Effects of fire intensity on groundcover shrubs in a frequently burned longleaf pine savanna

Abstract

Variation in fire intensity may affect the structure and composition of frequently burned plant communities. I hypothesize that small-scale variation in fuel loads affects fire intensity within single fires in frequently burned savannas. Furthermore, I expect that local differences in fire intensity produce predictable patterns to the demography of groundcover shrubs. I tested these hypotheses by manipulating fuel loads in a longleaf pine savanna. Fuel load treatments consisted of fine fuel removal, pine needle addition, wood addition and unaltered control, mimicking naturally occurring fuel load variation. Treatments were repeated in 1m2 subplots over two sets of biennial burns and shrub responses were followed for 5 years. Addition of fuels increased fire intensity (maximum fire temperature, fuel consumption, heat output), while removal of fuels decreased fire intensity relative to controls. For all species pooled, addition of fuels, particularly wood, increased damage to shrubs and decreased resprouting relative to control. Removal of fuels did not affect shrub resprouting. Similarly, fuel addition increased the probability of species loss (genet mortality), while fuel removal had no effect. Fuel addition decreased resprouting of native rhizomatous resprouting species relatively more than that of native root-crown resprouting species or non-native species. However, density of rhizome sprouting shrubs tended to increase over the course of the study. The demography of Rhus copallinum, a common rhizomatous species, was affected by fuel load treatments. Matrix projection models indicated that population growth was positive in the control treatment, nearly stable in the pine needle addition treatment, and negative in the fuel removal and wood addition treatments. In all treatments, stasis of large ramets was the largest contributor to population growth. Effects of fuel loads and fire intensity on population dynamics of R. copallinum may result from an interaction between damage to rhizomes and post-fire environmental conditions. My overall results indicate that small-scale variation in fuel loads predictably affects local fire intensity in pine savannas. By altering fire intensity, fuel loads alter patterns to the local demography of shrubs. Small-scale variation in fire intensity may contribute significantly to the observed patterns of shrub abundance in frequently burned pine savannas.