Longleaf pine (Pinus palustris Mill.) woodlands are noted for exceptionally high understory plant species richness. Niche- and dispersal-based mechanisms likely interact to control patterns of understory plant diversity at multiple scales in these frequently-burned systems. While dispersal limitation is likely important for structuring ground cover communities at the regional scale, local diversity could be controlled by a number of filtering mechanisms. Fire intensity is often assumed to be the main niche-based driver of fine-scale ground cover diversity, but the role of other deterministic factors in controlling community assembly at small scales remains largely unexplored. Our objective was to determine if the relatively low tree density created fine-scale resource gradients strong enough to affect species richness. We hypothesized that fine-scale spatial heterogeneity in radiation and throughfall influenced local understory plant richness. We sampled vascular plant species richness in ~200 1m2 quadrats in each of two 0.25 ha plots with similar canopy cover and soils. All trees above 10 cm dbh were mapped and distance to the nearest tree was calculated for each quadrat. Canopy cover for each quadrat was estimated from densiometer readings.
Results/Conclusions
We found a relatively large range of species richness among quadrats in both plots (5-37 species). The most frequently occurring species displayed weak dominance when relative frequencies were plotted against frequency rank. While the most species-rich quadrats were generally located in areas of lower canopy cover (<60% shaded), spatial variation in richness was not explained well by canopy openness or distance to nearest canopy tree, and patterns of species dominance did not appear to be linked to nearness to trees. While some species were more likely to be found when only a subset of quadrats nearest and furthest from trees was considered, species occurrence frequencies largely remained the same along a tree nearness gradient. These initial results do support the assumption that dispersal-based mechanisms play a large role in structuring fine-scale understory plant diversity in longleaf pine. Canopy modelling and further community analysis is ongoing to better quantify fine-scale variations in specific resources, such as light, and to determine whether or not fine-scale resource gradients affect species richness.