The natural vegetation covering about 70% of the US land surface is strongly influenced both by the climate and by the atmospheric carbon dioxide (CO₂) concentration. To provide a common base of information about potential changes in vegetation across the nation for use in the regional and sector studies, specialized ecosystem models were run using the two major climate model scenarios selected for this Assessment. A summary of the national level results follows. Agricultural production forestry systems are the focus of separate sections of this Overview report. and

What are Ecosystems?

Ecosystems are communities of plants, animals, microbes, and the physical environment in which they exist. They can be characterized by their biological richness, by the magnitude of flows of energy and materials between their constituent species and their physical environment, and by the interactions among the biological species themselves, that is, by which species are predators and prey, which are competitors, and which are symbiotic.

Ecologists often categorize ecosystems by their dominant vegetation -- the deciduous broad-leafed forest ecosystems of New England, the short-grass prairie ecosystems of the Great Plains, the desert ecosystems of the Southwest. The term "ecosystem" is used not only to describe natural systems (such as coral reefs, alpine meadows, old growth forests, or riparian habitats), but also for plantation forests and agricultural systems, although these ecosystems obviously differ in many important ways from the natural ecosystems they have replaced.

Ecosystems Supply Vital Goods and Services

While we value natural ecosystems in their own right, ecosystems of all types, from the most natural to the most extensively managed, produce a variety of goods and services that benefit humans. Some of these enter the market and contribute directly to the economy. Thus, forests as sources of timber and pulpwood, and agro-ecosystems as sources of food are important to us. But ecosystems also provide a set of un-priced services that are valuable, but that typically are not traded in the marketplace. There is no current market, for example, for the services that forests and wetlands provide for improving water quality, regulating stream flow, and providing some measure of protection from floods. However, these services are very valuable to society.

Ecosystems are also valued for recreational, aesthetic, and ethical reasons. These are also difficult to value monetarily, but are nevertheless important. The bird life of the coastal marshes of the Southeast and the brilliant autumn colors of the New England forests are treasured components of our regional heritages, and important elements of our quality of life.

Climate and Ecosystems

Climatic conditions determine where individual species of plants and animals can live, grow, and reproduce. Thus, the collections of species that we are familiar with -- the southeastern mixed deciduous forest, the desert ecosystems of the arid Southwest, or the productive grasslands of the Great Plains -- are influenced by climate as well as other factors such as land-use. The species in some ecosystems are so strongly influenced by the climate to which they are adapted that they are vulnerable even to modest climate changes. For example, alpine meadows at high elevations in the West exist where they do entirely because the plants that comprise them are adapted to the cold conditions that would be too harsh for other species in the region. The desert vegetation of the Southwest is adapted to the high summer temperatures and aridity of the region. Forests in the east are 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.

There are also many freshwater and marine examples of sensitivities to climate variability and change. In aquatic ecosystems, for example, many fish can breed only in water that falls within a narrow range of temperatures. Thus, species of fish that are adapted to cool waters can quickly become unable to breed successfully if water temperatures rise. Wetland plant species can adjust to rising sea levels by dispersing to new locations, within limits. Too rapid sea-level rise can surpass the ability of the plants to disperse, making it impossible for coastal wetland ecosystems to re-establish themselves.

Both temperature and precipitation limit the distribution of plant communities. The climate (temperature and precipitation) zones of some of the major plant communities (such as temperate forests, grasslands, and deserts) in the US are shown in this figure. Note that grasslands' zone encloses a wide range of environments. This zone can include a mixture of woody plants with the grasses. The shrublands and woodlands of the West are examples of grass/woody vegetation mixes that occur in the zone designated as grasslands.

With climate change, the areas occupied by these zones will shift relative to their current distribution. Plant species are expected to shift with their climate zones. The new plant communities that result from these shifts are likely to be different from current plant communities because individual species will very likely migrate at different rates and have different degrees of success in establishing themselves in new places.

Effects of Increased CO₂ Concentration on Plants

The ecosystem models used in this Assessment consider not only changes in climate, but also increases in atmospheric CO₂. The atmospheric concentration of CO₂ affects plant species in ecosystems since it has a direct physiological effect on photosynthesis, the process by which plants use CO₂ to create new biological material. Higher concentrations of CO₂ generally enhance plant growth if the plants also have sufficient water and nutrients, such as nitrogen, to sustain this enhanced growth.

For this reason, the CO₂ levels in commercial greenhouses are sometimes boosted in order to stimulate plant growth. In addition, higher CO₂ levels can raise the efficiency with which plants use water. Different types of plants respond at different rates to increases in atmospheric CO₂, resulting in a divergence of growth rates due to CO₂ increase. Some species grow faster, but provide reduced nutritional value. The effects of increased CO₂ level off at some point; thus, continuing to increase CO₂ levels will not result in increased plant growth indefinitely. There is still much we do not understand about the CO₂ �fertilization -- effect, its limits, and its direct and indirect implications.

Species Responses to Changes in Climate and CO₂

The responses of ecosystems to 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 of changes in temperature and precipitation, and in their responses to changes in CO₂; thus, new climate conditions are very likely to result in current ecosystems breaking apart, and new assemblages of species being created. Current ecosystem models have great difficulty in predicting these kinds of biological and ecological responses, thus leading to large uncertainties in projections.

What the Models Project

Modeling results to date indicate that natural ecosystems on land are very likely to be highly sensitive to changes in surface temperature, precipitation patterns, other climate parameters, and atmospheric CO₂ concentrations. Two types of models utilized in this Assessment to examine the ecological effects of climate change are biogeochemistry models and biogeography models. Biogeochemistry models simulate changes in basic ecosystem processes such as the cycling of carbon, nutrients, and water (ecosystem function). Biogeography models simulate shifts in the geographic distribution of major plant species and communities (ecosystem structure).

The biogeochemistry models used in this analysis generally simulate increases in the amount of carbon in vegetation and soils over the next 30 years for the continental US as a whole. These probable increases are small -- in the range of 10% or less, and are not uniform across the country. In fact, for some regions the models simulate carbon losses over the next 30 years. One of the biogeochemistry models, when operating with the Canadian climate scenario, simulates that by about 2030, parts of the Southeast will likely lose up to 20% of the carbon from their forests. A carbon loss by a forest is treated as an indication that it is in decline. The same biogeochemistry model, when operating with the Hadley climate scenario, simulates that forests in the same part of the Southeast will likely gain between 5 and 10% in carbon in trees over the next 30 years.

Why do the two climate scenarios result in opposite ecosystem responses in the Southeast? The Canadian climate scenario shows the Southeast as a hotter and drier place in the early decades of the 21st century than does the Hadley scenario. With the Canadian scenario, forests will be under stress due to insufficient moisture, which causes them to lose more carbon in respiration than they gain in photosynthesis. In contrast, the Hadley scenario simulates relatively plentiful soil moisture, robust tree growth, and forests that accumulate carbon.