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Physical Factors

Primary Question (chapter 18): How have abiotic factors including environmental stressors such as air pollution influenced the overall health of the South’s forests, and what are future effects likely to be?

Southern forests have evolved under the influence of a number of physical disturbances. Among the most important is fire. Fire-adapted ecosystems are maintained by frequent, low-intensity fires but may be substantially altered by intense fires. The suppression of fire—an objective of public forestry programs throughout much of the 20^th century—has altered these systems. The effective use of fire in forested ecosystems is an ongoing challenge for forest managers as humans continue to populate forested areas, and safety and smoke management concerns restrict burning options.

Wind-related disturbances also influence forests in much of the South. Hurricanes alter forest structure over large areas of the Coastal Plain and Piedmont, and microbursts and tornadoes create smaller openings throughout the region. Ice storms limit the northern range of some tree species.

In addition, the growth, development, and species composition of forests depend on the ambient environment. Alterations to the environment, especially through air pollutants such as atmospheric ozone, have the potential to affect forest health and vigor. The natural geographic range in which forest species occur and thrive can be influenced by temperature and moisture regimes. Some model scenarios forecast that potential climate changes could alter the ranges of commercially and ecologically important species. There are some ecotypes that may be especially sensitive to small changes in climate and environment.

We examined trends and potential future conditions for various physical stressors in the South (chapter 18). We found that:

• While effects are variable, acid deposition does not pose a significant threat to forest vegetation or to stream chemistry through a vast majority of the South. The major exception is the Southern Appalachians where, because of soil conditions, continued acid deposition derived from sulfur dioxide and the oxides of nitrogen is expected to alter plant communities and stream chemistry.

• Ozone pollution is forecast to increase anywhere from 20 to 50 percent between 1990 and 2025, but concentrations are and will continue to be highly variable across the South (fig. 12). Ozone has reduced and will continue to reduce the growth rates of pine species in the South, and future ozone damage could be exacerbated by a warming climate.

• Atmospheric carbon (CO₂) is expected to rise throughout the 21^st century, and current models forecast a resulting increase in air temperatures. Overall, the moderate increases that are projected would enhance the growth of trees in the South. However, these models suggest that if temperatures were to rise somewhat more than expected, then the area and the productivity of forests would decline. These changes are uncertain and could vary across the region.

• There is a high degree of uncertainty about the direction and magnitude of forest impacts from projected climate changes due to the importance of interactions among precipitation, air temperature, and forest growth.

• Various studies show that there could be a general increase in the number and/or severity of extreme weather events with climate change. However, we lack the knowledge to predict with confidence the magnitudes of these changes.

• Southern forests could play an important role in sequestering atmospheric carbon (fig. 13), which could aid in mitigating the negative effects of climate change.

• Southern vegetation has long been associated with fire regimes. As in other regions, fire suppression has altered species composition and forest structure and elevated the risk of catastrophic fire. Reestablishing historical fire regimes is an important part of efforts to restore longleaf pine systems, but other forest types—including Table Mountain pine, shortleaf pine-bluestem, and various oak types—could also benefit from effective reintroduction of fire. Increasing population densities in rural areas and regulations limiting smoke emissions will make these activities more complicated.