, with the remainder attributed to increases in host susceptibility due to drought stress.²⁹ The biomass of ponderosa pines in California may not return to levels that occurred prior to the drought due to future warming, droughts, and western pine beetle outbreaks.⁶²

Warming has allowed mountain pine beetles to erupt at elevations and latitudes where winters historically were cold enough to kill most mountain pine beetle brood within the host tree.²⁶ In New York and New England, a recent climate-driven range expansion of southern pine beetle resulted in a new bark beetle-host interaction in pitch pine forests.⁶³ In Alaska, an ongoing spruce beetle outbreak has affected more than 1.6 million acres since 2016 and expanded into the Alaska Range,²⁷ threatening spruce forests in interior Alaska, where spruce beetle populations were historically regulated by cold winter temperatures.⁶⁴

Bark beetle outbreaks are often detrimental to the provision of ecosystem services,⁶⁵ and can affect other disturbances and their effects on ecosystem services. For example, in some forests, mountain pine beetle outbreaks have increased the severity of wildfires⁶⁶ and the abundance of invasive weeds.⁶⁷ Silvicultural interventions such as thinning to reduce tree densities can be used to increase the resistance and resilience of some forests to bark beetles (KM 7.3), a relationship attributed to decreases in tree competition and associated increases in tree vigor, among other factors.²⁷

Outbreaks of spruce and pine beetles, partly attributable to climate change, are killing trees in the West.

Figure 7.6. These photos show a spruce beetle outbreak on the Bridger–Teton National Forest in Wyoming (left) and a western pine beetle outbreak on the Sierra National Forest in California (right). Discolored trees were colonized and killed by bark beetles. Warming and drought have increased the impacts of some bark beetles in US forests, affecting many ecosystem services. Photo credits: Christopher J. Fettig, USDA Forest Service.

Climate Change Affects Ecosystem Services Provided by Forests

Climate change threatens the ecosystem services forests provide that enrich human lives and sustain life more broadly. Increasing temperatures, changing precipitation patterns, and altered disturbances are affecting the capacity of forest ecosystems to sequester and store carbon , provide clean water and clean air , produce timber and non-timber products , and provide recreation , among other benefits. Future climate effects will interact with societal changes to determine the capacity of forests to provide ecosystem services .

Some effects of climate change and climate-related disturbances on ecosystem services and their associated economic benefits are gradual, driven by annual or seasonal warming, altered precipitation patterns, or sea level rise. Others are more rapid, driven by extreme events such as droughts, hurricanes, or heatwaves. Co-occurrence and/or interactions among disturbances (compound disturbances) can amplify the effects of individual disturbances on ecosystem services (Box 7.1; Figure 7.7; KM 2.3).⁶⁸^,⁶⁹

Climate change is projected to affect forest growth domestically and internationally, wood and paper markets (e.g., Tian et al. 2016⁷⁰), and the amount of carbon stored in harvested wood products (Box 7.2; KM 12.2; e.g., Johnston and Radeloff 2019⁷¹). However, the strength of these effects is uncertain due to disturbances, such as droughts, wildfires, insects, and diseases, that limit forest growth.⁷² Forest management actions taken in response to climate change can also affect timber product outputs, carbon, and associated ecosystem services (e.g., Creutzburg et al. 2017⁷³). Sea level rise also directly and indirectly affects timber output and carbon storage through loss of coastal forests to saltwater intrusion and housing losses and rebuilding, with a projected 800,000 new residential units needed in the US by 2050 to accommodate relocations due to sea level rise under a very high scenario (RCP8.5).⁷⁴

Climate change is altering the ranges and abundances of some plants and fungi used for food, medicine, and other purposes. Reduced snow depth, for example, can increase plant injury and mortality through increased exposure of tissues to frosts,⁷⁵^,⁷⁶^,⁷⁷ as well as reduced microbial biomass and activity.⁷⁸^,⁷⁹ Many plants and fungi have precise ecological requirements and narrow geographic ranges, leaving them vulnerable to climate change.⁸⁰ Some species are at their range limits, unable to adapt to rapidly changing conditions.⁸¹ Effects differ by plant and fungal species, relating to their sensitivity and adaptive capacity.

Climate change affects heritage values, cultural identity, and spiritual connections associated with forests, exacerbating environmental injustices affecting Indigenous and Tribal food sovereignty, health, cultural practices, and knowledge transmission (Chs. 16, 20).⁸² Examples of culturally significant species affected by climate change include salmon, brook trout, oaks, pinyon pine, and whitebark pine (Box 7.3).⁸³^,⁸⁴^,⁸⁵^,⁸⁶^,⁸⁷ In 2023, whitebark pine was listed as a threatened species by the US Fish and Wildlife Service, with white pine blister rust, mountain pine beetle, altered wildfire patterns, and climate change identified as major threats to its existence.⁸⁸ Climate-related changes highlight fluctuating consistency, timing, and availability of culturally significant foods, fibers, and medicines.⁸⁹^,⁹⁰^,⁹¹

Climate change decreases some forest-based recreational activities and increases others. For example, warming and reduced snowpack have had negative effects on winter sports (e.g., cross-country skiing, snowshoeing, and snowmobiling) and positive effects on warm-weather activities, with mixed effects on water-based activities.⁹² Participation in fishing and motorized water activities is projected to increase in the North, while motorized water activities are projected to decrease in parts of the West.⁹³

Increases in the amount of forest burned by wildfires are creating negative human health effects and growing economic losses.⁹⁴ Increases in wildfire smoke are increasing respiratory and cardiovascular-associated hospitalizations (KM 14.2; Focus on Western Wildfires )⁹⁵ and out-of-hospital cases of cardiac arrest.⁹⁶ Chemicals mobilized into the environment from wildfire-ignited structures and infrastructure can differ from those emitted from burning forest fuels, potentially increasing human health concerns (KM 14.2).⁹⁷^,⁹⁸

Maps of the contiguous United States, Alaska, Hawaii, and US Caribbean, as well as 6 time series graphs, illustrate how climate change has affected the provision of forest ecosystem goods and services, as explained in the text and caption. The maps show forest areas as green and other areas in gray, with extensive forest cover across the Northeast, Southeast, US Caribbean, the upper Midwest, eastern portions of the Southern Great Plains, the far western Northern Great Plains, mountainous areas of the Southwest and Northwest, most of central Alaska, and eastern portions of the Hawaiian islands. In panel (a), a time series shows the percentage of atmospheric PM2.5 from wildfire smoke from 2006 to 2018, with y-axis values from 0 to 30 percent. Values were between about 6% and 10% through 2011, spiked at about 20% in 2012, and then peaked at more than 20% in 2018. In panel (b), a time series shows the estimated moose population in Northeast Minnesota from 2005 to 2021, with the y-axis showing the number of moose from 2,000 to 10,000. The population was about 8,000 at the start of the period but declined rapidly thereafter, remaining at roughly 4,000 from 2012 to 2021. In panel (c), a time series shows US forest net CO2 flux from 1990 to 2021, with the y-axis showing values from negative 600 up to 0 metric tons of CO2 equivalent. A black line, labeled total public, rises steadily over the period from minus 300 to about minus 100. Dashed line, labeled total private, drops from just under minus 400 to just under minus 500. In panel (d), a time series shows the share of mangrove area in Florida located at 28°N and higher from 1984 to 2011, with data for 1992 missing. Y-axis values range from 0 to 0.25 decimal share. Share starts at just over 0.10, peaks at about 0.22 in 1993, and then stays in the range of 0.15 to 0.20 for the rest of the period of record. In panel (e), a time series shows the area impacted by western bark beetles from 2000 to 2019, with the y-axis showing area from 0 to 12 million acres. Area impacted rises from about 1 million acres in 2000 to about 10 million acres in 2009, dips to about 4 million in 2013, and then rises to more than 8 million in 2016 before dipping down to about 4 million again. In panel (f), a time series shows residences lost to wildfire in the US from 1990 to 2021, with y-axis values ranging from 0 to 20,000 residences. A vertical bar shows the 1990 to 2010 average was close to zero. Losses remained below 5,000 through 2016, rose quickly to a peak of nearly 20,000 in 2018, plunged close to 0 in 2019, rose to about 10,000 in 2020, and dropped below 5,000 in 2021.

Climate change has affected the provisioning of forest ecosystem goods and services in the United States.

Figure 7.7. (a) Increases in fine particulate matter air pollutants (PM_2.5) caused by wildfires degrade air quality and increase human health risks (data for Alaska, Hawai‘i, Puerto Rico, US Virgin Islands, and US-Affiliated Pacific Islands are not available). (b) Reduction in moose hunting opportunities stems from climate-related increases in parasites; declines in hunting opportunities have also been noted in the western US.⁹⁹ (c) Increased tree mortality is shrinking carbon sequestration in public forests (Mt is millions of metric tons). (d) Northward migration of mangroves is displacing saltwater marshes, altering coastal storm protection and affecting recreation (data for 1992 are not available). (e) Increased tree mortality by western bark beetles lowers home values through reduced environmental amenities.¹⁰⁰ (f) Housing losses due to wildfires increased by fivefold in the 2010s compared to the average from 1990 to 2010, with substantial interannual variability; data exclude losses from US-Affiliated Pacific Islands. Notes: Map excludes depictions of forest area for the US Virgin Islands and US-Affiliated Pacific Islands due to the large scale of the map. US Virgin Islands forest area is 46,967 acres.¹⁰¹ Forest areas for US-Affiliated Pacific Islands are as follows: Federated States of Micronesia (143,466 acres), Marshall Islands (23,252 acres), Northern Mariana Islands (75,407 acres), Palau (90,685 acres), American Sāmoa (43,631 acres), and Guam (63,833 acres); US Affiliated Pacific Island summary data are from sources cited in Lugo et al. 2022.¹⁰² Figure credits: (top) USDA Forest Service; (a) adapted with permission from Burke et al. 2021;¹⁰³ (b) USDA Forest Service; (c) adapted from Domke et al. 2023;³ (d) USDA Forest Service; (e) adapted from Fettig et al. 2022;²⁷ (f) USDA Forest Service.

Extreme events have been linked to declines in populations of amphibians, birds, fish, invertebrates, mammals, plants, and reptiles (KM 8.2).¹⁰⁴ Recent insect population declines have been attributed, in part, to climate change, with wide-reaching consequences.¹⁰⁵^,¹⁰⁶ Climate change is increasing the intensity of hurricanes in the East and their associated rainfall,¹⁰⁷^,¹⁰⁸ although projected changes in the frequency of hurricanes due to warming are uncertain (Chs. 2, 3; e.g., Sobel et al. 2021¹⁰⁹). Increased intensities of hurricanes could affect the structure and function of forests and wildlife habitat (e.g., Brown et al. 2011¹¹⁰). Mobile species or species capable of rapid population growth (e.g., invasives) generally benefit from extreme events and abrupt disturbances.¹⁰⁴

Box 7.2. Forests and Carbon

Carbon is continuously cycled between the Earth and atmosphere. Forests help regulate climate, as live plants remove carbon dioxide (CO₂) from the atmosphere through photosynthesis, facilitating maintenance and growth, and release some of that carbon through respiration. Forest ecosystems are the largest terrestrial carbon sink on Earth.¹¹¹ In the US, the amount of carbon stored in forests (primarily in soils and trees), as well as in harvested wood products that are either in use (e.g., paper, plywood) or in solid waste disposal sites (e.g., landfills), is equivalent to nearly three decades of fossil fuel emissions. On average, between 2017 and 2021, forest ecosystem carbon uptake has offset the equivalent of more than 13% of economy-wide CO₂ emissions each year.³ In recent decades, the rate of forest carbon sequestration has slowly declined, in part due to increasing frequency and severity of climate-related disturbances, leading to interannual variability in the forest carbon sink and abrupt (e.g., wildfire) and/or gradual (e.g., insect outbreak) transfers of carbon to the atmosphere, dead organic matter pools (dead wood, litter), and soils (Figure 7.8; KM 6.1).¹¹²^,¹¹³ In some cases, climate-related disturbances have slowed or stopped recovery of forests, reducing their capacity to store carbon (Figure 7.8),⁶²^,¹¹⁴^,¹¹⁵^,¹¹⁶ and these trends are projected to continue under multiple climate, land-use, and socioeconomic scenarios. Human activities such as forest management (e.g., timber harvesting, prescribed fire, and other silvicultural interventions) are also major drivers of forest ecosystem carbon dynamics. Harvesting, for example, results in the transfer of some carbon stored in live and dead trees to the atmosphere as well as to harvested wood products and may alter the capacity of forest ecosystems to store new carbon.¹¹²^,¹¹⁷ Land-use changes, including cropland expansion and urbanization, have also contributed to the decline in carbon sequestration and/or storage.¹¹⁵^,¹¹⁸ Yet managing forest ecosystems, including forest soils,¹¹⁹ for the purposes of carbon sequestration and/or storage, along with many other ecosystem services, remains a relatively cost-effective strategy for mitigating climate change (KM 6.3; Ch. 32).¹¹²^,¹²⁰^,¹²¹^,¹²²

Two stacked bar charts at left and one flow chart at right illustrate forest carbon sources and sinks, as described in the text and caption. In panel (a), a stacked bar chart shows forest carbon sources and sinks from 1990 to 2021, with y-axis values ranging from about negative 1250 to about positive 500 million tons carbon dioxide equivalent (abbreviated M t C O 2 - e q). Total sources are about 500 MtCO2-eq through 2005, dip to about 400 in 2007, then rise to about 500 again by 2015, where they remain through 2021. Of that total, about 100 is attributed to land use and land cover loss, with most of the rest from harvested wood products. Fire emissions are negligible, accounting for 20 or less MtCO2-eq in about half of the years, and none in the others. Total sinks are about negative 1350 MtCO2-eq in 1990, then show a declining trend for the next two decades, to about negative 1100 in 2009 and then remaining at about that level through 2021. Land use and land cover gain represent less than negative 100 MtCO2-eq throughout the period of record, and net ecosystem exchange (abbreviated N E E) is consistent at about negative 500. The greatest variation is in harvested wood products, which is generally about negative 500 but dips to about negative 400 for several years around 2010. A black line shows the forest net sink, which starts at about negative 800 in 1990 and reaches about negative 600 by 2021. Panel (b), another stacked bar chart, shows forest carbon sink variability by region from 1990 to 2021, with y-axis values from negative 800 to positive 400 MtCO2-eq. In the positive portion of the chart, all regions accounted for less than 75 MtCO2-eq each year through 2001; after that, much more interannual variability is evident, with several years reaching 200 or more. Alaska, the Northern Great Plains, and the Southwest are the greatest sources, with Alaska showing dramatic spikes of 200 to more than 300 MtCO2-eq for several years. In the negative portion of the chart, the Southeast serves as the largest sink, sequestering about 400 MtCO2-eq each year over the period of record. A black line shows the forest net sink, which starts at about negative 800 in 1990 and trends upward, ending at about negative 600 in 2021. Panel (c), a flow chart, represents the carbon cycle in US forests, showing total forest carbon stocks and average annual transfers between land and atmosphere in 2021. There is a net transfer of 25 MtCO2-eq to the atmosphere from fires and 371 MtCO2-eq from decay, burning, and energy. There is a net uptake from the atmosphere of 593 MtCO2-eq from net ecosystem exchange. Carbon stocks are as follows: Live trees (leaves, branches, stems, roots), 69,188 MtC02-eq; dead trees (standing and down deadwood), 11,442 MtC02-eq; soil and litter (decomposed organic carbon and inorganic carbon stored in soils), 128,776 MtC02-eq; harvested products (logs, building material, furniture, pellets, pulp and paper, wood chips, solid waste disposal sites), 9,979 MtC02-eq. Land-use and land-cover change account for a net sink of negative 98 MtC02-eq. When these sinks and sources are added together, the net ecosystem carbon balance—that is, the long-term carbon storage in forest lands and harvested wood products—shows a balance of negative 637 MtC02-eq in 2021.

The forest carbon sink has declined in recent decades in the United States, with substantial interannual variability.

Figure 7.8. The figure shows (a) the interannual variability in forest carbon sources and sinks during the period 1990–2021; (b) the interannual variability in the forest carbon sink in the US by National Climate Assessment region during 1990–2021; and (c) the total forest carbon stocks by ecosystem pool (boxes) and mean annual transfers among the atmosphere and forest ecosystem pools, harvested wood products, and land conversions (arrows) in 2021. Negative estimates indicate net carbon uptake (i.e., a net removal of carbon from the atmosphere or transfer between ecosystem pools or land categories). Forest ecosystems are the largest carbon sink in the US. There is substantial interannual variability in greenhouse gas emissions and removals from forest land that is driven, in large part, by climate-related disturbances, land use, and land-use change. Figure credit: USDA Forest Service and USGS.

Box 7.3. Climate Change Effects on Forest Water Resources

Forests are a critical source of water in the US (Figure 7.9),¹²³^,¹²⁴ and climate change and climate-related disturbances are directly and indirectly affecting the availability and quality of water from forests. Warming across the West has resulted in reduced snowpack and earlier snowmelt and spring runoff, decreasing downstream water availability (KM 4.1).¹²⁵ In the Southwest, higher temperatures and reduced precipitation have decreased streamflow in recent decades (Figure 7.9).¹²⁶ Shifts in precipitation and declining snowpacks are decreasing the magnitude and/or frequency of flooding in some areas but increasing them in others (Figure A4.8).¹²⁷^,¹²⁸ Models indicate that climate change will affect flood events as some watersheds transition from snow-dominated precipitation, or mixed rain and snow, to rain-dominated precipitation.¹²⁹^,¹³⁰ Wildfires and other disturbances (e.g., bark beetle outbreaks) can also result in changes in the availability and quality of water from forests.¹³¹^,¹³² Following wildfires, tree mortality decreases evapotranspiration, thereby increasing water runoff and supply.¹³³ However, wildfire also increases the runoff of sediments, metals, and other chemicals into water bodies for several years or more after a fire,¹³⁴ with higher rates of drinking-water standard violations occurring in burned versus unburned watersheds.¹³⁵

Climate change impacts on water quantity and quality in turn affect aquatic life. Warming, drought, and declines in snowpack increase stream temperatures,¹³⁶ decreasing coldwater fish habitat (Figure 7.9).¹³⁷^,¹³⁸^,¹³⁹ Shifts in precipitation from snow to rain are projected in much of southern Alaska.¹⁴⁰ Anticipated effects include changes in streamflow timing and magnitude, with negative effects on salmon production and salmon habitat.¹⁴¹

Adaptation measures can reduce some of the effects of climate change on water resources. Management practices, such as reintroducing American beaver, have increased water storage in some landscapes¹⁴² but have had mixed effects on water quality for salmon in the Northwest.¹⁴³ Maximizing riparian forest buffers reduces erosion and sedimentation, provides habitat for wildlife, and is projected to delay or reduce stream warming through enhanced shading.¹⁴⁴ Thinning and surface-fuel reduction can lessen the risk of high-severity wildfires in fire-prone buffers and adjacent forests in the West,¹⁴⁵^,¹⁴⁶ with the potential to reduce fire severity and consequently the effects of wildfire on water resources.¹³²

Map of the contiguous US and two bar charts show the effects of climate on forest water resources, as described in the text and caption. Panel (a), a map of contiguous US with NCA regions and Southwest drainage basins delineated, shows the percentage of surface water originating on forest lands using the following colors: 76 to 100 percent, purple; 51 to 75 percent, blue; 26 to 50 percent, green; 11 to 25 percent, yellow; greater than zero to 10 percent, gray. Among NCA regions, the Northeast and Southeast show the highest percentages, with most of the regions above 26 percent and mountainous areas above 76 percent; the exception is Florida, much of which shows 10 percent or less. Also showing 10 percent or less are central parts of the Midwest, the eastern half of the Northern Great Plains, and the western part of the Southern Great Plains. The Southwest and Northwest are highly variable, with mountainous areas showing 76 percent or more and deserts showing 10 percent or less. Panel (b), a bar chart, shows average annual streamflows for the following Southwest drainage basins: Upper Colorado, Lower Colorado, Rio Grande, and Great Basin. For each basin, separate bars show the streamflow by decade for the 1950s to the 2020s. The y-axis shows average annual streamflow from 0 to 1,600 cubic feet per second. Upper Colorado flows are generally between 800 and 900 with peaks of around 1,100 and 1,000 in the 1980s and 1990s respectively. Lower Colorado flows are around 200 for the earlier and later decades, with values peaking above 300 for the 1970s through 1990s. The Rio Grande values are generally between 300 and 400 but were more than 500 in the 1980s and above 400 in the 1990s. Great Basin flows are around 200 for all decades except for a peak of around 300 in the 1980s. Panel (c), a bar chart, shows suitable habitat for coldwater fish in the southern Appalachians for the contemporary period and under two future warming scenarios: a 3.6°F rise and a 7.2°F rise. The y-axis values range from 0 to 10,000 miles of stream. Currently there are nearly 9,000 miles of suitable stream. That number drops to about 6,500 under the 3.6°F scenario and to about 5,000 at 7.2°F.

Climate change and climate-related disturbances are affecting the availability and quality of water from forests in the United States.

Figure 7.9. Panel (a) shows the percent of surface water originating on forest lands across the contiguous US, illustrating that forests are a critical source of water. Panel (b) shows decadal average variations in average annual streamflow (measured in cubic feet per second [ft³/s]) from Hydrologic Unit Code 8 (HUC8) watersheds with greater than 50% forest cover, no impoundments above the streamflow gauges, at least four gauges per basin, and complete records back to 1950 in the Great Basin (number of HUC8 gauges = 6), Upper Colorado (16), Lower Colorado (5), and Rio Grande (4). Data generally show annual streamflow has been comparatively lower in more recent years compared to earlier decades. Panel (c) shows projected changes in suitable coldwater fish habitat in the Southeast under 3.6°F and 7.2°F warming air temperature over contemporary (2012) air temperature. Projections suggest suitable coldwater fish habitat will decline in the future as air temperatures increase. Figure credits: (a) adapted from Liu et al. 2022;¹²⁴ (b, c) USDA Forest Service.

Adaptation Actions Are Necessary for Maintaining Resilient Forest Ecosystems

Climate change creates challenges for natural resource managers charged with preserving the function, health, and productivity of forest ecosystems . Forest landowners, managers, and policymakers working at local, state, Tribal, and federal levels are preparing for climate change through the development and implementation of vulnerability assessments and adaptation plans . Proactive adaptation of management strategies that create, maintain, and restore resilient forest ecosystems are critical to maintaining equitable provisioning of ecosystem services .

Proactive adaptation of forest management can help maintain the continued provisioning of ecosystem services from forests (e.g., Peterson and Halofsky 2018; Voggesser et al. 2013⁸⁷^,¹⁴⁷). Since 2017, the development of assessments, frameworks, and tools to guide adaptation in forests has accelerated. Climate change vulnerability assessments and adaptation plans for federal (e.g., Halofsky et al. 2016, Timberlake and Schultz 2019¹⁴⁸^,¹⁴⁹), state (Figure 31.1; e.g., Ontl et al. 2018; PADEP 2021¹⁵⁰^,¹⁵¹), private,¹⁵² and Tribal lands⁹¹^,¹⁵³ have proliferated. Similarly, many guides and frameworks for adaptation have been introduced (e.g., Adaptation Partners 2023; Schuurman et al. 2020¹⁵⁴^,¹⁵⁵). Examples of implementation of adaptation practices in forests are now much more widespread (Figure 7.10; Table 7.1).

Map of the contiguous United States shows that adaptation actions occur across many types of forest ownership and management, as explained in the text, caption, and Table 7.1. A legend shows forest ownership or management in the following categories: family (light green), corporate (yellow), timber investment management organization / real estate investment trust (orange), other private (brown), federal (dark green), state (blue), local (purple), tribal (pink), and non-forest (white). Overall, forests in the eastern US are mostly privately owned, and the majority of forests in the western US are federally managed. In addition to large areas of federal forest, the western US also has significant amounts of Tribal- and corporate-owned forest. Texas and West Virginia have significant amounts of corporate-owned forest. Timber investment management organization / real estate investment trust control is prominent in Maine and common throughout the Southeast. Family-owned forest is common throughout the East and Midwest. There are large areas of state-owned forest in New York State and the northern Midwest. Numbers corresponding to the “Map location” column in Table 7.1 are placed on the map as follows: 1, northern California and western Washington; 2, northern Ohio and western Washington; 3, Hudson Valley of New York; 4, northern Wisconsin; 5, eastern Missouri.

Adaptation actions occur across many types of forest ownership and management in the United States.

Figure 7.10. Forests in the eastern US are mostly privately owned, whereas the majority of forests in the western US are federally managed. Climate change adaptation actions have been implemented in diverse forest ownership settings. The numbers on the map correspond to locations of adaptation examples listed in Table 7.1 (example 6 is not depicted as it focuses on Puerto Rico, which is not shown on the map). TIMO = timber investment management organization; REIT = real estate investment trust. These data are not available for Alaska, Hawai‘i, the US Caribbean, or the US-Affiliated Pacific Islands. Adapted from Sass et al. 2020.¹⁵⁶

Table 7.1. Examples of Climate Change Adaptation Actions in Forest Ecosystems

Map Location	Climate Change Effect	Adaptation Response	References and Resources
1	Longer wildfire season and increased area burned.	Thin forests and conduct prescribed and cultural burns to reduce the risk of high-severity wildfire and promote valued plants.	Long et al. 2021¹⁵⁷ Marks-Block et al. 2019¹⁵⁸ WA DNR 2020¹⁵⁹
2	Species and genotypes may be maladapted to future climate.	Plant genotypes and species considered more tolerant of increased temperatures and changing disturbance regimes.	St. Clair et al. 2022¹⁶⁰ NIACS 2022¹⁶¹ NIACS 2022¹⁶²
3	Increasing temperatures and other stressors threaten urban forests.	Develop silvicultural techniques to maintain urban forests.	Piana et al. 2021¹⁶³ Piana et al. 2021¹⁶⁴ Pregitzer et al. 2019¹⁶⁵
4	Climate change adaptation plans are not typically suited to the needs of Indigenous Peoples.	Develop a Tribal climate change adaptation menu to incorporate Tribal values and cultural considerations into climate adaptation planning.	Tribal Adaptation Menu Team 2019¹⁶⁶
5	More frequent and severe flooding.	Relocate and restore recreation-related infrastructure in vulnerable floodplains.	NIACS 2022¹⁶⁷
6	More intense hurricanes increasing downed and damaged trees.	Increase capacity to learn from disaster and manage vegetative debris in order to recover value and sequester carbon.	Álvarez-Berríos et al. 2021¹⁶⁸ Wiener et al. 2020¹⁶⁹

Taking into account all lands and people improves climate change vulnerability assessments, because of the many federal, state, territorial, municipal, private, Tribal, and Indigenous management policies and practices governing forests. Adaptation options differ by region, ownership, and management objectives, reflecting differences in regional climate and ecology, management history, and local values. However, general principles for adaptation hold across geographies and ownerships and are consistent with the principles of sustainable forest management. For example, in drought-prone temperate forests, reducing tree densities increases resistance (the ability to remain largely unaltered by disturbance) and resilience (the ability to recover after disturbance) to bark beetles and drought¹⁷⁰^,¹⁷¹ and, when combined with fuel reduction treatments (e.g., prescribed fire), resilience to wildfire.¹⁴⁵^,¹⁴⁶ In fire-prone forests, reintroducing low- to mixed-severity fire and incorporating Indigenous Knowledge into fire management can reduce the risk of high-severity wildfire and promote valued ecosystem services¹⁷²^,¹⁷³ (Table 7.1; KM 16.3). Promoting biological diversity is also a common adaptation strategy,¹⁵⁰ as forest areas of high diversity are better at maintaining ecosystem functions.¹⁷⁴ Increasing the diversity of functional traits, such as shade tolerance, seed size, specific leaf area, ability to resprout, and bark thickness, may give forests a better chance to adapt to climate-related disturbances.¹⁷⁴^,¹⁷⁵

Opportunities to better integrate social considerations of climate-driven changes in forests and forest management are emerging, as socioecological vulnerability frameworks and assessments expand their treatment of social dimensions.¹⁷⁶^,¹⁷⁷ For example, assessments can consider ecological changes and altered ecosystem services in light of the socioeconomic characteristics (e.g., social vulnerability) and welfare of the beneficiaries of ecosystem services, including social capacity to adapt to novel conditions. Access to forests and associated ecosystem services, including recreation, differs from urban to rural settings and with socioeconomic characteristics such as racial and ethnic identity. For example, Black landowners in the South face a legacy of unequal access to forestry extension and management, as well as insecure property ownership and minimal economic return for land inherited without a will.¹⁷⁸ Environmental justice analyses can be used to consider access to forests, technical assistance for forest management, and ecosystem services, as well as hazardous occurrences such as wildfire smoke.¹⁷⁹^,¹⁸⁰

Forest health and management are tied to socioeconomic well-being among Indigenous and Tribal Peoples, where stewardship of forest ecosystems is interrelated with cultural identity (KM 16.3).¹⁸¹^,¹⁸² Such perspectives lead to different adaptation options with emphasis on active management designed to maintain reciprocal relationships. For example, many Diné (Navajo) depend directly on the land for their livelihoods and cultural traditions, and forests provide social, cultural, spiritual, and economic resources. Under continued warming, substantial forest losses are projected for the Diné. Ambitious tree planting strategies have been proposed to offset these losses and meet future resource needs (e.g., for fuelwood; 50% of Diné households use wood as a primary heating source).¹⁸³

Adapting reforestation practices, including where species are planted and which species and genotypes are planted, will facilitate adaptation to future climatic conditions. Assisted migration can help address the effects of climate change by promoting tree species and genotypes expected to survive future climates and disturbance regimes.¹⁸⁴ Assisted migration encompasses 1) assisted population migration within a species range, 2) assisted range expansion adjacent to a species range, and 3) assisted species migration that moves species far outside their range.¹⁸⁵ Specific guidance on assisted migration is rare, but tools such as hierarchical decision-making,¹⁸⁶ the Seedlot Selection Tool,¹⁶⁰ and the Managed Relocation Ecological Risk Assessment Tool¹⁸⁷ provide guidance.

Assisted migration and reforestation efforts are constrained by unreliable availability of climate-adapted seedling stock or other resources.¹⁸⁸ Adaptation interventions can focus on altering the exposure of forests to climate change or the demand for ecosystem services. Interventions include forest management that alters stand structures or composition, strengthening of disturbance response, and bolstering post-disturbance restoration.¹⁸⁹^,¹⁹⁰ Private forest owners’ actions to adapt to climate change are socially, institutionally, and economically constrained (e.g., Andersson and Keskitalo 2018¹⁹¹); therefore, policy and market-based incentives have the potential to increase adaptation on private lands (e.g., Anderson et al. 2019¹⁹²). Potential policies include regulations that require adaptation actions; subsidies (direct payments and tax reductions) that reduce private costs of actions or account for public benefits of private actions;¹⁹³ and taxes that increase the private costs of inaction or of actions that make forests less resilient to climate change (e.g., Hashida et al. 2020¹⁹⁴). Given that future benefits from a private intervention are uncertain, subsidies (e.g., for hazardous fuels management) also reduce financial risks (e.g., Amacher et al. 2006¹⁹⁵).

Effective implementation of climate adaptation requires working across landscapes with complex governances.¹⁹⁶ Equitable outcomes are enhanced by coproduction of knowledge (i.e., involving multiple knowledge sources and capacities from different groups of people) that determines expected risks, desired future benefits, and the capacity for implementing adaptation actions.¹⁹⁷

Process Description

Author selection centered on scientific expertise and ensuring that, to the extent possible, the author team represented a broad array of experiences. First, an outline of broad themes was developed by the chapter lead (CL) and federal coordinating lead author (CLA) based on review of previous assessments, a US Global Change Research Program (USGCRP) gap analysis, and new findings since the last National Climate Assessment.¹¹ The outline served as the basis for identifying the expertise necessary to complete the chapter. Next, the CL and CLA independently developed initial author lists following diversity criteria and guidance provided by USGCRP. It is important to note that prior to author selection, including the CL and CLA, the chapter was designated an all-federal-author chapter by the Federal Steering Committee, with the option to include non-federal authors as technical contributors. The CL and CLA relied heavily on the prepopulated list of individuals nominated through the USGCRP public call for authors to compile the initial author lists. The CL and CLA then worked through their initial lists of authors by chapter theme using the diversity criteria to arrive at the final list of chapter authors.

The author team met weekly to discuss chapter developments, comments, and timelines. Additional chapter authors and technical contributors were identified and added to increase depth and diversify perspectives. These decisions were informed by author team meetings, reviews and revisions, and comments received from US government agencies and the public. Consensus was built leveraging the specific expertise of chapter authors and by referring to the peer-reviewed literature, which was heavily weighted to articles published in the last five years. Engagement with the public occurred through a workshop held in January 2022 and opportunities for public review. Engagement with other chapters occurred through meetings among chapter leadership.

KEY MESSAGES

KEY MESSAGE 7.1

Forests Are Increasingly Affected by Climate Change and Disturbances

Read about Confidence and Likelihood

Description of Evidence Base

Abundant peer-reviewed literature indicates that climate change has increased the frequency, spatial scale, and severity of some disturbances that drive forest change.⁵⁶^,¹⁹⁸^,¹⁹⁹ Notable examples include area burned by wildfires in the West,²⁰⁰ area burned by large wildfires in the West,¹³ and area burned at high severity in the West.¹⁸ Over half (55%) of the changes in fuel aridity in western US forests are attributable directly to climate change,¹² and relationships between wildfire and climate (low precipitation/drought or interactions between temperature and precipitation) explain the trends and most of the variation in area burned.¹⁴^,¹⁵^,²⁰⁰^,²⁰¹ Projections of future wildfire¹⁴^,²⁰² indicate climatic drivers, and forest responses are expected to differ with forest type and fuels, with the potential for fuel feedbacks to eventually limit increases in area burned (Kitzberger et al. 2017,²⁰³ but see Abatzoglou et al. 2021¹⁶). There is also strong and increasing evidence that warming is reducing overwintering mortality and increasing voltinism (number of generations) of some bark beetles,²⁶^,²⁹^,⁶³ resulting in large impacts, especially in the West, and expansions of geographic and host ranges in both the Northeast and West.²⁶^,⁶³ In the Southwest, exceptional drought has compromised host tree defenses, resulting in increased bark beetle impacts.⁵⁸^,⁶⁰^,⁶¹^,²⁰⁴ Despite well-described physical changes to forest fuels following bark beetle outbreaks, effects on wildfires are mixed. These contradictions are largely explained by the different metrics used to assess wildfires, time since the outbreak, the spatial scale of studies, and the confounding effects of fire weather and beetle impacts.⁶⁶ Pathogens, extreme weather events (hurricanes, wind, flooding), and sea level rise have less evidence supporting widespread connections between disturbance and climate, in part because attribution is difficult for phenomena that occur rarely. Continued warming and regional changes in precipitation are expected to amplify interactions among disturbance agents and further alter forest ecosystem structure and function. Evidence for shifts in tree species ranges as affected by climate change is available for some areas;²⁰⁵^,²⁰⁶^,²⁰⁷ however, understory species range shifts will depend on whether the canopy is affected.⁴⁹^,²⁰⁸

Major Uncertainties and Research Gaps

A major uncertainty is the role of climate-related disturbance in landscape transformation, or the permanent transition to a different vegetation type, perhaps non-forested. Projecting future forest changes and resulting effects on ecosystem services should be predicated on an ability to simulate and anticipate the (sometimes rapid) emergence of novel vegetation types. A key research gap is whether and how much management actions may alter climate-driven disturbance effects and resulting forest ecosystem trajectories. For example, differences in forest type and management history affect fuels available to fire and how climate changes alter their flammability or susceptibility to insects or other disturbances. As a result, forest-specific management may be capable of altering some or most of the projected climate effects, all other things being equal, primarily for wildfire. Another research gap is the ability to model the hydrological responses in forested watersheds under novel combinations of climate and disturbance.

Description of Confidence and Likelihood

The vast majority of the scientific literature on forest disturbance supports a high confidence statement for the role of climate-related disturbance in structuring forest ecosystems, although estimates of the strength of relationships between climate and disturbance vary with mechanisms and methods. However, the mechanisms by which disturbances interact and the degrees to which they will chronically or acutely affect forests differ greatly with the type and nature of the disturbance. For example, interactions between wildfire and bark beetles are well documented and may be amplified over time, whereas interactions between drought and pathogens are poorly understood. Disturbances can rapidly alter forest structure and dynamics, as well as other resource values (e.g., recreation) and socioeconomic conditions. It is likely that such dynamics will continue, but it is difficult to project how much they will resemble the dynamics with which we have experience. Therefore, high confidence exists that future disturbances will further alter forest structure and function.

KEY MESSAGE 7.2

Climate Change Affects Ecosystem Services Provided by Forests

Read about Confidence and Likelihood

Description of Evidence Base

Abundant peer-reviewed literature underpins how climate change is affecting ecosystem services. Most research supports that increases in air temperature and changing precipitation patterns are reducing the capacity of US forests to sequester and store carbon, especially in the West.³ There is also strong evidence that climate change is reducing snowpacks and decreasing water supplies in the West.¹²⁵ Stream temperatures are increasing in multiple regions, reducing coldwater fish habitat.¹³⁷^,¹³⁸ Increases in the frequency of large wildfires in the West reduce air and water quality¹⁰³^,¹³²^,¹³⁴^,²⁰⁹^,²¹⁰ and, when combined with an expanding wildland–urban interface (WUI),⁵ likely increase structure losses.

Climate change affects timber by increasing the area burned by wildfires²¹¹ and by increasing the area affected by, and the severity of, bark beetle outbreaks,²⁶^,⁵⁸ leading to increased timber salvage and lower timber values. Beetle outbreaks have also lowered scenic beauty, property values, and property tax revenues for local governments in some areas.¹⁰⁰ Non-timber products in some parts of the US are increasingly subjected to variable output, due to climate-related increases in disturbances and variability in temperatures and seasonality,²¹² affecting benefits of cultural ecosystem services, particularly for Indigenous and Tribal Peoples.⁹⁰^,²¹³ For example, in the Midwest and Northeast, rising average winter temperatures and reduced snow depth have increased the severity of winter tick (Dermacentor albipictus) and brainworm (Parelaphostrongylus tenuis) infestations²¹⁴^,²¹⁵ in moose, increasing adult and calf mortality and reducing hunting opportunities.²¹⁶^,²¹⁷^,²¹⁸ Rising sea levels are leading to ghost forests, thereby changing recreation and affecting storm surge buffering capacity.²¹⁹^,²²⁰ Sea level rise is also projected to exceed mangrove accretion rates in future decades, leading to loss of mangrove forests in many places.³⁸^,³⁹^,⁴⁰ In response to warmer winters, however, mangrove expansion is currently occurring along the US Gulf and Atlantic coasts, enhancing coastal protection from storms and rising seas, adding biomass carbon, improving pelican habitat, creating loss of coastal views, increasing insects, reducing fishing access, and reducing habitat for whooping cranes, an endangered species.⁴¹ Skiing in undeveloped areas and motorized snow-based activities in the continental US are projected to be affected by climate change by midcentury, with effects varying by region, model scenario, and participation measure.⁹³^,²²¹

Major Uncertainties and Research Gaps

Climate change effects on recreation values are uncertain because human values change over time and because overall effects will depend on how temperature and precipitation patterns change across forested landscapes. Furthermore, imprecise information on historical recreation activities has led to a lack of statistical significances for quantitative estimates of how climate may be affecting particular activities. Research gaps exist regarding the effects of climate change on cultural goods and services important to Indigenous and Tribal Peoples in the US.

Description of Confidence and Likelihood

Recent research provides ample evidence of the direct and indirect effects of climate change on forest ecosystem services. There is high confidence that multiple ecosystem services are being impacted by climate change, including coldwater fishing, multiple recreation activities, amenities from coastal forests lost to rising seas, Indigenous forest values, consumptive and nonconsumptive wildlife provisioning, and many nontimber forest products. There is high confidence that climate change is affecting forest carbon sequestration; the provisioning of clean water from forests; the occurrence of wildfires that destroy structures, alter habitats, and increase PM_2.5 concentrations in the atmosphere; and snowpacks in the West and Northeast. There is medium confidence that climate change is changing the availability of water-based and snow-based recreation. The capacity of forests to continue providing ecosystem services will be determined, in part, by changes in society and how those changes interact with future climate effects (likely, high confidence).

KEY MESSAGE 7.3

Adaptation Actions Are Necessary for Maintaining Resilient Forest Ecosystems

Read about Confidence and Likelihood

Description of Evidence Base

Climate change vulnerability assessments provide the basis for adaptation, and there are many examples of vulnerability assessments that have been conducted for a variety of forest landowners and managers in recent years. Although frameworks and examples of adaptation planning are still more numerous than adaptation actions, the recent literature contains an increasing number of adaptation actions implemented to increase forest resistance and resilience to climate change (Table 7.1). Research on climate change effects and adaptation efforts increasingly draws on coproduction, iterative, and collaborative processes that combine different types of knowledge and participants to produce effective climate adaptation science.²²²

Although considering local context and management objectives is critical in identifying climate change adaptation options, there are general principles applicable across forest types. Adaptation principles that receive strong support in the scientific literature include promoting diversity, modifying planting practices, implementing assisted migration, and increasing resilience to disturbance. A mixture of tree species and functional traits²²³ in a forest stand increases the likelihood that disturbances, such as insect and disease outbreaks, will not result in complete stand mortality; that forests will be better able to withstand changing environmental conditions; and that multiple ecosystem services can be provided.¹⁷⁴^,²²⁴ Strong and increasing evidence exists for lowering stand density in many fire-prone forest types to 1) increase resistance and resilience to disturbances, including droughts, bark beetles, and wildfires;¹⁴⁵^,¹⁴⁶^,¹⁵⁷^,¹⁷⁰^,²²⁵ and 2) improve residual tree growth by decreasing fuel loads, decreasing tree competition, and increasing water availability.²²⁵^,²²⁶^,²²⁷

Major Uncertainties and Research Gaps

Identification of climate change adaptation actions is based on our current understanding of ecosystem function and how management actions affect ecosystem function. However, understanding of the effectiveness of climate change adaptation actions is limited by a lack of long-term monitoring over several decades. Future monitoring will be critical for evaluating the effectiveness of adaptation actions in different contexts, especially because interactions among multiple disturbances could result in unexpected effects on ecosystems and their response to adaptation actions. Implementation of adaptation actions is still in the relatively early stages, and barriers to adaptation implementation are expected to persist, limiting the future pace, scale, and effectiveness of adaptation.

Description of Confidence and Likelihood

There is high confidence that changes in climate over the last several decades are already affecting the ability of forest owners and managers to meet management objectives. This is primarily because of the increasing extent of severe disturbances, primarily wildfires, bark beetle outbreaks in the West, and storms in coastal locations in the East. This is based on the proliferation of peer-reviewed literature to support climate-informed management and planning, as well as various guidelines and sources of adaptation options developed by agencies and non-governmental organizations. Based on understanding of forest ecosystems and effects of management actions, there is medium confidence that adaptation actions will be effective in helping maintain the provisioning of ecosystem services. Continued monitoring is needed to assess the effectiveness of adaptation actions.

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Likelihood

Virtually Certain	Very Likely	Likely	As Likely as Not	Unlikely	Very Unikely	Exceptionally Unlikely
99%–100%	90%–100%	66%–100%	33%–66%	0%–33%	0%–10%	0%–1%

Confidence Level

Very High	High	Medium	Low
Strong evidence (established theory, multiple sources, well-documented and accepted methods, etc.) High consensus	Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.) Medium consensus	Suggestive evidence (a few sources, limited consistency, methods emerging, etc.) Competing schools of thought	Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.) Disagreement or lack of opinions among experts

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INTRODUCTION

Forests Are Increasingly Affected by Climate Change and Disturbances

Climate Change Affects Ecosystem Services Provided by Forests

Adaptation Actions Are Necessary for Maintaining Resilient Forest Ecosystems

TRACEABLE ACCOUNTS

Process Description

KEY MESSAGES

KM 7.1

Forests Are Increasingly Affected by Climate Change and Disturbances

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 7.2

Climate Change Affects Ecosystem Services Provided by Forests

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 7.3

Adaptation Actions Are Necessary for Maintaining Resilient Forest Ecosystems

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

REFERENCES

Likelihood

Confidence Level