Rapid Forest Change—Wildfire

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 temperatures^44,45 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

Rapid Forest Change—Insects and Pathogens

Climate change is expected to increase the effects of some insect species in U.S. forests^23,61,62 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. 2014^78,79,80). 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

Long-Term Forest Change

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 Carbon Dynamics

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

Forest Water Resources

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.