Fine-scale variation in surface fire environment and legume germination in the longleaf pine ecosystem

Fine-scale heterogeneity in fuels influences fire behavior and, in turn, may influence patterns of plant recruitment and regeneration from soil seed banks. In particular, hard-seeded species, such as many members of the plant family Fabaceae, may experience differential germination and mortality rates as a result of heterogeneous fire intensities mediated by variable distribution of fine and coarse fuels. Post-fire germination was examined for nine legume species native to longleaf pine (Pinus palustris) stands of the southeastern United States in response to fuel variation using a paired laboratory and field study. For both studies, fine fuel loads (longleaf pine needles) and coarse fuels (longleaf pine cones) were manipulated. Germination of seven species was assessed for four months in the field following experimental burns of small plots. Germination and mortality were assessed for six species after exposure to experimental laboratory fires. Burning reduced germination compared to unburned controls in both studies. Furthermore, legumes exposed to burning pine cones experienced greater reductions in germination than when exposed to burning pine needles alone. Manipulation of fine fuel loads did not affect germination in either study. Results suggest that small pieces of coarse woody debris are responsible for differential germination following fire, rather than variation in fine fuels. We present a conceptual model of fine-scale fire effects to explain how long-duration smoldering creates localized mortality events that may drive variation in recruitment from the soil seed bank for longleaf pine ecosystems.