Predictors of fire-tolerant oak and fire-sensitive hardwood distribution in a fire-maintained longleaf pine ecosystem

Abstract

The longleaf pine (Pinus palustris) ecosystem has been reduced to a fraction of its original extent, and where this ecosystem does occur, it is often degraded by hardwood encroachment. The reduction of hardwood tree cover is often a desirable longleaf pine community restoration outcome, though hardwood midstory and overstory trees have been recognized as an important natural component of the communities. Moreover, the appropriate amount of hardwood tree cover in a restored longleaf pine community is debated, as more hardwood tree cover can benefit mixed forest and mast-dependent wildlife (e.g., fox squirrels [Sciurus niger], white-tailed deer [Odocoileus virginianus]), and less hardwood tree cover is critical to the federally endangered red-cockaded woodpecker (Leuconotopicus borealis). To inform the debate, we assessed the environmental (e.g., topography, edaphic conditions, and pine basal area) and management (e.g., distance to firebreaks, prescribed fire history) factors that influenced abundance of upland hardwood trees in xeric longleaf pine communities on a site where frequent growing-season fire has been ongoing since 1991. We counted upland hardwoods ≥5 cm diameter at breast height (DBH) at 307 random field plots (0.04 ha) and categorized all hardwood trees as belonging to either a guild of fire-tolerant oaks or a guild of fire-sensitive hardwood species. We used generalized linear models (GLM) to determine the most important predictors of abundance for both guilds. The predictors of abundance differed between the two guilds, with fire-tolerant oak abundance increasing with greater slope and proximity to ignition sources and decreasing with greater pine basal area. Fire-sensitive hardwood abundance increased with mesic site conditions and decreased with the number of growing-season fires and greater pine basal area. Although seasonality in fire history was an important predictor of fire-sensitive hardwood abundance, variables related to long-term fire-history were not important predictors of fire-tolerant oak abundance in longleaf pine communities. However, with limited variation in fire return interval across the study area, our ability to draw inferences regarding the role of fire return interval was limited. Where hardwood encroachment is not a problem, and hardwood levels are below desired, balanced target levels, hardwood abundance in longleaf pine communities can be increased by reducing pine basal area and reducing prescribed fire intensity.