Experimentally linking disturbance, resources and productivity to diversity in forest ground-layer plant communities

Disturbance can function to maintain diversity within forest communities; however, specific mechanisms and the relationship to productivity are not well understood. We examined these linkages in forest ground-layer plant communities using a replicated, manipulative field experiment. Treatments included a range of gap sizes and untreated controls. We assessed spatial and temporal responses over the first three years following gap creation. Light transmittance and soil water content increased with gap size, while rates of colonization and species richness increased after a critical threshold. Subsequent increases in productivity were associated with declines in species richness, increased rates of local extirpation and a unimodal relationship between species richness and productivity at the individual quadrat scale (4 m(2)). The richness and productivity of vines, shrubs and especially graminoids, increased within 200-380m(2) gaps treatments. However, the productivity of forbs and tree seedlings did not, showing possible drought sensitivity overriding treatments. Spatial and temporal partitioning of gaps occurred as a result of interactions between species traits and environmental conditions. Significantly, productivity and richness showed complex relationships with canopy structure.Synthesis. Our results show that richness increases to an asymptote after a critical threshold in disturbance severity initially. Decreases in species richness over time associated with increases in productivity may eventually result in the unimodal relationship predicted by the intermediate disturbance hypothesis. However, species composition continues to differ with canopy gap size, suggesting a range of canopy gap sizes is required to maintain the greatest diversity of plant species over broader spatial and temporal scales.