Well managed pine savannas of the southeastern US harbor some of the highest diversity groundcover on Earth. Our research group is interested in the ecological and evolutionary processes that gave rise to and maintain high diversity in these ecosystems. Fire is the key disturbance in these ecosystems and groundcover in the best managed sites is burned every few years. Fire keeps woody shrubs and hardwood trees at bay. Even so, the roles fire plays beyond maintaining bunchgrass dominated groundcover are not fully understood. For example, does fire heterogeneity promote coexistence, or otherwise shape groundcover assembly? Regional influences undoubtedly also shape local groundcover communities, yet we do not fully understand the relative contributions that regional and local processes make. For example, do larger regional species pools support higher local species richness in pine savannas? Finally, the relatively few physically dominant foundational bunchgrass species are iconic elements of these ecosystems, but how do they influence the diverse components of the groundcover community? We examined these questions through observations and experiments (fuel manipulation, seed addition, bunchgrass removal, etc.) in Louisiana and Florida.
Results/Conclusions
In support of an influence of fire heterogeneity on groundcover assembly, groundcover biomass (fuel) is heterogeneous and contributes toward small scale fire heterogeneity, which in turn contributes toward small scale plant heterogeneity. In support of an influence of the regional species pool on local groundcover, seed addition boosts species richness. In support of the influence of dominant foundational bunchgrasses, tussocks are overdispersed and subordinate species are released from competition in their absence. Our observations and experiments motivate a working model for locally high diversity pine savanna groundcover. Key elements of the model include limited seed dispersal from a large regional species pool, frequent fire whose influence is mediated by groundcover fuels, nonrandom spatial patterning of dominant bunchgrasses, and the asymmetric influence of bunchgrasses on subordinate species.