Effects of prescribed fire, invasive species, and geographic distribution patterns on granite rock outcrop plant communities in the Georgia Piedmont
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The granite rock outcrops of the southeastern United States are a unique assemblage of island-like habitats that support many endemic and rare species in their unique plant communities. The flora associated with the ecotone for many of these outcrops has shifted over time. Fire suppression, exotic plant invasion, and anthropogenic impacts are creating a dense understory environment of high competition and low light levels that discourage establishment and persistence of native outcrop flora. This research project includes two small-scale field studies and a broader landscape-level assessment of disturbance effects on species diversity and composition in the ecotone of rock outcrop environments. Prescribed burns were applied to the ecotone plant community of two rock outcrops in the Piedmont of Georgia. Two other outcrop sites were chosen to assess community response to removal of invasives in the ecotone. Occurrence of vegetation classes and species richness were measured along random stratified belt transects. Hemispherical photographs, soil nutrients, and soil depth were used to monitor changes initiated by the prescribed burn. Neither fire, nor removal of invasives had a significant effect on vegetation class occurrence, including invasive and rare plant occurrence, or species richness. A site-specific evaluation, including the consideration for disturbance history and potential rare species’ response, would be necessary to determine whether prescribed fire should be an appropriate management tool on rock outcrop ecotones. Data for tree canopy, impervious surface, and landcover produced by the University of Georgia Natural Resources Spatial Analysis Laboratory were used to observe a 33-year snapshot of landcover change surrounding Georgia outcrops. Predictive models of species geographic distributions were used to determine the key environmental variables that determine local and regional species patterns. Landcover trends reveal increasing urban development and deforestation in all areas that were analyzed. Impervious surface surrounding rock outcrops significantly increased from 1991 to 2005. Rare plant occurrence increased with latitude and outcrop spatial complexity and did not appear to be tied to outcrop size or isolation. Geology, population density, and outcrop shape complexity were predictive of quarrying. These models may be applied to specific habitats to assist in identifying threats and informing conservation decisions.