It is very likely that more frequent extreme weather events will increase the frequency and magnitude of severe ecological disturbances, driving rapid (months to years) and often persistent changes in forest structure and function across large landscapes. It is also likely that other changes, resulting from gradual climate change and less severe disturbances, will alter forest productivity and health and the distribution and abundance of species at longer timescales (decades to centuries).
It is very likely that climate change will decrease the ability of many forest ecosystems to provide important ecosystem services to society. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperatures, more frequent drought, and increased disturbances. The onset and magnitude of climate change effects on water resources in forest ecosystems will vary but are already occurring in some regions.
Forest management activities that increase the resilience of U.S. forests to climate change are being implemented, with a broad range of adaptation options for different resources, including applications in planning. The future pace of adaptation will depend on how effectively social, organizational, and economic conditions support implementation.
It is very likely that more frequent extreme weather events will increase the frequency and magnitude of severe ecological disturbances, driving rapid (months to years) and often persistent changes in forest structure and function across large landscapes. It is also likely that other changes, resulting from gradual climate change and less severe disturbances, will alter forest productivity and health and the distribution and abundance of species at longer timescales (decades to centuries).
It is very likely that climate change will decrease the ability of many forest ecosystems to provide important ecosystem services to society. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperatures, more frequent drought, and increased disturbances. The onset and magnitude of climate change effects on water resources in forest ecosystems will vary but are already occurring in some regions.
Forest management activities that increase the resilience of U.S. forests to climate change are being implemented, with a broad range of adaptation options for different resources, including applications in planning. The future pace of adaptation will depend on how effectively social, organizational, and economic conditions support implementation.
Virtually Certain | Extremely Likely | Very Likely | Likely | About as Likely as Not | Unlikely | Very Unikely | Extremely Unlikely | Exceptionally Unlikely |
---|---|---|---|---|---|---|---|---|
99%–100% | 95%–100% | 90%–100% | 66%-100% | 33%-66% | 0%-33% | 0%-10% | 0%-5% | 0%-1% |
Very High | Hoch | Medium | Niedrig |
---|---|---|---|
Strong evidence (established theory, multiple sources, consistent results, 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, models incomplete, 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 |
Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Guide to this Report for more on assessments of likelihood and confidence.
Forests on public and private lands provide benefits to the natural environment, as well as economic benefits and ecosystem services to people in the United States and globally. The ability of U.S. forests to continue to provide goods and services is threatened by climate change and associated increases in extreme events and disturbances.1 For example, severe drought and insect outbreaks have killed hundreds of millions of trees across the United States over the past 20 years,2 and wildfires have burned at least 3.7 million acres annually in all but 3 years from 2000 to 2016. Recent insect-caused mortality appears to be outside the historical context3,4 and is likely related to climate change; however, it is unclear if the apparent climate-related increase in fire-caused tree mortality is outside the range of what has been observed over centuries of wildfire occurrence.5
A warmer climate will decrease tree growth in most forests that are water limited (for example, low-elevation ponderosa pine forests) but will likely increase growth in forests that are energy limited (for example, subalpine forests, where long-lasting snowpack and cold temperatures limit the growing season).6 Drought and extreme high temperatures can cause heat-related stress in vegetation and, in turn, reduce forest productivity and increase mortality.7,8 The rate of climate warming is likely to influence forest health (that is, the extent to which ecosystem processes are functioning within their range of historic variation)9 and competition between trees, which will affect the distributions of some species.10,11
Large-scale disturbances (over thousands to hundreds of thousands of acres) that cause rapid change (over days to years) and more gradual climate change effects (over decades) will alter the ability of forests to provide ecosystem services, although alterations will vary greatly depending on the tree species and local biophysical conditions. For example, whereas crown fires (forest fires that spread from treetop to treetop) will cause extensive areas of tree mortality in dense, dry forests in the western United States that have not experienced wildfire for several decades, increased fire frequency is expected to facilitate the persistence of sprouting hardwood species such as quaking aspen in western mountains and fire tolerant pine and hardwood species in the eastern United States (see regional chapters for more detail on variation across the United States). Drought, heavy rainfall, altered snowpack, and changing forest conditions are increasing the frequency of low summer streamflow, winter and spring flooding, and low water quality in some locations, with potential negative impacts on aquatic resources and on water supplies for human communities.12,13
From 1990 to 2015, U.S. forests sequestered 742 teragrams (Tg) of carbon dioxide (CO2) per year, offsetting approximately 11% of the Nation’s CO2 emissions.14 U.S. forests are projected to continue to store carbon but at declining rates, as affected by both land use and lower CO2 uptake as forests get older.15,16,17,18 However, carbon accumulation in surface soils (at depths of 0–4 inches) can mitigate the declining carbon sink of U.S. forests if reforestation is routinely implemented at large spatial scales.
Implementation of climate-informed resource planning and management on forestlands has progressed significantly over the past decade. The ability of society and resource management to continue to adapt to climate change will be determined primarily by socioeconomic factors and organizational capacity. A viable forest-based workforce can facilitate timely actions that minimize negative effects of climate change. Ensuring the continuing health of forest ecosystems and, where desired and feasible, keeping forestland in forest cover are key challenges for society.
<b>Vose</b>, J.M., D.L. Peterson, G.M. Domke, C.J. Fettig, L.A. Joyce, R.E. Keane, C.H. Luce, J.P. Prestemon, L.E. Band, J.S. Clark, N.E. Cooley, A. D’Amato, and J.E. Halofsky, 2018: Forests. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 232–267. doi: 10.7930/NCA4.2018.CH6
Forests are distributed across the spectrum of rural to urban environments, covering 896 million acres (including approximately 130 million acres in urban, suburban, and developed areas), or 33% of land in the contiguous United States, Alaska, and Hawai‘i. The structure and function of these forests vary considerably across the Nation due to differences in environmental conditions (for example, soil fertility; temperature; and precipitation amount, type, and distribution), historical and contemporary disturbances, and forest management and land-use activities.
Forests on public and private lands provide benefits to the natural environment, as well as economic benefits and ecosystem services (for example, water, fiber and wood products, fish and wildlife habitat, biodiversity, recreational opportunities, spiritual renewal, and carbon storage) to people in the United States and globally. Public forests are mostly managed for non-timber resources or for multiple uses; private lands owned by corporations are mostly managed for timber production, whereas private lands owned by individuals are typically managed for multiple uses. To date, assessments of climate change vulnerability and development of adaptation options in the western United States have occurred mostly on public lands, whereas assessment and adaptation planning and implementation in the eastern United States span public and private lands, with documented examples of adaptation on most ownership types.19,20 The ability of U.S. forests to continue to provide goods and services is threatened by climate and environmental change and associated increases in extreme weather events and disturbances (for example, drought, wildfire, and insect outbreaks; Figure 6.1), which can pose risks to forest health (that is, the extent to which ecosystem processes are functioning within their natural range of historic variation)9 and conditions across large landscapes for years to centuries.1
The effects of climate change on forests in specific regions are discussed in many of the regional chapters (for example, Ch. 18: Northeast, KM 1 and 2; Ch. 19: Southeast, KM 3 and 4; Ch. 21: Midwest, KM 2; Ch. 24: Northwest, KM 1; Ch. 25: Southwest, KM 2; Ch. 27: Hawai‘i & Pacific Islands, KM 2 and 5). Rapid changes have been driven by severe drought in combination with insect outbreaks, which have killed more than 300 million trees in Texas in 201121 and more than 129 million trees in California from 2010 to 2017.22 Also, mountain pine beetles have caused tree mortality across more than 25 million acres in the western United States since 2010, representing almost half of the total area impacted by all bark beetles combined in that region. Recent warming has allowed mountain pine beetles to erupt at elevations and latitudes where winters historically were cold enough to keep them in check.4,23,24 Wildfire burned at least 3.7 million acres nationwide in 14 of the 17 years from 2000 to 2016—an area larger than the entire state of Connecticut—including a record 10.2 million acres in 2015 (an area greater than Maryland and Delaware combined). Over this same time span, annual federal wildfire suppression expenditures ranged from $809 million to $2.1 billion (Figure 6.4).
Recent insect-caused mortality appears to be far outside what has been documented since Euro-American settlement3 and is likely related to climate change. It is unclear if the apparent climate-related increase in area burned by wildfire is outside the range of what has been observed over centuries of fire occurrence.5 Drought, heavy rainfall, altered snowpack, and changing forest conditions are increasing the risk of low summer streamflow, winter flooding, and reduced water quality, with potential negative impacts on aquatic resources and human communities.12,13 A changing climate and forest disturbances also interact with chronic stressors (such as fungal pathogens and nonnative species) to affect the scale and magnitude of forest responses to climate change.25,26
The ability of society in general and resource managers in particular to adapt to climate change will be determined primarily by socioeconomic factors, technological developments, and organizational capacity (Ch. 28: Adaptation). Although some general principles apply to adaptation (defined here as adjustments in natural systems in response to actual or expected climatic effects that moderate harm or exploit benefits) across all forests, it is biophysical variability, socioeconomic conditions, and organizational objectives that dictate local management approaches. A viable forest-based workforce in local communities can facilitate timely actions that minimize the negative effects of climate change, as long as this workforce can support the objectives of treatments aimed at building forest resilience and provide a justification for treatments (for example, prescribed fire—the purposeful ignition of low-intensity fires in a controlled setting) that help minimize potential economic loss. Reduction in forestland associated with human land-use decisions, especially conversion of forests to nonforests on private lands, is a significant impediment to providing desired ecosystem services from forests. Hence, ensuring the continuing health of forest ecosystems and, where desired and feasible, keeping forestland in forest cover are key challenges for society.
Forests in the United States vary in their susceptibility to climate change due to differences in biophysical conditions and anticipated changes in future climate (see regional chapters for specific discussions). For example, eastern forests are largely expected to undergo gradual change, punctuated by rapid changes from small-scale disturbances.26 Across most U.S. forests, an increased frequency of large-scale disturbances is expected to be the primary challenge to maintaining healthy, functional forest ecosystems in a warmer climate; however, forest disturbances resulting from human activity can add to the effects of climate in some parts of the United States.27 Over the past decade, several large-scale disturbances have killed hundreds of millions of trees at different locations in the United States. The two Case Studies in this chapter illustrate how disturbances can cause rapid changes in the ecology and structure of forests that can result in significant social and economic effects.
Southeastern landscapes are dominated by private lands and relatively high human populations, so changes in social behavior (for example, human-caused fire ignitions), policy (for example, fire suppression), and climate can affect wildfire activity.27 Modeling studies suggest that the southeastern United States will experience increased fire risk and a longer fire season.34,35 Although projections vary by state and ecoregion,36 on average, the annual area burned by lightning-ignited wildfire is expected to increase by at least 30% by 2060, whereas human-ignited wildfire is expected to decrease slightly due to changes in factors driving human-ignited wildfire, including projected losses of forestland and increased efforts to suppress and prevent wildfires. Although native vegetation is well-adapted to periodic wildfire, most people living near wildlands are not. More frequent and larger wildfires, combined with increasing development at the wildland–urban interface (where people live in and near forested areas), portend increasing risks to property and human life. For example, a prolonged dry period in the southern Appalachian region in 2016 resulted in widespread wildfires that caused 15 deaths and damaged or destroyed nearly 2,500 structures in Gatlinburg, Tennessee (Figure 6.3). In a warmer climate, increased fire frequency will damage local economies and degrade air quality in the Southeast.
Most fire-prone forests (forests that are likely to burn at least once every few decades) have the ability to persist as more fires occur, but the resilience of these ecosystems depends on three factors: 1) continued presence of fire-adapted species, 2) fire intensity (the amount of heat energy released) and frequency of future fires, and 3) societal responses to increased fires. A century of fire exclusion in fire-prone forest ecosystems in the United States (especially lower-elevation ponderosa pine forests and mixed conifer forests in dry locations in the West) has created landscapes of dense forests with high flammability and heavy surface and canopy fuel loads (combustible dead and live vegetation).37 Over the past 20 years, a warm, dry climate has increased the area burned across the Nation.38 Large, intense wildfires in some locations39 (Figure 6.4) have been difficult to suppress, increasing risk to property and lives, including those of firefighters.40,41 The cost of fire suppression has also increased over time, partially driven by the high cost of protecting property in the wildland–urban interface.42,43
The duration of the season during which wildfires occur has increased throughout the western United States as a result of increased temperatures44,45 and earlier snowmelt.46,47 Increased vapor pressure deficit (Ch. 21: Midwest, Figure 21.3)48 and reduced summer precipitation49 have deepened summer droughts in the West and thus increased wildfire risk.50 By the middle of this century, the annual area burned in the western United States could increase 2–6 times from the present, depending on the geographic area, ecosystem, and local climate.51,52 An increase in the area burned, however, does not necessarily translate to negative impacts to ecosystems (Figure 6.5). As the spatial extent of wildfires increases, previously burned areas will in some cases provide fuel breaks that influence the pattern, extent, and severity (the degree to which fire causes vegetation damage and mortality) of future fires.53 Future wildfire regimes will be determined not only by climate but also by topography, fuel accumulation (as affected by plant growth and frequency of disturbances), and efforts to suppress and prevent fires.54,55
Wildfire risk can be reduced in low-elevation, dry conifer forests in the West and conifer forests in the South by reducing stand density (thinning), using prescribed burning, and letting some fires burn if they will not affect people. Frequent prescribed burning in fire-prone and fire-dependent (forests that require fire to maintain structure and function) southern forests has been a socially accepted practice for decades, illustrating how wildfire risk can be reduced. However, health risks from smoke produced by prescribed burning are a growing concern in the wildland–urban interface (see Ch. 19: Southeast for additional discussion about fire in the southeastern United States and Ch. 13: Air Quality, KM 2 on the effects of wildfires on health).56
Climate change is expected to increase the effects of some insect species in U.S. forests23,61,62 but reduce the effects of others.63 For example, drought increases populations of some defoliating insect species64 but decreases populations of other defoliators.65 In some cases, fire exclusion in fire-prone forests has exacerbated the effects of insects by increasing forest density, thus reducing tree vigor (the capacity of a tree to resist stress) and resistance to insect attack.3 Higher damage from native insects on trees with reduced vigor is expected to be one of the biggest effects of a warmer climate. Altered thermal conditions, including varying temporal patterns, will disrupt some insect life cycles, causing seasonal mismatches between insect species and tree hosts in some systems.66
Over the past 30 years, tree mortality caused by bark beetles in the western United States has exceeded tree mortality caused by wildfire,2 raising concerns about the sustainability of some western forests to provide ecological goods and services over time.67,68 Bark beetle epidemics in forests with commercially valuable tree species can negatively affect timber prices and the economic well-being of forest landowners and wood processors.69 Many bark beetle outbreaks have been associated with drought and elevated temperature.23,63 Recently, western pine beetles contributed to the mortality of 129 million trees weakened by a period of severe drought in California (see Case Study “Large-Scale Tree Mortality”). The southern pine beetle is the only bark beetle species in the eastern United States that causes extensive tree mortality. Although little evidence exists for drought-caused outbreaks of this beetle,63 a recent increase in its range into the northeastern United States, facilitated by increasing winter temperatures, now threatens pine barrens in New York and Massachusetts.70
The northward expansion of the hemlock woolly adelgid, a nonnative species that attacks eastern hemlock, has been facilitated by higher minimum winter temperatures.71 Similarly, the range of mountain pine beetles is expanding with warming; new breeding populations are now found in parts of the western plains and in jack pine in boreal forests in Alberta, Canada.24,72,73 Mountain pine beetle populations are also expanding in high-elevation forests of the western United States, affecting whitebark pine and other high-elevation pine species.4,23 Whitebark pine serves as a keystone species that quickly establishes after a disturbance and provides critical food sources for birds and mammals. Whitebark pine is expected to suffer significant mortality in the future due to the combined effects of white pine blister rust, mountain pine beetles, and a warmer climate.74
Fungal pathogens, especially those that depend on stressed plant hosts for colonization, are expected to perform better and have greater effects on forests as a result of climate change.63,75,76 For example, increasing annual temperatures and precipitation in portions of New England have provided ideal conditions for outbreaks of leaf diseases in eastern white pine,77 whereas the effects of some pathogens directly affected by climate (such as needle blights) are typically reduced in areas with decreased precipitation.75 Timing of pathogen life cycles relative to seasonal changes in temperature and precipitation will be critical in determining where and how damage might change.
Insect and disease outbreaks often interact with other disturbances, compounding their potential effects on ecosystem services. For example, in lodgepole pine forests attacked by mountain pine beetles, the intensity of surface and crown fires increases in stands impacted by outbreaks, but typically for less than 10 years (e.g., Page and Jenkins 2007, Hicke et al. 2012, Jenkins et al. 201478,79,80). Beetles have minimal effects on fire severity in some locations due to variability in topography, fuels, and fire weather.81 A recent study in California in areas heavily affected by drought and western pine beetles (see Case Study “Large-Scale Tree Mortality”) reported a greater potential for large-scale wildfires driven by the amount and continuity of combustible woody material from dying trees.31
Forests that frequently run out of water stored in the soil during the growing season are considered water limited, whereas forests where the growing season length or productivity rate is limited by snowpack and cool temperatures are considered energy limited. A warmer climate will generally decrease tree growth in water-limited forests (many semiarid and low-elevation forests in the western United States) but may increase growth in some energy-limited forests (the majority of forests in the eastern United States and coastal Alaska and high-mountain forests with short growing seasons).6,82 Experimental evidence shows that elevated atmospheric carbon dioxide (CO2) can increase tree growth (especially where soil nutrients are adequate), but it is uncertain whether this increase will occur in mature forests or will continue as younger forests age.83 Positive effects of CO2 on growth will be negated in some species and locations (such as near urban areas) by air pollutants such as ground-level ozone (not the protective layer of ozone high in the atmosphere), where concentrations of those pollutants are high enough to cause toxic effects in plants.84 Drought and extreme temperatures can cause heat-related stress in vegetation, in turn reducing forest productivity and reducing tree vigor.7,8 Although the effects are complex and variable among forests, warming and elevated CO2 can also impact below-ground processes, such as nitrogen and carbon cycling,85 with feedbacks that may impact forest productivity.86
The direct effects of climate change on tree mortality and forest health will likely be obscured by the slow response times of long-lived tree species.87 In some cases, climate-related stresses weaken trees, predisposing them to additional stresses.88 Variability in the drought response of tree species (for example, due to differences in hydraulic characteristics) is expected to influence how some forests deal with water stress.89 A lagged response and variability among species can make it difficult to attribute growth reductions to episodic drought, and growth reductions can persist for years.7,90,91 For species in which seed crops depend on resources stored over several growing seasons, reproductive responses are likely to lag behind climatic variation.92
The rate of climate warming will influence the rate and magnitude of potential changes in forest health, competition for resources among tree species, structure, and function, affecting the growth and distribution of some tree species.10,11 Negative effects on some species can benefit other species, and reorganization and changes in the structure of forest communities depend on the capacity of locally adapted populations to occupy new areas that become suitable as a result of climate change. For example, warming in the coastal region of the southern United States may result in the replacement of salt grass with mangrove forests (see Ch. 19: Southeast for additional information on mangrove forests).93
Canopy phenology (seasonal patterns of leaf emergence and flowering) responds to annual-to-decadal variation in climate,94,95 and evidence exists that changes in canopy phenology are contributing to altered species ranges and potential increases in water and nutrient limitations.96 Some studies report shifts in elevation ranges of terrestrial plant species in general,97,98,99 whereas many of the studies that focus on tree species do not.100,101,102,103 If large-scale latitudinal shifts in tree distributions are occurring, they are ambiguous at present;10,104 however, some evidence suggests that some boreal species are shifting poleward as reproduction fails on the southern edge of their range.105
The Millennium Ecosystem Assessment106 defines four categories of ecosystem services: supporting, provisioning, regulating, and cultural. Recent studies have focused on defining and quantifying the full range of services provided by forests including recreation, wildlife habitat, biodiversity, cultural values, and non-timber forest products.107,108 Here, we focus on climate change effects on two of the most important forest-based services: forest carbon dynamics (regulating and provisioning) and forest water resources (regulating and provisioning). (For additional discussion on the effects of climate on ecosystem services, see Ch. 7: Ecosystems and the regional chapters.)
Forest productivity (Key Message 1) is one of many factors that determine carbon storage potential.109 Typically, soil carbon is the largest and most stable carbon pool in forest ecosystems,14,110,111,112 but increased above-ground biomass production in forests is not necessarily accompanied by higher soil carbon content. In some locations, heavy rainfall events will result in flood-related tree mortality, leading to soil erosion and losses of particulate and dissolved organic carbon from forests.113 Increased disturbances such as harvesting, wildfire, and insect and disease damage can also release carbon stored in soils, especially where multiple disturbances occur over a short time span (Figure 6.6).114
The fate of carbon in forests depends, in large part, on the type, extent, frequency, and severity of the disturbance.114,115 Severe disturbances, such as stand-replacing wildfire, typically result in the immediate release of carbon to the atmosphere,32 a reduction in stand productivity, the transfer of carbon from live to dead pools, and an increase in decomposition.114,115 Productivity will gradually increase following a disturbance, and decomposition will decrease as the forest recovers. The abrupt release of carbon after a disturbance transitions to net carbon uptake through forest regrowth. However, the full effect of the disturbance on atmospheric CO2 depends on the timing of disturbance-induced CO2 releases. Although carbon storage in biomass will increase in areas where tree growth rates rise, those increases will be small compared to the reduced storage that occurs in response to more disturbances.18
Economic and population growth will affect land-use decisions that influence forest-based carbon storage. Over the last several decades, conversion of forestland to other land uses has contributed to CO2 emissions,14,116 and this trend is likely to continue, although this is among the most significant sources of uncertainty in the forest carbon sink in the United States.18,117,118 The current (2017) U.S. deforestation rate (the conversion from forest to nonforest land use) of 0.12% per year is more than offset by forest gain from afforestation (the establishment of a forest where there was no previous tree cover) and reforestation, for a net gain of forest area of 0.09% per year (679,000 acres).14 Gains occur mostly through a transition from grasslands and croplands to shrublands, woodlands, and forests, and losses occur mostly in urban areas (see Ch. 5: Land Changes for details on forest land-use trends).14 While some individual states have lost forestland, overall, each region of the United States (for example, northern, southern, Rocky Mountain, and Pacific coast) has gained forestland area over the past 20 years.14,16
Net storage of atmospheric carbon by forests (742 teragrams, or Tg, of CO2 per year from 1990 to 2015) has offset approximately 11% of U.S. CO2 emissions.14 Assuming no policy intervention—and accounting for land-use change, management, disturbance, and forest aging—U.S. forests are projected to continue to store carbon but at declining rates (35% less than 2013 levels by 2037) as a result of both land use and lower CO2 uptake as forests grow older.15,16,17,18,42
Although forest area has increased over the last few decades (Ch. 5: Land Changes, Figure 5.1), this trend is projected to level off by 2030, then decline gradually as human population expands and afforestation on agricultural lands slows,18,42 with more rapid leveling in the West compared to the East. However, carbon accumulation in surface soils (at depths of 0–4 inches) resulting from reforestation activities can help mitigate declining carbon storage in U.S. forests over the long term. Surface soils in reforested areas are currently accumulating 13–21 Tg carbon per year, with the potential to accumulate hundreds more Tg of carbon within a century.112,119
Economic and population trends will affect national and global production and consumption of wood products, which can temporarily store carbon. The storage of carbon in and emissions from wood products contribute to carbon stores and exchanges with the atmosphere; the carbon stored in wood products accumulates as wood is harvested from forests at a rate that exceeds carbon releases from the decay and combustion of wood products already in use. The harvested wood products pool alone is not a direct sink for atmospheric carbon, but losses from the pool are a direct source of atmospheric carbon. Although the contribution of harvested wood products is uncertain, the worldwide net surplus of carbon in wood products is estimated to be approximately 8% of the established global forest sink (189 Tg carbon per year).120 In the United States, 76% of the annual domestic harvest input to the wood products pool in 2015 (110 Tg carbon per year) was offset by release processes (84 Tg carbon per year), resulting in an increase in wood products of 26 Tg carbon.14
Forested watersheds provide water for municipal water supplies, agricultural irrigation, recreation, spiritual values, and in-stream flows for aquatic ecosystems. Changes in snowfall amount, timing, and melt dynamics are affecting water availability and stream water quality. In the western United States (especially the Pacific Northwest), less precipitation is falling as snow and more as rain in winter months, leading to a longer and drier summer season (Ch. 24: Northwest).121 Persistence of winter snowpacks has also decreased in the northeastern United States over the last few decades, with more mid-winter thaws (Ch. 18: Northeast). Changing snowmelt patterns are likely to alter snowmelt contributions to the flushing of soil nutrients into streams in both western122 and eastern forests.123
Forest watersheds moderate the effects of extreme climate events such as drought and heavy rainfall, thus minimizing downstream impacts on aquatic ecosystems and human communities such as flooding, low flows, and reduced water quality. Disturbances and periodic droughts affect streamflow and water quality,12,13,124 as do changes in forest structure that are influenced by climatic variability and change, such as leaf area and species distribution and abundance.33 For example, drought-related bark beetle outbreaks and wildfire kill trees, reducing water uptake and evapotranspiration and potentially increasing water yield,125 although water yield can decrease if regrowing species have higher water-use demands than did the insect- or fire-killed trees.126
Wildfires can also increase forest openness by killing midstory and overstory trees, which promotes earlier snowmelt from increased solar radiation. This, in turn, leads to more winter runoff and exacerbates dry summer conditions, especially in cooler interior mountains.127,128 In warmer forests, typically in wetter climates where wildfire is currently rare, increased forest openness can in some cases increase snowpack retention.129 Wildfires can increase erosion and sediment in western U.S. rivers,130 as well as reduce tree cover adjacent to rivers and streams and thus increase stream temperature.131,132 In eastern U.S. forests, the proportion of tree species with moderate water demands (mesophytes) is increasing in many areas as a result of fire exclusion, less logging and other disturbances, and possibly a warmer climate.133,134 Mesophytes transpire more water than other species occupying the same area, thus reducing streamflow.135,136
Decisions about how to address climate change in the context of forest management need to be informed by a better understanding of the risks of potential climate change effects on natural resources and the organizations that manage those resources. For example, risks posed by ecological disturbances can be reduced by first assessing specific disturbance components (such as wildfire exposure) and second identifying forest management activities that can be implemented to reduce risk.52 However, identifying how climate change will alter biophysical conditions (risk assessment) and how forest management organizations will respond to future changes (risk management) is complex. Describing operational (technical and financial), economic, and political risks is even more difficult. Furthermore, identifying interactions among all types of risks at regional and local scales will provide land managers with the information needed to manage forests sustainably across large landscapes (Ch. 28: Adaptation).137 To that end, recent nationwide projects examining site-specific adaptation practices help inform forest management focused on maintaining long-term productivity under future climatic conditions.20,138,139
Assessments of climate change effects and adaptation actions are being incorporated into resource management plans, environmental assessments, and monitoring programs of public agencies.42,140 Adaptation planning tools and compendia of adaptation options for forest resources are now institutionalized in public land management in much of the United States (Ch. 28: Adaptation).19,141 Adaptation actions are also being implemented by Native American tribes and communities, with an emphasis on culturally significant forest resources, such as flora and fauna, which in turn affect sovereignty and economic sustainability.142 Adaptation is especially urgent for Native American communities affiliated with reservations where place-based traditional medicine, ceremonial practices, and methods of gathering and hunting for food contribute to cultural identity (Ch. 15: Tribes).143
Implementing climate change adaptation measures in forest management requires an understanding of the effects of climate change on different types of forests, forest-related enterprises, and resource-dependent communities (Figure 6.7). However, even if the potential magnitude and consequences of climate change are well understood and viable management responses exist, adaptation measures cannot occur unless management organizations (on public and private lands) have the capacity (people and financial resources, enabled by policy) to implement management responses.144
Fortunately, many ongoing practices that address existing forest management needs—stand density management, surface fuel reduction, control of invasive species, and aquatic habitat restoration—contribute to the goal of increasing resilience to higher temperatures, drought, and disturbances.127,144,145,146,147 Fuel treatments across large landscapes have the additional benefit of creating defensible space for fire suppression, especially near the wildland–urban interface. Resource managers are evaluating how these practices can be modified and implemented to address future climate risks.141 For example, forest managers in dry western U.S. forests are considering greater reductions in stand density to increase forest resistance and resilience to fire, insects, and drought.148 Implementation of these practices can be costly, often confront legal and administrative barriers,149 and must consider economic tradeoffs associated with management of other natural resources.55
Applications of these and other practices vary as a function of ownership objectives, timber and non-timber wood product markets, policy constraints, and setting (urban, rural, or wildland–urban interface). For example, land managers in regions where short-rotation, plantation management of forest tree species is common (for example, private lands in the southern United States and Pacific Northwest) have the flexibility to periodically shift species and genetic composition of trees to align with future changes in climate and disturbance regimes.150 A significant amount of adaptation has occurred on public lands, including actions that reduce climate-related risks to water resources such as 1) design of sustainable forest road systems that take into account increased flooding hazard, including upsizing culverts to match projected streamflows; 2) joint planning and design of fuel treatments (including prescribed burning) and watershed restoration to create resilient terrestrial and aquatic ecosystems;127 3) comprehensive mapping of projected stream temperatures to set priorities for riparian restoration and coldwater fish conservation;151 and 4) supporting viable American beaver populations to facilitate retention of cool water in forested aquatic systems (Figure 6.8).140
Applying climate change adaptation management activities over large areas of forestland will be challenged by projected declines in the size of the forest sector workforce and receding timber product outputs in some parts of the country.42 Declines in the workforce mean fewer skilled workers who can carry out management actions, although collaborative efforts by nongovernmental organizations are emerging to assist with climate change adaptation.152 Low timber product output, the result of abundant supplies of timber and low demand for primary and secondary timber products,153 means lower prices for timber, which have trended downward since the late 1990s (e.g., Timber Mart-South 2018154), thereby providing fewer opportunities to offset treatment costs with sales of timber removed. As a result, weak timber markets mean reduced incentives for private forest owners to actively manage forests in ways that enhance climate resilience. However, multiorganization collaboration, widespread availability of adaptation options,155,156 and a growing list of examples of on-the-ground implementation bode well for the future of climate-informed forest management. Flexible management approaches that promote learning and sharing among interested parties can help accelerate implementation.
Lead authors, chapter authors, and technical contributors engaged in multiple technical discussions via teleconference between September 2016 and March 2018, which included a review of technical inputs provided by the public and a broad range of published literature as well as professional judgment. Discussions were followed by expert deliberation on draft Key Messages by the authors and targeted consultation with additional experts by the authors and technical contributors. A public engagement webinar on May 11, 2017, solicited additional feedback on the report outline. Webinar attendees provided comments and suggestions online and through follow-up emails. Strong emphasis was placed on recent findings reported in the scientific literature and relevance to specific applications in the management of forest resources.