PS 32-37
The relative importance of fire versus hydrology in shaping species distributions along a hydrological gradient
In longleaf pine savannas, fire plays an important role in the maintenance of species composition. Small inclusional wetlands with drastically different vegetation often occur within these fire-adapted savannas, creating a hydrological gradient. Most species found in these wetlands are restricted to the wetlands and the ecotone between the relatively dry upland savanna and the wetland. Within this ecotone, species abundances vary along the hydrological gradient. Fires that burn in the savannas often burn through the ecotone and occasionally into the wetlands. We undertook a demographic study to understand the relative importance of fire versus hydrology in shaping species distributions along hydrological gradients. Seven dominant woody species, which comprise a large proportion of the biomass along the hydrological gradient, were chosen for study. Permanent plots were established along the hydrological gradient at 25 ecotone sites at Fort Bragg, North Carolina, and subjected to fire suppression, annual burning, biannual burning, or triannual burning. Three annual censuses were performed for each of 13,000 individual plants. Data were collected from each plant in order to quantify survival and growth. To quantify seedling establishment, separate plots were established along the gradient, seeded with locally collected seeds, and monitored for seedling germination and survival.
Results/Conclusions
Across all species, size had a positive effect on survival and growth. For most species, position along the gradient had a positive effect on survival and growth, with individuals in wetlands having both the highest survival and growth. Fire had a negative effect on all species; survival was higher in unburned than burned sites. Additionally, the probability of survival of burned individuals also depends on the size of the individual. Larger individuals were more likely to survive fire than smaller ones. Position along the hydrological gradient had a significant positive effect on seedling establishment for all species. More seedlings established in the wetlands and very few established in the drier savannas. However, a seedling’s position along the gradient did not influence subsequent survival. Fire had a significant negative effect on seedling establishment and survival for most species with few seedlings surviving fires. Overall, fire tended to reinforce the species’ response to hydrology along the transects, sharpening the gradient in species success. While the general patterns of survival, growth, and seedling establishment with respect to fire and position along the gradient are consistent across species, quantitative differences in these vital rates between species results in different patterns of abundance.