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Potential Interactions with Land Cover
The natural vegetative cover of the United States is largely determined by the prevailing climate and soil. Where not altered by societal activities, climate conditions largely determine where individual species of plants and animals can live, grow, and reproduce. Thus, the collections of species that we are familiar with -- e.g., the southeastern mixed deciduous forest, the desert ecosystems of the arid Southwest, the productive grasslands of the Great Plains -- are primarily a consequence of present climate conditions. Past changes in ecosystems indicate that some species are so strongly influenced by the climate to which they are adapted that they are vulnerable even to modest changes in climate. For example, alpine meadows at high elevations in the West exist where they do entirely because the plants that comprise them are adapted to cold conditions that are too harsh for other species in the region. The desert vegetation of the Southwest is adapted to the region's high summer temperatures and aridity. Similarly, the forests in the East tend to have adapted to relatively high rainfall and soil moisture; if drought conditions were to persist, grasses and shrubs could begin to out-compete tree seedlings, leading to completely different ecosystems.
To provide a common base of information about potential changes in vegetation across the nation for use in the National Assessment (NAST 2000), specialized ecosystem models were used to evaluate the potential consequences of climate change and an increasing CO2 concentration for the dominant vegetation types. Biogeography models were used to simulate potential shifts in the geographic distribution of major plant species and communities (ecosystem structure). And biogeochemistry models were used to simulate changes in basic ecosystem processes, such as the cycling of carbon, nutrients, and water (ecosystem function). Each type of model was used in considering the potential consequences of the two primary model-based climate scenarios. These scenarios represented conditions across much of the United States that were generally either warmer and moister, or hotter and drier. The results from both types of models indicated that changes in ecosystems would be likely to be significant.
Climate changes that affect the land surface and terrestrial vegetation will also have implications for fresh-water and coastal marine ecosystems that depend on the temperature of runoff water, on the amount of erosion, and on other factors dependent on the land cover. For example, in aquatic ecosystems, many fish can breed only in water that falls within a narrow range of temperatures. As a result, species of fish that are adapted to cool waters can quickly become unable to breed successfully if water temperatures rise. As another example, because washed-off soil and nutrients can benefit wetland species (within limits) and harm estuarine ecosystems, changes in the frequency or intensity of runoff events caused by changes in land cover can be important. Such impacts are described in the subsections dealing with climate change interactions with water resources and the coastal environment, while issues affecting terrestrial land cover are covered in the following subsection.
Redistribution of Land Cover
The responses of ecosystems to projected changes in climate and CO2 are made up of the individual responses of their constituent species and how they interact with each other. Species in current ecosystems can differ substantially in their tolerances to changes in temperature and precipitation, and in their responses to changes in the CO2 concentration. As a result, the ranges of individual species are likely to shift at different rates, and different species are likely to have different degrees of success in establishing themselves in new locations and environments. While changes in climate projected for the coming hundred years are very likely to alter current ecosystems, projecting these kinds of biological and ecological responses and the structure and functioning of the new plant communities is very difficult.
Analyses of present ecosystem distributions and of past shifts indicate that natural ecosystems are sensitive to changes in surface temperature, precipitation patterns, and other climate parameters and changes in the atmospheric CO2 concentration. For example, changes in temperature and precipitation of the magnitude being projected are likely to cause shifts in the areas occupied by dominant vegetation types relative to their current distribution. Some ecosystems that are already constrained by climate, such as alpine meadows in the Rocky Mountains, are likely to face extreme stress and disappear entirely in some places. Other more widespread ecosystems are also likely to be sensitive to climate change. For example, both climate model scenarios suggest that the southwestern United States will become moister, allowing more vegetation to grow (Figure 6-6). Such a change is likely to transform desert landscapes into grasslands or shrublands, altering both their potential use and the likelihood of fire. In the northeastern United States, both climate scenarios suggest changes mainly in the species composition of the forests, including the northward displacement of sugar maples, which could lead to loss in some areas. However, the studies also indicate that conditions in this region will remain conducive to maintaining a forested landscape, mainly oak and hickory. In the southeastern United States, however, there was less agreement among the models: the hot-dry climate scenario was projected to lead to conditions that would be conducive to the potential breakup of the forest landscape into a mosaic of forests, savannas, and grasslands; in contrast, the warm-moist scenario was projected to lead to a northward expansion of the southeastern mixed forest cover. (See additional discussion in the Forest subsection.)
Basically, changes in land cover were projected to occur, at least to some degree, in all locations, and these changes cannot generally be prevented if the climate changes and vegetation responds as much as projected.
Effects on the Supply of Vital Ecosystem Goods and Services
In addition to the value of natural ecosystems in their own right, ecosystems of all types, from the most natural to the most extensively managed, provide a variety of goods and services that benefit society. Some products of ecosystems enter the market and contribute directly to the economy. For example, forests serve as sources of timber and pulpwood, and agro-ecosystems serve as sources of food. Ecosystems also provide a set of unpriced services that are valuable but that typically are not traded in the marketplace. Although there is no current market, for example, for the services that forests and wetlands provide for improving water quality, regulating stream flow, providing some measure of protection from floods, and sequestering carbon, some of these services are very valuable to society. Ecosystems are also valued for recreational, aesthetic, and ethical reasons. For example, the bird life of the coastal marshes of the Southeast and the brilliant autumn colors of the New England forests are treasured components of the nation's regional heritages and important elements of our quality of life.
Based on the studies carried out, changes in land cover induced by climate change, along with an increased level of disturbances, could have varied impacts on ecosystem services, including the abilities of ecosystems to cleanse the air and water, stabilize landscapes against erosion, and store carbon. Even in such regions as the Southwest, where vegetation is expected to increase as a result of increased rainfall and enhanced plant growth due to the rising CO2 concentration, an important potential consequence is likely to be a heightened frequency and intensity of fires during the prolonged summer season. Increased fire frequency would likely be a threat not only to the natural land cover, but also to the many residential structures being built in vulnerable suburban and rural areas, and later would increase vulnerability to mudslides as a result of denuded hills. Considering the full range of available results, it is plausible that climate change-induced alterations to natural ecosystems could affect the availability of some ecosystem goods and services.
Effects of an Increased CO2 Concentration on Plants
The ecosystem models used in the National Assessment considered the potential effects of increases in the atmospheric CO2 concentration be-cause the CO2 concentration affects plant species via a direct physiological effect on photosynthesis (the process by which plants use CO2 to create new biological material). Higher CO2 concentrations generally enhance plant growth if the plants also have sufficient water and nutrients (such as nitrogen) that are needed to sustain this enhanced growth. For example, the CO2 level in commercial greenhouses is sometimes boosted to stimulate plant growth. In addition to enhancing plant growth, higher CO2 levels can raise the efficiency with which plants use water and reduce their susceptibility to damage by air pollutants.
As a result of these various influences, different types of plants respond at different rates to increases in the atmospheric CO2 concentration, resulting in a divergence of growth rates. Most species grow faster and increase biomass; however, the nutritional value of some of these plants could be altered. Both because of biochemical processing and because warming temperatures increase plant respiration, the beneficial effects of increased CO2 on plants are also projected to flatten at some higher level of CO2 concentration, beyond which continuing increases in the CO2 concentration would not enhance plant growth.
While there is still much to be learned about the CO2 "fertilization" effect, including its limits and its direct and indirect implications, many ecosystems are projected to benefit from a higher CO2 concentration, and plants will use water more efficiently.
Effects on Storage of Carbon
In response to changes in climate and the CO2 concentration, the biogeochemistry models used in the National Assessment generally simulated increases in the amount of carbon stored in vegetation and soils for the continental United States. The calculated increases were relatively small, however, and not uniform across the country. For example, one of the biogeochemistry models, when simulating the effects of hotter and drier conditions, projected that the southeastern forests would lose more carbon by respiration than they would gain by increased photosynthesis, causing an overall carbon loss of up to 20 percent by 2030. Such a loss would indicate that the forests were in a state of decline. The same biogeochemistry model, however, when calculating the potential effects of the warmer and moister climate scenario, projected that forests in the same part of the Southeast would likely gain between 5 and 10 percent in carbon over the next 30 years, suggesting a more vigorous forest.
Susceptibility of Ecosystems to Disturbances
Prolonged stress due to insufficient soil moisture can make trees more susceptible to insect attack, lead to plant death, and increase the probability of fire as dead plant material adds to an ecosystem's "fuel load." The biogeography models used in this analysis simulated at least part of this sequence of climate-triggered events in ecosystems as a prelude to calculating shifts in the geographic distribution of major plant species.
For example, one of the biogeography models projected that a hot, dry climate in the Southeast would be likely to result in the replacement of the current mixed evergreen and deciduous forests by savanna/woodlands and grasslands, with much of the change effected by an increased incidence of fire. Yet the same biogeography model projected a slight northward expansion of the mixed evergreen and deciduous forests of the Southeast in response to the warm, moist climate scenario, with no significant contraction along the southern boundary. Thus, in this region, changes in the frequency and intensity of such disturbances as fire are likely to be major determinants of the type and rapidity of the conversion of the land cover to a new state.
As explained more fully in the sections on the interactions of climate change with coastal and water resources, aquatic ecosystems are also likely to be affected by both climate change and unusual disturbances, such as storms and storm surges.
Potential Adaptation Options to Preserve Prevailing Land Cover
The National Assessment concluded that the potential vulnerability of natural ecosystems is likely to be more important than other types of potential impacts affecting the U.S. environment and society. This importance arises because in many cases little can be done to help these ecosystems adapt to the projected rate and amount of climate change. While adjustments in how some systems are managed can perhaps reduce the potential impacts, the complex, interdependent webs that have been naturally generated over very long periods are not readily shifted from one place to another or easily recreated in new locations, even to regions of similar temperature and moisture. Although many regions have experienced changes in ecosystems as a result of human-induced changes in land cover, and people have generally become adapted to -- and have even become defenders of -- the altered conditions (e.g., reforestation of New England), the climate-induced changes during the 21st century are likely to affect virtually every region of the country -- both the ecosystems where people live, as well as those in the protected areas that have been created as refuges against change.