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Climate Change and Extreme Weather-Related Event Discussion and Conclusions

Wildfire—The rapid response of fire regimes to changes in climate can potentially overshadow the direct effects of climate change on species distribution, migration, or extinction (Flannigan and others 2000, Stocks and others 1998).

Hurricanes—The effects of hurricanes on forest vegetation include sudden, massive, and complex patterns of tree mortality and altered patterns of forest regeneration (Lugo and Scatena 1996). A likely result is lower aboveground biomass in mature stands (Lugo and Scatena 1995). Faster rates of decomposition and vegetation regrowth have been measured after hurricanes; species substitutions, with those species having faster nutrient and biomass turnover rates becoming more competitive, may result (Lugo 2000). Hurricanes can also bury vegetation in carbon sinks, increasing belowground carbon storage (Dale and others 2000, Lugo 2000). Overall, it has been suggested that the decadal variation in hurricane intensity and frequency may be great enough to mask any changes resulting from climate change (Lugo 2000).

Tornadoes—Damage resulting from tornadoes may shift forest species composition towards late-successional species, as early successional species often are large and shallow rooted, making individuals more vulnerable. Because late-successional species may share these traits, effects of tornadoes or other catastrophic winds on species composition may be more contingent on forest species and size characteristics (Peterson 2000). Wind disturbances often remove dominant trees from the forest, changing species richness or evenness and potentially altering species diversity (Peterson 2000).

Floods—It is difficult to translate changes in precipitation patterns to effects on flood probability or severity. Existing flood records suggest that monitoring runoff and stream-flow levels may provide more insight on future floods (Intergovernmental Panel on Climate Change 1998).

At predicted levels of increase, sea level rise would threaten coastal areas with more frequent flooding, salinization of coastal streams and aquifers, and increased beach erosion. It is important to consider that local sea levels are also affected by regional processes such as ocean tides and currents (Gornitz 2001).

Drought—Secondary effects of drought may occur. When reductions in growth are extreme or sustained over multiple growing seasons, increased susceptibility to insects or disease is possible, especially in dense stands (Negron 1998). Drought may also reduce decomposition rates, leading to a buildup of organic matter on the forest floor. This buildup may reduce nutrient cycling or increase fire frequency or intensity.

The consequences of drought depend on annual and seasonal climate changes and the ability of current drought adaptations to provide resistance or resilience to new conditions. Forests are likely to grow to a level of maximum leaf area, using nearly all the available soil water in the growing season (Neilson and Drapek 1998). A significant increase in growing season temperatures could increase evaporation and trigger moisture stress.

If changes in regional precipitation reduce soil moisture, there may be direct impacts on plant foliage water status that modify carbon assimilation (Hanson and Weltzin 2000).

Overall, reductions in total annual rainfall would not increase drought severity in most forests of the South because early season rainfall is the most important determinant of total growth (Hanson and Weltzin 2000). However, there are different responses to late-season drought for hardwoods and pines of the Eastern United States. Hardwood growth activity does not overlap with drought occurrence, and therefore basal area growth is relatively unaffected. Because conifer stems grow during a greater portion of the growing season, their drought susceptibility is greater (Hanson and Weltzin 2000).

Ice Storms—Though the weather conditions producing ice storms are well understood, it is uncertain how climate change will influence the frequency, location, extent, or intensity of these extreme weather events. Jagger and others (1999) state that warmer winter temperatures brought about by climate change may increase the probability of ice storms across portions of the United States. Continued atmospheric warming will likely shift the distribution of ice storms northward, potentially decreasing the frequency and severity of ice storm damage to southern forests (Dale and others 2000, Irland 2000).

Climate change—According to the PnET-II and HadCM2Sul predictions, forest productivity increased more for hardwood and mixed pine-hardwood forest types than for pine plantations. The primary reason for this conclusion is the greater annual water demands of pine forest types. Even with increasing WUE resulting from increasing atmospheric CO₂, evapotranspiration rates increase with air temperature, and pines are still water limited under the HadCM2Sul climate scenario. Sensitivity analyses completed for PnET-II and the HadCM2Sul scenario showed that substantial variation in temperature increase might lead to larger net losses in forest area and productivity (National Assessment Synthesis Team 2001).

Elevated CO₂ influences tree physiology, potentially increasing productivity, WUE, and nutrient-(nitrogen) use efficiency. Reviews of CO₂-enrichment studies have shown positive but variable biomass accumulation. Interactions between CO₂ and other environmental factors account for some of the wide response range (National Assessment Synthesis Team 2001). For example, in a recent North Carolina field experiment, growth of loblolly pine increased by 25 percent under continuous CO₂ elevation (National Assessment Synthesis Team 2001). Maintaining such responses on a decadal time scale could mean greater carbon storage potential and increased drought tolerance. For some species, however, acclimation to increased CO₂ levels has included a reduction in photosynthesis (Intergovernmental Panel on Climate Change 1998). Such down regulation may occur when nutrient availability does not increase with CO₂ (National Assessment Synthesis Team 2001). Recent studies point out that acclimation to CO₂ may not be as widespread when roots are unconstrained and that leaf conductance may not be reduced. In this case, forests might produce more leaf area under elevated CO₂, but, because transpiration could also increase under increased temperatures, soil drying and drought effects could result (Intergovernmental Panel on Climate Change 1998).

If precipitation patterns decrease across the region, rates of evaporation and transpiration could increase without offset, resulting in declines in runoff and consequent drops in river flows, groundwater levels, and recharge. Alternatively, if substantial increases in precipitation occur, increases in runoff and river flows could be expected (Intergovernmental Panel on Climate Change 1998).

Wetlands may be particularly affected by variability in the amount and seasonality of rainfall. As a result, flood protection, water filtering, carbon storage, and other wetland functions may be significantly altered (Intergovernmental Panel on Climate Change 1998).

Results from the biogeography models suggest a northward shift in forest productivity over the next century, but they do not consider changes in management that could potentially ameliorate adverse effects. In summary, forest productivity in the South will likely increase over the next century as a result of atmospheric CO₂ enrichment, provided that: (1) precipitation and temperature changes do not offset the enrichment benefits by inducing water stress, and (2) abiotic stressors such as O₃ do not reduce growth rates significantly. Strategies to increase WUE or water availability could be used to prepare for a potentially warmer and drier climate.

Interactions between climate, extreme weather-related events, and forest health—Disturbance effects often cascade. Drought may weaken tree vigor, leading to insect and disease infestations or fire. Disease and insect infestations promote future fires by increasing fuel loads. Fires then promote future infestations by compromising tree defenses.

Changes in forest management, land use, and atmospheric chemistry interact with natural disturbances. For example, in the Southern Appalachian Mountains, climate change, increased O₃ exposure, continued acid deposition, and infestations of non-native insects may increase stress and mortality in red spruce and Fraser fir forests. In some combinations, negative impacts from disturbances may be ameliorated: under drought conditions, leaf stomata tend to close, reducing the effects of elevated O₃ exposure on seedlings (McLaughlin and Percy 1999).

Interactions between extreme weather events are common in the South, and the impacts of multiple extreme events are greater than the sum of the individual events (Paine and others 1998). For example, although hurricanes rapidly lose strength after reaching land, sustained winds of over 40 miles per hour may occur hundreds of miles inland. Soil saturation, which occurs when large amounts of rain accompany the winds, can reduce tree-root support. Under these conditions, even a moderate wind can blow down a mature tree. Without these multiple stresses, little or no forest damage would have occurred.

Interactions between extreme weather events are further complicated by the effects of other forest ecosystem stressors. Drought often weakens tree vigor, increasing the potential for insect or disease attacks. If tree mortality results from these combined stresses, fuel loads and the likelihood of future wildfires can also increase. An example of interactions of this type can be observed in the Southern Appalachian Mountains, where increased O₃ exposure and periodic drought have increased the infestation rate of native and non-native insects in red spruce and Fraser fir forests. The combined stressor effects are partially responsible for increased mortality in these high-elevation tree species. Climate change may cause these integrated events and their compounded influences to occur slowly, unpredictably, and in unique configurations.

Understanding the effects of climate change on extreme weather events is critical for managing broad-scale disturbances before, during, and after they occur. Forest management could play a key role in minimizing negative forest responses, thus sustaining forests through long-term climate change and short-term intense weather events.