Longleaf pine (Pinus palustris Mill.) ecosystems are remarkably rich in understory plant species, which is driven by frequent (1-3 year interval) low-intensity fires. The co-existence of many small plants in small areas (1 m2) of very deep, excessively drained sands begs the question of whether niche-based processes are occurring at these fine-scales. We hypothesize that frequent fire continuously levels the playing field for maintaining such prominent plant species co-existence and that neutral processes explain a large portion of community dynamics at high fire frequencies. We aimed to quantify the proportion of neutral vs. niche processes that occur and examine the underlying species traits in the latter. To test this, we developed a simple autonomous agent model with probability based mortality and birth to examine mechanisms of neutrality in longleaf pine ground cover communities of Eglin Air Force Base in northwest Florida, USA. We used four years of empirical data collected at very fine-scales (10 cm2) for model parameterization. We examine how much plant diversity patterns are explained by neutral processes and spatial dispersal patterns. To identify the scale at which deviations from neutrality occur, we examine the effects of scaling input frequency distributions on modeled richness patterns.
From this modeling study, we found that neutral and niche-based processes work in tandem in these small plant communities; however, neutral processes explain the majority of simulated richness patterns. From our scaling analyses, we found that the proportion of species simulated is the same across scales, but coarser scale data maintain higher species richness through time. Dispersal limitation (immigration) from the metacommunity is, however, the main driver of simulated species richness, followed in general by the scale of inputs, mortality and birth (recruitment) probability, community size simulated, and local patterns of spatial dispersal. This study provides evidence of stochastic assembly patterns in these plant communities, but in this five year study, we have also found that spatial and temporal aspects of fire, fire intensity (fine-scale “hot spots”) and changes in fire frequency, respectively, have mechanistic impacts on community assembly. These linkages between fuel, fire, and biodiversity are keys to applying this model at management relevant scales (burn units up to large managed landscapes). We illustrate our approach to coupling this model to overstory structure and discuss the relevance in frequently burned systems globally.