Forests Impacts & Adaptation

Climate Impacts on Forests

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Extent and types of forests in the United States. Source: USDA (2006) (PDF)

In the United States, forests occupy approximately 751 million acres, about one third of the country's total land area. ^[1] America's forests provide many benefits and services to society, including clean water, recreation, wildlife habitat, carbon storage, and a variety of forest products. Climate influences the structure and function of forest ecosystems and plays an essential role in forest health. A changing climate may worsen many of the threats to forests, such as pest outbreaks, fires, human development, and drought.

Climate changes directly and indirectly affect the growth and productivity of forests: directly due to changes in atmospheric carbon dioxide and climate and indirectly through complex interactions in forest ecosystems. Climate also affects the frequency and severity of many forest disturbances.

In conjunction with the projected impacts of climate change, forests face impacts from land development, suppression of natural periodic forest fires, and air pollution. Although it is difficult to separate the effects of these different factors, the combined impact is already leading to changes in our forests. As these changes are likely to continue in the decades ahead, some of the valuable goods and services provided by forests may be compromised. To learn more about examples of projected regional changes in forests, see the Northeast, Southeast, Southwest, and Alaska regional impacts pages.

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Impacts on Forest Growth and Productivity

Many aspects of projected climate change will likely affect forest growth and productivity. Three examples are described below: increases in carbon dioxide (CO₂), increases in temperature, and changes in precipitation.

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Projected shifts in forest types. ^[3] The maps show current and projected forest types. Major changes are projected for many regions. For example, in the Northeast, under a mid-range warming scenario, the currently dominant maple-beech-birch forest type (red shading) is projected to be completely displaced by other forest types in a warmer future. Source: USGCRP (2009)

• Carbon dioxide is required for photosynthesis, the process by which green plants use sunlight to grow. Given sufficient water and nutrients, increases in atmospheric CO₂ may enable trees to be more productive. ^[2] Higher future CO₂ levels could benefit forests with fertile soils in the Northeast. However, increased CO₂ may not be as effective in promoting growth in the West and Southeast, where water is limited. ^[3]
• Warming temperatures could increase the length of the growing season. However, warming could also shift the geographic ranges of some tree species. Habitats of some types of trees are likely to move northward or to higher altitudes. Other species may be at risk locally or regionally if conditions in their current geographic range are no longer suitable. ^[2] For example, species that currently exist only on mountaintops in some regions may die out as the climate warms since they cannot shift to a higher altitude.
• Climate change will likely increase the risk of drought in some areas and the risk of extreme precipitation and flooding in others. Increased temperatures would alter the timing of snowmelt, affecting the seasonal availability of water. Although many trees are resilient to some degree of drought, increases in temperature could make future droughts more damaging than those experienced in the past. In addition, drought increases wildfire risk, since dry trees and shrubs provide fuel to fires. Drought also reduces trees' ability to produce sap, which protects them from destructive insects such as pine beetles. ^[3]

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Impacts of Disturbances

The brown and red trees in this picture have been infested by mountain pine beetles near Winter Park, Colorado in May 2007. Source: CCSP (2008) (PDF)

Climate change could alter the frequency and intensity of forest disturbances such as insect outbreaks, invasive species, wildfires, and storms. These disturbances can reduce forest productivity and change the distribution of tree species. In some cases, forests can recover from a disturbance. In other cases, existing species may shift their range or die out. In these cases, the new species of vegetation that colonize the area create a new type of forest.

• Insect outbreaks often defoliate, weaken, and kill trees. For example, pine beetles have damaged more than 1.5 million acres of forest in Colorado and spruce beetles have damaged more than 2.5 million acres in Alaska. ^[3] The hemlock woolly adelgid, an invasive species that is sensitive to cold weather and destroys Eastern hemlock, will likely extend its habitat north as the climate warms. ^[4] A lack of natural controls, such as predators, or pathogens, or inadequate defenses in trees, can allow insects to spread. Climate change could contribute to an increase in the severity of future insect outbreaks. Rising temperatures may enable some insect species to develop faster and expand their ranges northward. Invasive plant species can displace important native vegetation because the invasive species often lack natural predators. Climate change could benefit invasive plants, since they are generally more tolerant to a wider range of environmental conditions than are native plants. ^[3]
• In recent years, wildfires consumed more than 6.25 million acres of forest in Alaska (roughly equal to the area of Massachusetts). Warm temperatures and drought conditions during the early summer contributed to this event. ^[5] Climate change is projected to increase the extent, intensity, and frequency of wildfires in certain areas of the country. Warmer spring and summer temperatures, coupled with decreases in water availability, dry out woody materials in forests and increase the risk of wildfire. Fires can also contribute to climate change, since they can cause rapid, large releases of carbon dioxide to the atmosphere. ^[2]

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The size of wildfires in the United States increased from 1983 to 2008. Source: USGCRP (2009)

• Hurricanes, ice storms, and wind storms can cause damage to forests. Hurricanes Rita and Katrina in 2005 damaged a total of 5,500 acres of forest. The amount of carbon released by these trees as they decay is roughly equivalent to the net amount of carbon sequestered by all U.S. forests in a single year. ^[2]

Disturbances can interact with one another, or with changes in temperature and precipitation, to increase risks to forests. For example, drought can weaken trees and make a forest more susceptible to wildfire or insect outbreaks. Similarly, wildfire can make a forest more vulnerable to pests. ^{[2] [3]} The combination of drought and outbreaks of beetles has damaged piñon pine forests in the Southwest.

To learn more about forest adaptation measures, please see the Forest Adaptation section.

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References

1. U.S. Forest Service (undated). USDA Forest Service: An Overview (PDF) .

2. CCSP (2008). The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity in the United States . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Backlund, P. , A. Janetos, D. Schimel, J. Hatfield, K. Boote, P. Fay, L. Hahn, C. Izaurralde, B.A. Kimball, T. Mader, J. Morgan, D. Ort, W. Polley, A. Thomson, D. Wolfe, M. Ryan, S. Archer, R. Birdsey, C. Dahm, L. Heath, J. Hicke, D. Hollinger, T. Huxman, G. Okin, R. Oren, J. Randerson, W. Schlesinger, D. Lettenmaier, D. Major, L. Poff, S. Running, L. Hansen, D. Inouye, B.P. Kelly, L Meyerson, B. Peterson, and R. Shaw. U.S. Environmental Protection Agency, Washington, DC, USA.

3. USGCRP (2009). Global Climate Change Impacts in the United States . T.R. Karl, J.M. Melillo, and T.C. Peterson (eds.). United States Global Change Research Program. Cambridge University Press, New York, NY, USA.

4. U.S. Fish & Wildlife Service: Hemlock Woolly Adelgid . Accessed 3/08/2012.

5. CCSP (2008). Preliminary Review of Adaptation Options for Climate-Sensitive Ecosystems and Resources . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Julius, S.H., J.M. West (eds.), J.S. Baron, B. Griffith, L.A. Joyce, P. Kareiva, B.D. Keller, M.A. Palmer, C.H. Peterson, and J.M. Scott (authors). U.S. Environmental Protection Agency, Washington, DC, USA.

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Adaptation Examples in Forests

Changes in the amount of carbon dioxide in the atmosphere, temperature, and precipitation can affect how forests grow. Climate change will also likely alter the frequency and intensity of many forest disturbances, such as insect outbreaks, wildfires, droughts, and storms. Climate change may also worsen some existing stressors, such as habitat fragmentation, the abundance of invasive species, and pollution. To learn more about how climate change will likely impact forests, visit the Forests Impacts section.

Forests that are healthy tend to be more resilient to climate change. Forest adaptation measures related to climate change are often aimed to reduce the impacts of current ecosystem stressors. These measures include a wide variety of activities that are tailored to impacts occurring or anticipated to occur within a specific forest. Specific approaches include:

• Removing invasive species
• Promoting biodiversity and landscape diversity
• Collaborating across borders to create habitat linkages
• Managing wildfire risk through controlled burns and thinning

The following case studies, examples, and related links are illustrative and not intended to be comprehensive.

Park managers reduce existing stressors in Rocky Mountain National Park

Sprague Lake, Rocky Mountain National Park. Source: National Park Service (2010)

Rocky Mountain National Park (RMNP) reduces existing stressors and enhances the Park's ability to adapt to climate change through effective management strategies. Park managers are proactive in removing or preventing the spread of invasive species. They also manage wildfire risk through adaptive fire management (e.g., removal of excessive vegetation and dead fuels through thinning, prescribed fire, and other methods) and reduce regional air pollution through partnerships with regulatory agencies. RMNP is conducting a study of its forest species to determine which will be most impacted by climate change. These efforts will help guide management to protect the park's forests and other ecosystems. ^[1] For more information about how managers are helping ecosystems adapt to climate change, visit the Ecosystem Impacts & Adaptation page.

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Managers maintain diversity in the Olympic National Forest

Olympic National Forest. Source: U.S. Global Change Research Program (2008)

Managers at the Olympic National Forest (ONF) are improving forest resilience by promoting biodiversity. ONF is a restoration forest, which means that managers focus on reintroducing landscape complexity and moving away from the dominance of a single species that results from timber production. For example, ONF is applying targeted thinning (where some trees are removed to improve the growth of others) to increase the diversity of tree species in an area, decrease stress from overcrowding, and reduce sensitivity to disturbances. Given uncertainty about how any one specific species or system will respond to climate change, promoting biodiversity and landscape diversity will lead to increased resilience to climate change. ^[1]

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Land managers reduce the impacts of forest disturbances in Alaska

In recent years, the Kenai Peninsula experienced a beetle infestation that affected more than a million acres of forest. Beetle infestations that kill or weaken trees make forests more susceptible to large wildfires. According to the Fish and Wildlife Service, the predicted increases in temperature may contribute to continued spread of beetle infestations. Land managers in the Kenai Peninsula in Alaska are already taking action to adapt to increased spruce bark beetle infestations and the related fire risk. ^[2]

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The U.S. Forest Service develops a roadmap for climate change adaptation

The U.S. Forest Service (USFS) is responsible for maintaining the integrity of ecosystems on national forests and grasslands. In the past 20 years, USFS has observed an increased range of challenges caused by wildfires, changes in the water cycle, and expanding insect infestations. ^[3] In order to create management practices that evolve with the changing climate, USDA Forest Service released the National Roadmap for Responding to Climate Change (PDF). The Roadmap encourages taking a landscape-scale, or all-lands, approach that considers ecosystem boundaries rather than property boundaries. The plan outlines three key adaptive management strategies: ^[3]

• Build resistance to short-term threats.
• Increase ecosystem resilience for long-term maintenance.
• Facilitate large-scale ecological transitions for permanent changes.

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The U.S. and Canada work to protect forest species

The Two Countries One Forest project is working to conserve and restore 80 million acres of forests in the Northern Appalachian Acadian ecoregion (or bioregion), an area defined by geography and ecology, not political boundaries. The area of interest includes forests in New York, Vermont, New Hampshire, Maine, Québec, New Brunswick, Prince Edward Island, and Nova Scotia—spanning several states and provinces and crossing an international border. The project protects forest ecosystems threatened by climate change and human activity. Forests in the region are young and fragmented. Biological corridors that allow species migration between areas of habitat are cut off by roads and development. This makes it difficult for plants and animals to maintain healthy populations and genetic diversity. By taking a regional approach that crosses political boundaries, Two Countries One Forest is working to improve the health of the forest and make it less sensitive to climate change.

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References

1. CCSP (2008). Preliminary Review of Adaptation Options for Climate-Sensitive Ecosystems and Resources . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. [Julius, S.H., J.M. West (eds.), J.S. Baron, B. Griffith, L.A. Joyce, P. Kareiva, B.D. Keller, M.A. Palmer, C.H. Peterson, and J.M. Scott (authors)]. U.S. Environmental Protection Agency, Washington, DC, USA.

2. FWS (2011). Alaska: Across the Wildest State, Climate Change Threatens Many Species and Habitats . U.S. Fish and Wildlife Service. Accessed March 26, 2012.

3. USDA Forest Service (2010). National Roadmap for Responding to Climate Change . USDA Forest Service.

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