Effects of fire on population dynamics of congeneric pine savanna reseeders and resprouters (Hypericum spp.)

Background/Question/Methods

Woody plants can be divided into reseeding and resprouting life histories based on responses to disturbances, such as fire. Reseeders are killed by fires, but germinate from dormant seeds; they are restricted to habitats with fire return intervals long enough to reach reproductive maturity. Resprouters are top-killed by fires, but underground structures survive and regrow; they can persist in habitats with short fire return intervals because they do not need a long time to reach reproductive maturity. Nonetheless, woody reseeders and resprouters of the genus Hypericum co-occur in pine savannas along the Gulf of Mexico coast that historically experienced frequent lightning fires (> twice a decade). How can these species with contrasting life histories coexist under a regime of frequent disturbances? We examined population dynamics of Hypericum microsepalum, a resprouter, and H. chapmanii, a reseeder, following prescribed fire and treatments that simulated different aspects of fire. We established the following plots: controls (no manipulations), burned, vegetation clipped at ground level, vegetation clipped at ground level then ash added, and ash added to unburned/unclipped plots. This allowed us to determine whether populations of Hypericum reseeders and resprouters require: 1) no disturbances, 2) nutrient release and heat/smoke from fires, 3) only open habitat (i.e., clipping at ground level), 4) open habitat and additional nutrients (ash), or 5) only additional nutrients. Treatments were applied to 1x1m plots with H. microsepalum or H. chapmanii in eight sites located in the St. Joe Bay State Buffer Preserve, FL during the growing season of 2006. Plants were sampled twice yearly for recruitment, growth, mortality, and fecundity.

Results/Conclusions

There was insignificant recruitment in the controls and unburned/unclipped plots with nutrients added. In the burned, clipped, and clipped plus nutrients added plots, the resprouter produced new ramets quickly, and numerous seedlings of the reseeder appeared. Nonetheless, the reseeder experienced high mortality (>90%) in the first two years. We conclude that disturbances creating open space are needed for recruitment and growth, but there was no effect of nutrient additions. Sprouts of H. microsepalum were reproductive within 2-3 years after treatments were applied, while the few surviving seedlings of H. chapmanii showed no evidence of reproductive maturity. Thus, the reseeder needs a longer disturbance-free interval to reach reproductive maturity. We conclude that if frequent fires are heterogeneous over time and space, reseeders might reach reproductive maturity in patches that remain unburned and thereby coexist with resprouters.