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.
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
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
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 ï¿½fertilization -- 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.