Savannas are ecological communities comprised of scattered trees in a grassland matrix. The mechanisms leading to the coexistence of grasses and trees and the persistence of savanna communities over long time periods are unclear. Opposing ecological theories have predicted either equilibrium savanna communities that result from niche partitioning or non-equilibrium savannas that result from repeated disturbances such as fire. This project will consider an alternative model to explain the structure of savannas, in which stability results from feedbacks between fire disturbance and vegetation. It will test the hypothesis that flammable savanna plants result in more frequent or intense fires and that these fires, in turn, promote the growth and establishment of savanna vegetation. Such positive feedbacks between savanna vegetation and fire could maintain the density of trees between the extremes of treeless prairies and the closed canopy of forests. The project will further explore whether less frequent, periodic disturbances such as hurricanes could have interactive effects with fire that could initiate vegetation-fire feedbacks and rescue savannas from a forest state. The underlying assumptions and predictions of a disturbance-feedback model will be tested using empirical and experimental data collected from sites that have been studied for the past 30 years in southern Florida, southwestern Georgia, and eastern Louisiana, and that broadly represent pine savannas in the southeastern United States. Demographic and spatial patterns predicted by a disturbance-feedback model will be compared to field observations and experimental data to ascertain the validity of the model.
This alternative model of savanna dynamics will contribute to a general understanding of the processes that structure savannas across a wide range of environments worldwide, and will facilitate predictions of savanna responses to disturbances such as climate warming and landscape fragmentation. Understanding savanna responses to perturbations is of broad significance as savannas comprise an eighth of the global land surface, provide essential ecological services, and are important reservoirs of biodiversity. Results from this project will also be broadly applicable to ecological restoration and management of ecosystems across the United States. This project will support a postdoctoral scholar and provide research training for graduate and undergraduate students, and Girl Scouts at a camp near the study site in Louisiana.
Savannas, terrestrial biomes with grasses and trees as co-dominant life forms, occur over wide ranges of rainfall on most continents. These are the most fire-frequented ecosystems worldwide. Natural lightning-ignited, landscape-level fires often occur during annual transitions from dry to wet seasons, especially in mesic (mean annual precipitation [MAP] > 650-800 mm) and high-rainfall savannas (>1200 mm MAP). Fires, alone or together with other disturbances have been proposed to maintain open canopy and grass-dominated ground layers in mesic and high-rainfall regions, where a closed woody canopy would be expected in a world without fire. Recurring fires produce "fire traps," demographic bottlenecks that block entry of woody plants into the overstory, preventing transitions to woodlands/forests. Nonetheless, many studies indicate that relative cover of woody plants and grasses appears intrinsically unstable. Extrinsically generated fires may not prevent escape from fire traps and thus closed canopies, especially in high rainfall areas. Key savanna species might play a critical role in maintaining savannas, especially in high rainfall regions. Recent models, building on older concepts regarding effects of plants on amounts, location, and flammability of fuels have indicated that savannas could be stabilized by fire-vegetation feedbacks that involve changes in amounts of flammable fine fuels and thus increased total heat produced in fires. We envision different relationships involving savanna trees whose flammable shed leaves modify specific characteristics of fires at ground level, thereby providing protection against less frequent, more intense fires and facilitating opportunities for regeneration. The resulting fire effects produced by such pyrogenic plants also should limit growth of plants in the ground layer, thereby maintaining savanna and blocking transition to forest. We addressed these hypotheses via study of high MAP savannas in which longleaf pine (Pinus palustris) is the primary pyrogenic species. We experimentally manipulated pine needles in experimental plots, generating additions and removal treatments that could be compared to unmanipulated controls in replicated plots. We measured characteristics of fine fuels before and after fires to estimate combustion. We measured characteristics of fires at ground level using thermocouple wires attached to data loggers and place so that the probe tips were next to target oaks or grasses. We measured the sizes of plants as numbers of stems and leaves before and after fires, both at 2 months and 8 months. We conducted statistical analyses (MANOVA) and structural equation modeling on the results to test a priori hypotheses and explore possible mechanisms whereby the flammable fuels affected vegetation. Results indicated that flammable pine needles had large effects on the characteristics of fires and ground level and on the vegetation. Presence of flammable pine needles affected fire characteristics at ground level. Compared to when grass fuels alone were present, temperature increases recorded by thermocouples at ground level were 25-100% greater and durations >60oC were 30-100% greater when pine needles were present. The effects of the pine needles increased with amount of pine needles present, and the presence of pine needles increased fuel consumption. These recorded changes in fire characteristics had negative effects on post-fire responses of dominant groundcover plants, both grasses and oaks. Resprouting grasses contained fewer culms, and fewer culms flowered when pine needles were present. Resprouting oaks tended to be smaller when pine needles were present, and resprouting also tended to be delayed. Some effects appeared permanent. Further, effects of pine fuels on groundcover plants appeared to result from specific fire effects at ground level and were not associated with traditional fire ecology measurements based on fine fuel consumption in fires (such as would be produced by combustion of grasses and litter). These results have important worldwide implications for ecological concepts regarding how fires influence savanna dynamics in high rainfall regions. We propose that pyrogenic vegetation generates a predictable mechanism whereby savannas are maintained at high MAP. Depressant effects models that include vegetation-fire feedbacks on production of flammable fine fuels have resulted in predictions of stable equilibria. Such predictions, based in changes in biomass of fuels, assume that effects of fire on vegetation change with total heat produced in fires. Because heat rises during fires, increasing fuel biomass should affect vegetation located above-ground (e.g., trees), but effects on the ground layers are problematic. Indeed, structure and flammability of fuels close to the ground have been hypothesized to influence patterns of heat production during combustion, differentially affecting local plant species. Resulting effects of such fire-adapted species should depend on how changes in fuels affect fire characteristics and how altered fire characteristics affect aspects of ground layer vegetation that are involved in post-fire responses.
