FUELS CONSUMPTION AND NITROGEN LOSS FOLLOWING PRESCRIBED FIRE: A COMPARISON OF PRESCRIPTION TyPES IN THE SOUTHERN APPALACHIANS

Prescribed burning is frequently used as a tool for restoration of plant communities, wildlife habitat improvement, and site preparation. We compared and contrasted the effects of four burning prescriptions on forest floor and fine fuels consumption, and nitrogen loss. The burning treatments included dry (DU) and mesic (MU) understory burns, stand replacement (SR) burning, and fell and burn (F ) site preparation. On all sites, forest floor was sampled before and immediately after burning. It was separated into woody fuels (< 7.5 cm in diameter), the Oi layer (litter), and the Oa + Oe layer (fermentation plus humus), and dry weight and nitrogen content of each component was determined. Tiles with heat-sensitive chalk and paint were used to estimate flame intensity at 30 cm above the forest floor. Mean peak flame temperatures ranged from 700 oC for the F treatment to 169 oC for the MU burn. Except in the F treatment, which had a substantial amount of woody mass on the forest floor as a result of felling overstory trees and shrubs, the majority of pre-burn mass and nitrogen was contained in the humus layer. Following burning, mass loss ranged from 88 Mg ha (90 percent wood, 10 percent litter, < 5 percent humus) on the F treatment to 5 Mg ha (5 percent wood, 55 percent litter, 40 percent humus) on the MU burns. Nitrogen losses followed similar patterns – 292 kg ha (70 percent from wood, 28 percent litter, < 5 percent humus) on the F to 30 kg ha (65 percent from wood, 35 percent litter, < 5 percent humus) on the SR site. High-intensity ground fires may result in greater losses of site nutrients, and this may have negative shortand long-term consequences. INTRODUCTION In many ecosystems, fire is used as a management tool to enhance and protect overall stand health and productivity by reducing fuel loadings, thereby reducing the threat of catastrophic fire (Sanders and Van Lear 1987, Van Lear and Waldrop 1989). Prescribed fire can also be used to reduce competition that affects commercially desirable tree species and improve habitat for terrestrial wildlife (Cooper 1971). The use of fire to accomplish these goals is considered an attractive alternative to mechanical techniques of stand improvement, primarily because of reduced costs (Cooper 1971, bercrombie and Sims 1986). The continued (and expanded) use of prescribed fire in the southern ppalachians and elsewhere has raised interest in its effects on ecosystem integrity. Scientists are especially interested in determining how fire influences losses of key plant nutrients such as nitrogen (N). lthough total N pools are frequently in greater supply than is needed for plant growth, N is commonly a limiting nutrient to forest productivity (Keeney 1980, Vitousek and others 1982) because most is in unavailable organic forms (Vose 2000). Total ecosystem nitrogen may decrease in forested systems following fire (Neary and others 1984, Rapp 1990) due to volatilization of nitrogen stored in coarse and fine fuels, or to increased leaching of released NO 3 from the system, or to both of these causes (Knoepp and Swank 1993). In contrast, increases in total N following burning can result from a combination of increased abundance of symbiotic and non-symbiotic N-fixers. Similarly, N availability often increases due to increased N mineralization and decreased plant uptake (De ano 1991). Nitrogen losses during and following prescribed fire in southern forests range from 20 kg ha for low-intensity understory burns (Kodama and Van Lear 1980) to > 400 kg ha for high-intensity site preparation burns in heavy fuels (Vose and Swank 1993, Clinton and others 1996). The significance of 1 arton D. Clinton, Research Ecologist, and James M. Vose, Project Leader, USD Forest Service, Southern Research Station, Coweeta Hydrologic Laboratory, Otto, NC 28763.