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5.1 Extreme Weather-Related Event Methodology: Current Conditions

Climate effects on forest conditions are most strongly expressed by extreme events such as fire, hurricanes, tornadoes, floods, drought, and ice storms (Dale and others 2000). Each type of event affects forests differently; some cause large-scale tree mortality, whereas others, such as ice storms, impact community structure and organization without causing massive mortality.

5.1.1 Wildfire

The frequency, seasonality, size, intensity, and type of wildfires depend on weather phenomena and forest structure and composition. Fire initiation and spread also depend on fuel availability, the presence of ignition agents, and topography.

Across the Southern Coastal Plain, forest shrub and brush species can create highly flammable fuel conditions in just 5 years under the right climatic conditions, if fuel loads are not managed. Fuel management, therefore, is necessary. Each year, across all land ownership classes, 5.4 million acres are managed with prescribed fire. Seventy-five percent of the prescribed burning occurs in the States of Alabama, Florida, and Georgia. All 34 national forests in the region have prescribed fire programs, and since 1944, approximately 21 million acres have been treated to minimize wildfire risk (Forest Health Protection Program 2000). Fire management would be more prevalent were it not for smoke problems associated with controlled burns. Criteria included in the Environmental Protection Agency's National Ambient Air Quality Standards for Particulate Matter (U.S. EPA 1997b) limit the amount and extent of prescribed fire programs, because smoke can impair road visibility and breathing in sensitive individuals.

Wildfire can substantially influence forest structure and function. Ecological effects of forest fires include mortality of individual trees, shifts in successional direction, induced seed germination, acceleration of nutrient cycling, death of seeds stored in the soil, changes in surface-soil organic layers and underground plant root and reproductive tissues, volatilization of soil nutrients, and increased landscape heterogeneity (Whelan 1995). As a result of these effects, the capacity of forests to provide wildlife habitat, timber, and recreation may be diminished (Flannigan and others 2000).

5.1.2 Hurricanes

Hurricanes disturb forests along the coastlines of the South. Ocean temperatures and regional weather influence the path, size, frequency, and intensity of hurricanes (Emanuel 1987). An average of two hurricanes strike land every 3 years in the United States (NOAA 1997). Some scientists have hypothesized that hurricane impacts on forests, including mortality, may be related to soil characteristics (Duever and McCollum 1993).

5.1.3 Tornadoes

Tornadoes are one of the most important agents of abiotic disturbance in eastern deciduous forests. Nearly 1,000 tornadoes occur each year in the Conterminous United States (Peterson 2000). In the South, tornadoes are very common in Oklahoma and Texas and frequent in Alabama, Florida, Louisiana, and Mississippi. Tornadoes can cause severe mortality, reduce tree density, alter stand size structure, and modify local environmental conditions via soil erosion or nutrient loss (Dale and others 2000). The resulting disturbance may bring about the release of advance regeneration, seed germination, or accelerated seedling growth (Peterson and Pickett 1995). These effects can change gap dynamics, successional patterns, and other ecosystem-level processes such as water use. The relationship between wind strength and severity of disturbance varies by tree species and forest type. Shallow-rooted species and thinned stands tend to be more vulnerable, but multiple factors influence tree response to windstorms.

5.1.4 Floods

Floods occur throughout the South but are most concentrated in coastal and floodplain areas. On average, floods cause almost $4 billion dollars in damage each year (NOAA 2000). Upland forest ecosystems that experience flooding respond with reduced photosynthetic rates; over extended periods, changes in tree species composition are possible, as some species are more flood-tolerant that others (Burke and others 1999, Iles 1993). Most trees can withstand 1 to 4 months of flooding duration without significant injury (Bratkovich and others 1993). In extreme situations, higher mortality rates may occur (Iles 1993). Anaerobic soil conditions in flooded areas cause physiological stress and influence nutrient availability (Burke and others 1999). Secondary effects of flooding include elevated soil erosion and sedimentation rates (Iles 1993). At the regional scale, there is high variability in the spatial location and amount of disturbance associated with floods.

5.1.5 Drought

Droughts occur in most forest ecosystems in the South. Occurrence is irregular in forests east of the Mississippi River, occasional across most of the South, and more common in late summer on the Coastal Plain (Hanson and Weltzin 2000). Consequences long-term drought or, flooding are generally proportional to the area affected; during the past few decades, an increasing portion of the United States has experienced either severe drought or flooding (Karl and others 1995b). Drought effects are influenced by soil texture and depth, exposure, species composition, life stage, and the frequency, duration, and severity of drought. The immediate response of forests to drought is to reduce water use and growth. Small plants, including seedlings and saplings, are usually the first to succumb to moderate drought conditions. Deep rooting and stored carbohydrates and nutrients make large trees susceptible only to severe droughts (Dale and others 2001).

5.1.6 Ice Storms

Ice storms occur throughout the South. They are produced when rain falls through subfreezing air masses, freezing when contact is made with objects on the ground. Ice accumulation varies with topography, elevation, and area of exposure. Ice storms may sever twigs and bend or break stems, causing moderate crown loss. Damage to forest stands can range from light and patchy to the breaking of all mature stems, depending on stand composition, past disturbances, and the amount of ice accumulation (Irland 2000). Effects of ice storms on forest stands include stem damage, loss of growth until leaf area is restored, and possible shifts in tree species composition towards trees more resistant to ice damage.

Recently thinned stands may have increased vulnerability to ice storm damage, because tree crowns have spread into openings but branch strength has not increased yet. Potentially, there are several secondary consequences of ice damage. Susceptibility to insects and diseases may be increased, and fuel loads may accrue, heightening wildfire risk in some areas (Irland 2000).