WINTER FIRES PROMOTE GREATER VITAL RATES IN THE FLORIDA KEYS THAN SUMMER FIRES

Abstract

Because fire season may influence how plant populations respond to fire, quantifying effects of different fire seasons may be essential for conservation of fire-dependent species. In this paper, we report results from a three-year study on the effects of replicated experimental burns in summer (wet) vs. winter (dry) seasons, on vital rates of Chamaecrista keyensis, a narrowly endemic understory perennial herb of pine rockland forests of the Lower Florida Keys. C. keyensis in the burned treatments had lower first-year survival than in unburned treatments, and plants in the summer-burn treatments had lower first-year postburn survival than those in the winter-burn treatments. These annual survival patterns, however, were reversed by the third year postfire, when plants in summer-burn plots had the highest survival. The number of plants lost due to winter, but not summer fires, was replenished to the control unburned population level by three years postburn. Fires, particularly summer fires, stimulated vegetative growth of C. keyensis. In addition, summer- and winter-burn treatments each stimulated C. keyensis fruit production when compared to unburned treatments. In general, plants that existed before the fires had greater annual survival and fruit production, but smaller vegetative growth, than plants that came into existence after the fires. These two cohorts of plants have nonetheless similar responses to the three fire treatments. The winter-burn treatments produced greater seedling recruitment than the summer-burn or control treatments over three years postburn. Slight differences in the timing of burns within the wet season caused large differences in the demographic responses of C. keyensis, suggesting that the definition of summer season vs. winter season might be too broad for management purposes. Overall, winter burns and early summer burns had advantages over late summer burns and no-burn treatments. This suggests that C. keyensis may have evolved under a fire regime that included lightning-caused early summer burns, as well as non-lightning winter burns.