The natural vegetation
covering about 70% of the US land surface is strongly influenced
the climate and by the atmospheric carbon dioxide (CO2)
provide a common base of information about potential changes in
across the nation for use in the regional and sector studies,
ecosystem models were run using the two
major climate model scenarios selected for this Assessment. A
of the national level results follows. Agricultural and 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
exist. They can be characterized by their biological richness, by
magnitude of flows of energy and materials between their
species and their physical environment, and by the interactions
biological species themselves, that is, by which species are
prey, which are competitors, and which are symbiotic.
Ecologists often categorize
ecosystems by their dominant vegetation -- the deciduous
forest ecosystems of New England, the short-grass prairie
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
but also for plantation forests and agricultural systems, although
ecosystems obviously differ in many important ways from the
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
natural to the most extensively managed, produce a variety of
services that benefit humans. Some of these enter the market and
contribute directly to the economy. Thus, forests as sources of
pulpwood, and agro-ecosystems as sources of food are important to
ecosystems also provide a set of un-priced services that are
that typically are not traded in the marketplace. There is no
market, for example, for the services that forests and wetlands
for improving water quality, regulating stream flow, and providing
measure of protection from floods. However, these services are
valuable to society.
Ecosystems are also valued
for recreational, aesthetic, and ethical reasons. These are also
to value monetarily, but are nevertheless important. The bird life
coastal marshes of the Southeast and the brilliant autumn colors
New England forests are treasured components of our regional
and important elements of our quality of life.
Climate and Ecosystems
determine where individual species of plants and animals can live,
and reproduce. Thus, the collections of species that we are
-- the southeastern mixed deciduous forest, the desert
ecosystems of the
arid Southwest, or the productive grasslands of the Great Plains
influenced by climate as well as other factors such as land-use.
species in some ecosystems are so strongly influenced by the
which they are adapted that they are vulnerable even to modest
changes. For example, alpine meadows at high elevations in the
where they do entirely because the plants that comprise them are
to the cold conditions that would be too harsh for other species
region. The desert vegetation of the Southwest is adapted to the
summer temperatures and aridity of the region. Forests in the east
adapted to relatively high rainfall and soil moisture; if drought
conditions were to persist, grasses and shrubs could begin to
tree seedlings, leading to completely different ecosystems.
There are also many
freshwater and marine examples of sensitivities to climate
change. In aquatic ecosystems, for example, many fish can breed
water that falls within a narrow range of temperatures. Thus,
fish that are adapted to cool waters can quickly become unable to
successfully if water temperatures rise. Wetland plant species can
to rising sea levels by dispersing to new locations, within
rapid sea-level rise can surpass the ability of the plants to
making it impossible for coastal wetland ecosystems to
Both temperature and
precipitation limit the distribution of plant communities.
climate (temperature and precipitation) zones of some of
plant communities (such as temperate forests, grasslands,
deserts) in the US are shown in this figure. Note that
zone encloses a wide range of environments. This zone can
a mixture of woody plants with the grasses. The shrublands
woodlands of the West are examples of grass/woody
that occur in the zone designated as grasslands.
With climate change,
the areas occupied by these zones will shift relative to
current distribution. Plant species are expected to shift
their climate zones. The new plant communities that result
these shifts are likely to be different from current plant
communities because individual species will very likely
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
in atmospheric CO2. The atmospheric concentration of CO2 affects
species in ecosystems since it has a direct physiological effect
photosynthesis, the process by which plants use CO2 to create new
biological material. Higher concentrations of CO2 generally
growth if the plants also have sufficient water and nutrients,
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
efficiency with which plants use water. Different types of plants
at different rates to increases in atmospheric CO2, resulting in a
divergence of growth rates due to CO2 increase. Some species grow
but provide reduced nutritional value. The effects of increased
off at some point; thus, continuing to increase CO2 levels will
in increased plant growth indefinitely. There is still much we do
understand about the CO2 fertilization -- effect, its limits,
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
their constituent species and how they interact with each other.
in current ecosystems can differ substantially in their tolerances
changes in temperature and precipitation, and in their responses
changes in CO2; thus, new climate conditions are very likely to
current ecosystems breaking apart, and new assemblages of species
created. Current ecosystem models have great difficulty in
these kinds of biological and ecological responses, thus leading
uncertainties in projections.
What the Models Project
Modeling results to date
indicate that natural ecosystems on land are very likely to be
sensitive to changes in surface temperature, precipitation
climate parameters, and atmospheric CO2 concentrations. Two types
models utilized in this Assessment to examine the ecological
climate change are biogeochemistry models and biogeography models.
Biogeochemistry models simulate changes in basic ecosystem
as the cycling of carbon, nutrients, and water (ecosystem
Biogeography models simulate shifts in the geographic distribution
major plant species and communities (ecosystem structure).
The biogeochemistry models
used in this analysis generally simulate increases in the amount
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%
and are not uniform across the country. In fact, for some regions
models simulate carbon losses over the next 30 years. One of the
biogeochemistry models, when operating with the Canadian climate
simulates that by about 2030, parts of the Southeast will likely
to 20% of the carbon from their forests. A carbon loss by a forest
treated as an indication that it is in decline. The same
model, when operating with the Hadley climate scenario, simulates
forests in the same part of the Southeast will likely gain between
10% in carbon in trees over the next 30 years.
Why do the two climate
scenarios result in opposite ecosystem responses in the Southeast?
Canadian climate scenario shows the Southeast as a hotter and
in the early decades of the 21st century than does the Hadley
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
scenario simulates relatively plentiful soil moisture, robust tree
and forests that accumulate carbon.
The maps above show
projections of relative changes in vegetation carbon
and the 2030s for two climate scenarios. Under the
scenario, vegetation carbon losses of up to 20% are
some forested areas of the Southeast in response to
drying of the region by the 2030s. A carbon loss by
treated as an indication that they are in decline. Under
scenario, vegetation carbon increases of up to 20% are
in the forested areas in the West that receive substantial
increases in precipitation. Output
from Terrestrial Ecosystem Model (TEM) as part of the Vegetation
Modeling and Analysis Project (VEMAP II) study.
Prolonged stress due to
insufficient soil moisture can make trees more susceptible to
attack, lead to plant death, and increase the probability of fire
plant material adds to an ecosystem's "fuel load." The
biogeography models used in this analysis simulate at least part
sequence of climate-triggered events in ecosystems as a prelude to
in the geographic distribution of major plant species. One of the
biogeography models, when operating with the Canadian climate
simulates that towards the end of the 21st century, a hot dry
the Southeast will result in the replacement of the current mixed
evergreen and deciduous forests by savanna/woodlands and
much of the change involving fire. This change in habitat type in
Southeast would imply that the animal populations of the region
change, although the biogeography models are not designed to
these changes. The same biogeography model, when operating with
scenario, simulates a slight northward expansion of the mixed
and deciduous forests of the Southeast with no significant
along the southern boundary. Other biogeography models show
Major uncertainties exist in
the biogeochemistry and biogeography models. For example,
uncertain about how increases in atmospheric CO2 affect the carbon
water cycles in ecosystems. What they assume about these CO2
significantly influence model simulation results. One of these
used to show the importance of testing these assumptions.
climate change alone results in a 10% decrease in plant
Consideration of both climate and CO2 effects results in an
plant productivity of 10%. This illustrates the importance of
uncertainties about the effects of CO2 on ecosystems.
With respect to biogeography
models, scientists are uncertain about the frequency and size of
disturbances produced by factors such as fire and pests that
changes in the distribution of major plant and animal species.
disturbances caused by climate change be regular and small or will
episodic and large? The latter category of disturbances is likely
a negative impact on ecosystems services; the ability of
cleanse the air and water, stabilize landscapes against erosion,
carbon, for example, are very likely to be diminished.
Maps of current and
projected potential vegetation distribution for the
US. Potential vegetation means the vegetation that would
in the absence of human activity. Changes in vegetation
distribution by the end of the 21st century are in
response to two
climate scenarios, the Canadian and the Hadley. Output is
Atmosphere-Plant-Soil System (MAPSS).
portion of the Southeast's mixed forest is replaced by a
combination of savanna and grassland in response to fire
warming and drying of the region as projected by the
model. The Hadley climate projection leads to a simulated
northward expansion of the mixed forest.
model runs show the response of vegetation to atmospheric
concentrations of CO2 that have stabilized at about 700
million, approximately twice the present level.
In the Southwest,
large areas of arid lands are replaced with grassland or
shrub/woodland in response to increases in precipitation
by both models.