Plant community assembly depends on the interactions that occur among coexisting species.  Neighboring species may differentiate in their resource use (niche partitioning), share similar resource requirements (environmental filtering) or represent a random sample from the species pool.  These processes determine which species coexist and comprise plant communities.  Large-scale logging disturbance can alter plant community composition and the processes driving assembly.  In the absence of disturbance does niche partitioning, environmental filtering or random assembly drive species coexistence at fine spatial scales?  How do the processes change after long term recovery from logging disturbance?  We used a trait based approach to determine whether neighboring individuals in the understory herbaceous community of Southern Appalachian cove forests exhibit niche partitioning, environmental filtering or random assembly.  Three forests logged over a century ago were compared to three old growth forests.  Within each forest, all individuals were mapped in ten 1.0 m² plots at each site. Functional trait measurements were used as proxies for a plant’s ability to use light, water, and nutrient resources. Trait similarity between nearest neighbors was analyzed based on a model in which species were randomly sampled and their trait distance determined. 

Results/Conclusions

Environmental filtering predominates at fine spatial scales in the diverse understory herb community.  Niche partitioning does not occur among nearest neighbors in this community.  Intraspecific clumping did not account for trait similarity at fine spatial scales because nearest interspecific neighbors also showed significant trait similarity.  More environmental filtering occurs in old growth forests than forests that were previously logged.  Only root diameter had greater similarity among co-occurring species in logged sites.  In old growth sites, there was greater similarity in specific leaf area and chlorophyll a:b ratio.  Patterns in trait similarity are tied to environmental gradients.  Small root diameters occur in more acidic soils in hundred year old forests.  In old growth forests, the light environment provides a filter for determining which species coexist at fine-spatial scales.  With increased canopy cover, co-occurring species have similar lower specific leaf areas and higher chlorophyll a:b ratios.  At fine spatial scales where species interact, environmental filtering is the predominate force for species coexistence.  After long term recovery from logging disturbance, fine-scale species assembly is due mostly to random processes rather than environmental filters.