Effects of long-term annual growing season prescribed fire on species richness and community composition and structure in longleaf pine-wiregrass savannas
Longleaf pine-wiregrass communities of the southeastern United States are adapted to frequent low-intensity surface fires. Though much debated, it is thought that the historical (i.e., pre-European settlement) peak of fire season occurred naturally during the summer due to increased incidence of lightning strikes. Today’s use of prescribed fire as a surrogate largely occurs prior to the growing season because late winter and early spring prescribed fires are safer to implement and control than those conducted in summer. Studies relating to long-term “season of burn” effects in longleaf pine-wiregrass ecosystems are still lacking. Therefore, we instituted a study to investigate the effects of long-term annual growing season fires on ground cover species richness and on community composition and structure. Species presence data has been collected at the end of each growing season since 2004 in ten 1 m2 plots in each of three burn units where growing season fires have been conducted annually since 2003. In 2012, ten “control” 1 m2 plots were established to record species presence in three burn units that are adjacent to the annually burned units and are generally burned every other year. Longleaf pine seedling and woody stem counts were conducted in all plots in 2015.
Ground cover species richness fluctuated annually, but there were no significant long-term trends in total richness in plots burned annually during the growing season. However, some functional groups did show significant change over time relative to annual growing season fire. The frequency of occurrence of woody and ruderal species decreased over time, while the frequency of fall-flowering asters increased. Abundance of some individual species changed over time, including a decrease in the frequency of three common C4 grasses. In addition to lower frequency of occurrence, the density of woody stems was significantly lower in annually burned plots relative to control plots. Annually burned plots were sites of substantial longleaf pine regeneration following a masting event in 2014, but it is hypothesized that longleaf pine seedling mortality will be higher at these sites than control sites in the future. The results of this study indicate that the long-term annual application of prescribed fire during the growing season can have desirable and possibly undesirable effects on both composition and structure. Annual growing season fires can be a tool to meet certain land management objectives, including hardwood reduction, but its effects and usage should be fully understood by land managers.