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The projected 2°C (3.6°F) warming could shift the ideal range for many North American forest species by about 300 km (200 mi.) to the north. If the climate changes slowly enough, warmer temperatures may enable the trees to colonize north into areas that are currently too cold, at about the same rate as southern areas became too hot and dry for the species to survive. If the earth warms 2°C (3.6°F) in 100 years, however, the species would have to migrate about 2 miles every year.
 Trees whose seeds are spread by birds may be able to spread at that rate. But neither trees whose seeds are carried by the wind, nor such nut-bearing trees such as oaks, are likely to spread by more than a few hundred feet per year. Poor soils may also limit the rate at which tree species can spread north. Thus, the range over which a particular species is found may tend to be squeezed as southern areas become inhospitably hot. The net result is that some forests may tend to have a less diverse mix of tree species.
Several other impacts associated with changing climate further complicate the picture. On the positive side, CO2 has a beneficial fertilization effect on plants, and also enables plants to use water more efficiently. These effects might enable some species to resist the adverse effects of warmer temperatures or drier soils. On the negative side, forest fires are likely to become more frequent and severe if soils become drier. Changes in pest populations could further increase the stress on forests. Managed forests may tend to be less vulnerable than unmanaged forests, because the managers will be able to shift to tree species appropriate for the warmer climate.
Perhaps the most important complicating factor is uncertainty (see U.S. Climate in the Future Climate section) whether particular regions will become wetter or drier. If climate becomes wetter, then forests are likely to expand toward rangelands and other areas that are dry today; if climate becomes drier, then forests will retreat away from those areas. Because of these fundamental uncertainties, existing studies of the impact of climate change have ambiguous results.
Two different types of computer models have been employed to estimate the impact of climate change on forests: The "biogeography models" analyze the essential environmental conditions over entire continents, to estimate the type of vegetation that is most likely to cover a given area. The "gap models" simulate all of the dynamic relationships (e.g., large trees shading small trees) for small representative areas. Both types of studies generally report the total leaf area of all the trees in a forest.
 The biogeography models provide much more optimistic results than the gap models. Biogeography models suggest that if the beneficial effect of carbon dioxide is disregarded, then 12-76% of the mixed evergreen and deciduous forests in the United States would become thinner (i.e., less leaf area), while 2-49% would experience increases by the year 2100. When the likely effect of CO2 fertilization is included, however, less than 7 percent of the mixed forests are likely to decline, and more than 92 percent of those forests are likely to increase. Studies using the gap models, however, suggest that a large number of areas may no longer be able to support forests, particularly if the climate becomes drier.
The potential impacts of climate change on forest wildlife are poorly understood. If habitats simply shift to cooler areas (i.e., higher latitudes or higher altitudes), many forms of wildlife could potentially adapt to global warming, just as they have adapted to the changes in climate that have occurred over the last several million years. Unlike previous climatic shifts, however, roads, development, and other modifications to the natural environment may block the migration routes. Nature reserves, often established to protect particular species, may no longer be located in a climate hospitable to that species.
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