Meteorological and Ecological Perspectives on the Patterns and Consequences of Wind Disturbance to Central Hardwood Forests
Wind disturbance is one of the most pervasive types of natural disturbance, perhaps second only to fire in the variety of forests impacted, and in the resulting changes for forest structure, composition and functioning. Here we attempt to synthesize available information on the size, frequency, seasonality, and broad-scale spatial patterns of several of the major types of wind disturbance - thunderstorms, derechos, tornadoes, hurricanes, and the recently-recognized mountain wave winds. Each of these types of storms has particular meteorological causes and dynamics, which we will describe in light of potential to impact forests. Each also has characteristic geographic and topographic trends, such that particular geographic and topographic locations are more or less likely to experience wind disturbance. The sizes of, and wind velocities within, the meteorological phenomena directly translate into spatial patterns, extent, and severity of impacts to forest, which again differ among the types of storms. We further summarize what is known about how such disturbances alter the structure, composition and functioning of southeastern hardwood forests. At the community level we explore how wind disturbances change species composition, relative abundances and dominance among species, as well as richness and diversity. At the ecosystem level we summarize known and hypothesized effects on trace gas fluxes, nutrient cycling, and carbon dynamics. In so doing, we will highlight existing gaps in our knowledge, and suggest fruitful areas for near-future research to answer important questions. Finally, we consider lessons of these wind events for management that seeks to emulate natural disturbances.
1 Dept. of Plant Biology, University of Georgia, Athens, GA; 2 Dept. of Atmospheric Sciences, Univ. of North Carolina at Asheville
36 • Henri Grissino-Mayer1, Charles Lafon2
Fire Regimes of the Southern Appalachian Central Hardwood Forests: Effects of Climatic and Human Influences
Forests of the central hardwood region include pine, oak, and other communities that benefit from fire. Some plant species (such as Table Mountain pine) exhibit adaptations to the effects of repeated wildfires over millennia. Direct and indirect evidence of these past fires can be found in many habitat types, especially those where yellow pines constitute part of the forest type. This evidence is in two forms. First, fire scars form when a fire damages the trunk of yellow pines and hardwoods; these can be dated to an exact year in the tree-ring record. Second, forests can change in species composition, age structure, and forest structure with changes in fire activity, whether caused by natural processes or by humans. In the southern Appalachian Mountains, we created a network of fire history sites that reveals fire to have been common from at least the 1600s/1700s until the fire-exclusion era, contradicting the perception that fire was an uncommon ecosystem process in the humid Appalachian region. Fire was not strongly related to drought conditions at most sites, possibly because of overriding influences of human ignitions in our data. However, spatial patterns of fire were consistent with greater fire activity in places with a more favorable fire climate. Current fire activity across the central hardwoods region varies spatially with climate; if pronounced spatial patterns existed across the region in the past, these patterns may account for some vegetation patterns we observe today.
1 Dept of Geography, University of Tennessee-Knoxville, Knoxville, TN; 2 Dept of Geography, Texas A&M University, College Station, TX
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37 • Charles Lafon
Ice Storms in Central Hardwood Forests: The Disturbance Regime and Vegetation Influences
Freezing rain occurs more commonly in eastern North America than elsewhere on earth. During a major freezing rain event, or ice storm, heavy ice loads accrete onto trees, stripping branches, snapping boles, and uprooting trees. Freezing rain commonly develops throughout a large area within a winter storm, and therefore a single storm can damage broad swaths of forest. These disturbances greatly exceed background gap formation in severity and extent, but they do not generate the catastrophic mortality associated with severe fires or windstorms. Ice storms create disturbances of intermediate severity. A paucity of appropriate weather data obscures freezing rain climatology, but some generalizations are possible. Ice storms occur most frequently in the northeastern and Midwestern U.S., and a zone of high frequency also extends southward along the Appalachian Mountains and Piedmont. In mountainous landscapes, disturbance severity varies strongly over elevational and topographic gradients. Ice storms appear to be a primary disturbance agent on slopes with greatest exposure to these periodic events. A number of studies have investigated ice storm damage at the scale of forest stands. Tree species vary in susceptibility, with pioneer species typically sustaining heaviest damage and mortality. Less is known about longer-term consequences for age structure and species composition. The large gaps appear to provide recruitment opportunities for light-demanding colonizers. At the same time, the damage-resistance of slow-growing, stress-tolerant trees such as hemlocks and oaks likely enable persistence in ice-prone forests. The ecological role of ice storms depends, in part, on interactions with fires and other disturbances.
Dept of Geography, Texas A&M University, College Station, TX
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38 • Titian Ghandforoush, Kathleen A. Kron
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