Seedling recruitment as a driver of species richness in the longleaf pine savanna

The regulation of species richness is often driven by complex interactions between biotic and abiotic factors. In the threatened and diverse longleaf pine ecosystem the observed correlation between understory species diversity and net aboveground primary productivity may best be explained by variability in seedling recruitment events. To understand the potential mechanistic controls on species richness in this system, we used a long-term resource manipulation study across a natural soil moisture and productivity gradient to assess environmental controls on seedling recruitment, as well as the facilitative role of the dominant groundcover species. We used a factorial design to manipulate water and nitrogen over five years under a two year burn regime. Natural and experimentally seeded recruitment rates were assessed over a growing season by functional group and species. The average number of seedling recruits per plot varied with resource manipulation treatment. Total recruitment rate and number of species present was greatest in response to water addition and least with nitrogen addition. However, recruitment patterns differed by functional group in response to season and the soil moisture gradient. Forb recruitment responses were similar to the overall response throughout the course of the study, whereas, this pattern did not appear in the monocot recruits until after summer. Leguminous species recruitment did not respond to resource manipulation yet it was affected by location on the gradient.