Effects of Fire on Biodiversity

Fire is an important agent for maintaining biodiversity at several different scales. Fire can alter species diversity, landscape diversity and dynamics, and ecosystem function. Only a few studies have examined the effects of fire on biodiversity in the southern Appalachians (Clinton and Vose 2000, Harrod and others 2000). The following discussion, therefore, mainly treats this topic on a general level, based on theories and observations made in more fire-prone systems.

Fires influence species richness and diversity in several ways. By acting as a disturbance, fire prevents species of late seral stages from excluding those of earlier stages (Connell 1978). According to Connells intermediate disturbance hypothesis, fires of intermediate frequency, intensity, season, and size would promote high levels of species diversity (Malanson 1987, Pickett and White 1985). However, since "intermediate" levels of disturbance differ among communities, knowledge of an areas fire history is crucial in understanding what fire regime would best maintain the full complement of native plant species. Variations in fire frequency alter the mixture of species that dominate a landscape (Shugart 1984), and thus maintain high levels of heterogeneity and prevent fire-dependent systems from reaching equilibrium (Buckner and Turrill 1999).

The few studies that have examined the effect of fire on biodiversity in the southern Appalachians have generally reported that single fires have increased plant species diversity, but results have been mixed. Clinton and Vose (2000) report that application of the fell-and-burn technique in the Nantahala National Forest increased both herbaceous and woody species diversity, but through different mechanisms. Herbaceous species diversity was increased primarily by increased species richness, while woody species diversity was increased through increased eveness. In an 18-year study on a xeric site in the southern Appalachians, Harrod and others (2000) found that summer and autumn fires increased plant species richness for the first decade after fire. A prescribed fire used to restore a degraded pine-hardwood community on a slope in the Nantahala National Forest had varying effects on species richness and diversity. On the ridge, where fire intensities were greatest, diversity significantly increased in the understory and herb-layer, but decreased in the overstory. On the middle of the slope, no change was observed in the overstory, but diversity significantly decreased in the understory. On the lower slope, where fire intensities were lowest, no change in diversity was observed in the overstory or understory (Elliott and others 1999).

Fire may also affect biodiversity at larger scales, increasing or decreasing landscape diversity. The effect of fires on landscape level diversity is thought to be a function of the size of burns. Smaller-scale fires are more sensitive to fuel moisture, fuel types, atmospheric humidity, wind, temperature, and topography, and thus create fire patches of varying intensity and severity. In contrast, large fires respond more to wind velocity and direction, are more uniformly severe, and therefore decrease landscape heterogeneity (Turner and others 1994). Fire alters landscape dynamics by creating internal patch boundaries that differ from those created by edaphic and topographic factors (Wiens and others 1985, Buckner and Turrill 1999).

Fire may also influence landscape diversity through its effects on ecosystem processes. Abiotic and biotic changes induced by fire alter nutrient cycling, successional processes, and stand composition, and, in turn, change landscape pattern (Turner 1989). Fire suppression eventually causes regional edaphic factors to become more uniform (Wiens and others 1985), making the landscape more homogeneous (Buckner and Turrill 1999).