Controls on ground cover species richness in the longleaf pine-wiregrass ecosystem

Background/Question/Methods
Longleaf pine-wiregrass ecosystems of the southeastern United States are adapted to frequent low-intensity fires, which maintain open, savanna-like landscapes and foster high species richness within the ground cover vascular plant community. The objective of this long-term project is to understand how resource (moisture and nutrients) manipulation and fire exclusion interact to regulate ground cover species diversity. In 2000, we established a 2 x 2 factorial design on 32 plots in which we manipulated moisture and nutrients at both mesic and xeric sites. In an additional 16 plots, we implemented a 2 x 2 factorial design utilizing fire exclusion and nutrient addition. Vascular plant species richness was documented prior to treatment initiation (2000 and 2001) and in 2011using a nested plot design at spatial scales ranging from 0.1 m² to 400 m² for the resource manipulation plots and at 0.1 and 1 m² scales in the burn exclusion sites. Aboveground biomass was measured at the same dates.

Results/Conclusions
Species richness in burned-fertilized plots decreased relative to burned-unfertilized plots at xeric and mesic sites; this effect was significant at spatial scales up to 10 m² for xeric plots, but only at the 0.1 m² scale for mesic plots. Species richness in burned-irrigated xeric sites increased relative to burned-unirrigated plots at the 0.1 m² scale. Species richness in mesic plots was unaffected by increased water availability. Burn exclusion led to a decrease in species richness regardless of spatial scale or site condition (mesic or xeric), and nutrient addition exacerbated these losses in 1 m² plots at xeric sites. Nutrient addition and irrigation increased aboveground biomass in both burned and unburned sites. These results suggest that the disturbance due to prescribed fire has a greater role in structuring biodiversity than does productivity.