Potential Interactions with Forests

Forests cover nearly one-third of the United States, providing wildlife habitat; clean air and water; carbon storage; and recreational opportunities, such as hiking, camping, and fishing. In addition, harvested products include timber, pulpwood, fuelwood, wild game, ferns, mushrooms, berries, and much more. This wealth of products and services depends on forest productivity and biodiversity, which are in turn strongly influenced by climate.

Across the country, native forests are adapted to the local climates in which they developed, such as the cold-tolerant boreal forests of Alaska, the summer drought-tolerant forests of the Pacific Northwest, and the drought-adapted piñon-juniper forests of the Southwest. Given the overall importance of the nation's forests, the potential impacts from climate change are receiving close attention, although it is only one of several factors meriting consideration.

A range of human activities causes changes in forests. For example, significant areas of native forests have been converted to agricultural use, and expansion of urban areas has fragmented forests into smaller, less contiguous patches. In some parts of the country, intensive management and favorable climates have resulted in development of highly productive forests, such as southern pine plantations, in place of the natural land cover. Fire suppression, particularly in southeastern, midwestern, and western forests, has also led to changes in forest area and in species composition. Harvesting methods have also changed species composition, while planting trees for aesthetic and landscaping purposes in urban and rural areas has expanded the presence of some species. In addition, large areas, particularly in the Northeast, have become reforested as forests have taken over abandoned agricultural lands, allowing reestablishment of the ranges of many wildlife species.

Changes in climate and in the CO₂ concentration are emerging as important human-induced influences that are affecting forests. These factors are interacting with factors already causing changes in forests to further affect the socioeconomic benefits and the goods and services forests provide, including the extent, composition, and productivity of forests; the frequency and intensity of such natural disturbances as fire; and the level of biodiversity (NFAG 2001). Based on model projections of moderate to large warming, Figure 6-8 gives an example of the general character of changes that could occur for forests in the eastern United States by the late 21st century.

Effects on Forest Productivity

A synthesis of laboratory and field studies and modeling indicates that the fertilizing effect of atmospheric CO₂ will increase forest productivity. However, increases are likely to be strongly tempered by local conditions, such as moisture stress and nutrient availability. Across a wide range of scenarios, modest warming is likely to result in increased carbon storage in most U.S. forests, although under some of the warmer model scenarios, forests in the Southeast and the Northwest could experience drought-induced losses of carbon, possibly exacerbated by increased fire disturbance. These potential gains and losses of carbon would be in addition to changes resulting from changes in land use, such as the conversion of forests to agricultural lands or development.

Other components of environmental change, such as nitrogen deposition and ground-level ozone concentrations, are also affecting forest processes. Models used in the forest sector assessment suggest a synergistic fertilization response between CO₂ and nitrogen enrichment, leading to further increases in productivity (NFAG 2001). However, ozone acts in the opposite direction. Current ozone levels, for example, have important effects on many herbaceous species and are estimated to decrease production in southern pine plantations by 5 percent, in northeastern forests by 10 percent, and in some western forests by even more. Interactions among these physical and chemical changes and other components of global change will be important in projecting the future state of U.S. forests. For example, a higher CO₂ concentration can tend to suppress the impacts of ozone on plants.

Effects on Natural Disturbances

Natural disturbances having the greatest effects on forests include insects, disease, non-native species, fires, droughts, hurricanes, landslides, wind storms, and ice storms. While some tree species are very susceptible to fire, others, such as lodgepole pine, are dependent on occasional fires for successful reproduction.

Over millennia, local, regional, and global-scale changes in temperature and precipitation have influenced the occurrence, frequency, and intensity of these natural disturbances. These changes in disturbance regimes are a natural part of all ecosystems. However, as a consequence of climate change, forests may soon be facing more rapid alterations in the nature of these disturbances. For example, unless there is a large increase in precipitation, the seasonal severity of fire hazard is projected to increase during the 21st century over much of the country, particularly in the Southeast and Alaska.

The consequences of drought depend on annual and seasonal climate changes and whether the current adaptations of forests to drought will offer resistance and resilience to new conditions. The ecological models used in the National Assessment indicated that increases in drought stresses are most likely to occur in the forests of the Southeast, southern Rocky Mountains, and parts of the Northwest. Hurricanes, ice storms, wind storms, landslides, insect infestations, disease, and introduced species are also likely to be climate-modulated influences that affect forests. However, projection of changes in the frequencies, intensities, and locations of such factors and their influences are difficult to project. What is clear is that, as climate changes, alterations in these disturbances and in their effects on forests are possible.

Effects on Forest Biodiversity

In addition to the very large influences of changes in land cover, changes in the distribution and abundance of plant and animal species are a result of both (1) the birth, growth, death, and dispersal rates of individuals in a population and (2) the competition between individuals of the same species and other species. These can all be influenced in turn by weather, climate, contaminants, nutrients, and other abiotic factors. When aggregated, these processes can result in the local disappearance or introduction of a species, and ultimately determine the species' range and influence its population.

Although climate and soils exert strong controls on the establishment and growth of plant species, the response of plant and animal species to climate change will be the result of many interacting and interrelated processes operating over several temporal and spatial scales. Movement and migration rates, changes in disturbance regimes and abiotic environmental variables, and interactions within and between species will all affect the distributions and populations of plants and animals.

Analyses conducted using ecological models indicate that plausible climate scenarios are very likely to cause shifts in the location and area of the potential habitats for many tree species. For example, potential habitats for trees acclimated to cool environments are very likely to shift northward. Habitats of alpine and sub-alpine spruce-fir in the contiguous United States are likely to be reduced and, possibly in the long term, eliminated as their mountain habitats warm. The extents of aspen, eastern birch, and sugar maple are likely to contract dramatically in the United States and largely shift into Canada, with the shift in sugar maple causing loss of syrup production in northern New York and New England. In contrast, oak/hickory and oak/pine could expand in the East, and Ponderosa pine and arid woodland communities could expand in the West. How well these species track changes in their potential habitats will be strongly influenced by the viability of their mechanisms for dispersal to other locations and the disturbances to these alternative environments.

Because of the dominance of non-forest land uses along migration routes, the northward shift of some native species to new habitats is likely to be disrupted if the rate of climate change is too rapid. For example, conifer encroachment, grazing, invasive species, and urban expansion are currently displacing sagebrush and aspen communities. The effects of climate change on the rate and magnitude of disturbance (forest damage and destruction associated with fires, storms, droughts, and pest outbreaks) will be important factors in determining whether transitions from one forest type to another will be gradual or abrupt. If the rate and type of disturbances in New England do not increase, for example, a smooth transition from the present maple, beech, and birch tree species to oak and hickory may occur. Where the frequency or intensity of disturbances increases, however, transitions are very likely to occur more rapidly. As these changes occur, invasive (weedy) species that disperse rapidly are likely to find opportunities in newly forming ecological communities. As a result, the species composition of these communities will likely differ significantly in some areas from those occupying similar habitats today.

Changes in the composition of ecosystems may, in turn, have important effects on wildlife. For example, to the extent that climate change and a higher CO₂ concentration increase forest productivity, this might result in reduced overall land disturbance and improved water quality, tending to help wildlife, at least in some areas. However, changes in composition can also affect predator -- prey relationships, pest types and populations, the potential for non-native species, links in the chain of migratory habitats, the health of keystone species, and other factors. Given these many possibilities, much remains to be examined in projecting influences of climate change on wildlife.

Socioeconomic Impacts

North America is the world's leading producer and consumer of wood products. U.S. forests provide for substantial exports of hardwood lumber, wood chips, logs, and some types of paper. Coming the other way, the United States imports, for example, about 35 percent of its softwood lumber and more than half of its newsprint from Canada.

The U.S. market for wood products will be highly dependent upon the future area in forests, the species composition of forests, future supplies of wood, technological changes in production and use, the availability of such substitutes as steel and vinyl, national and international demands for wood products, and competitiveness among major trading partners. Analyses indicate that, for a range of climate scenarios, forest productivity gains are very likely to increase timber inventories over the next 100 years (NFAG 2001). Under these scenarios, the increased wood supply leads to reductions in log prices, helping consumers, but decreasing producers' profits. The projected net effect on the economic welfare of participants in timber markets increases by about 1 percent above current values.

Analyses conducted for the forest sector assessment indicate that land use will likely shift between forestry and agriculture as these economic sectors adjust to climate-induced changes in production. U.S. hardwood and softwood production is projected to generally increase, although the projections indicate that softwood output will only increase under moderate warming. Timber output is also projected to increase more in the South than in the North, and saw-timber volume is projected to increase more than pulpwood volume.

Patterns and seasons of outdoor, forest-oriented recreation are likely to be modified by the projected changes in climate. For example, changes in forest-oriented recreation, as measured by aggregate days of activities and total economic value, are likely to be affected and are likely to vary by type of recreation and location. In some areas, higher temperatures are likely to shift typical summer recreation activities, such as hiking, northward or to higher elevations and into other seasons. In winter, downhill skiing opportunities are very likely to shift geographically because of fewer cold days and reduced snowpack in many existing ski areas. Therefore, costs to maintain skiing opportunities are likely to rise, especially for the more southern areas. Effects on fishing are also likely to vary. For example, warmer waters are likely to increase fish production and opportunities to fish for some warm-water species, but decrease habitat and opportunities to fish for cold-water species.

Possible Adaptation Strategies to Protect Forests

Even though forests are likely to be affected by the projected changes in climate, the motivation for adaptation strategies is likely to be most strongly influenced by the level of U.S. economic activity. This level, in turn, is intertwined with the rate of population growth, changes in taste, and general preferences, including society's perceptions about these changes. Market forces have proven to be powerful when it comes to decisions involving land use and forestry and, as such, will strongly influence adaptation on private lands. For forests valued for their current biodiversity, society and land managers will have to decide whether more intense management is necessary and appropriate for maintaining plant and animal species that may be affected by climate change and other factors.

If new technologies and markets are recognized in a timely manner, timber producers could adjust and adapt to climate change under plausible climate scenarios. One possible adaptation measure could be to salvage dead and dying timber and to replant species adapted to the changed climate conditions. The extent and pattern of U.S. timber harvesting and prices will also be influenced by the global changes in forest productivity and prices of overseas products.

Potential climate-induced changes in forests must also be put into the context of other human-induced pressures, which will undoubtedly change significantly over future decades. While the potential for rapid changes in natural disturbances could challenge current management strategies, these changes will occur simultaneously with human activities, such as agricultural and urban encroachment on forests, multiple uses of forests, and air pollution. Given these many interacting factors, climate-induced changes should be manageable if planning is proactive.