Some Like it Hot: Fire and Legume Germination in the Longleaf Pine Ecosystem

Abstract

Fire is a globally distributed disturbance and is important for structuring and maintaining the diversity of many frequently burned ecosystems. Competitive relationships and vegetation patterns may be affected by fire’s influence on survival, reproduction, and recruitment of plants. Each fire is unique and the effects of a given fire are not necessarily uniform throughout the area burned. Within the southeastern United States, the once-vast longleaf pine (Pinus palustris Mill.) ecosystem has been reduced to a mere fraction of its former range by overharvesting and land conversion. This fire-dependent system is also home to one of the most diverse plant assemblages known in North America. The effectiveness of restoration and management of this imperiled system may be improved by increased knowledge of fire-related germination and recruitment responses for functionally important groups of plant species. This project first investigated potential mechanisms of fire-induced germination and then sought to understand how fine-scale fire heterogeneity may influence germination of common native legume species. For the first part of the project, six legume species were used to examine the effects of exposure to various components of fire: 1) elevated temperatures (five levels with dry heat temperatures ranging from presumably non-lethal to lethal) over three durations; 2) duration (three intervals) of moist heat (i.e., steam); 3) mechanical scarification; and 4) smoke. Legume species used represented a range of seed hardness, varying from soft-seeded to extremely hard-seeded. Results indicate that response to both dry and moist heat was species-specific, but moist heat had the greatest potential to stimulate germination. The legumes studied also exhibited high heat tolerance, which may be a function of their low seed moisture contents. Smoke had no detectable effect on germination of most species, but may have inhibited Centrosema virginianum. Two studies were used for the second part of the project to examine post-fire germination of nine legume species in response to fuel variation in laboratory and field settings. For both studies, fine fuel loads (longleaf pine needles) and woody fuels (longleaf pine cones) were manipulated. Germination and mortality were assessed for six species after experimental laboratory fires. Germination of seven species was assessed for four months in the field following experimental burns of small plots. In both studies, exposure to fire reduced seed germination compared to unburned controls. Furthermore, germination of seeds that were under burned pine cones was reduced more than treatments burned with pine needles alone. Manipulation of biomass of fine fuel loads did not affect germination in either study. Together, the results of all experiments indicate the potential influence of fire on germination and mortality of legume seeds. Such impacts may, in turn, affect recruitment of these and similar species, thereby influencing post-fire vegetation patterns. Additionally, a hypothetical model is presented that describes the potential influence of pine cones on post-fire seedling recruitment.