High‐elevation rock outcrop vegetation of the Southern Appalachian Mountains

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Abstract
Species composition patterns and vegetation‐environment relationships were quantified for high‐elevation rock outcrops of the Southern Appalachian Mountains, an infrequent and insular habitat in a forested landscape. Outcrops occur over a wide geographic range encompassing extensive variation in both geology and climate. Geographic‐scale factors interact with site‐scale factors to produce variation in vegetation among outcrops. Similarly, site‐scale factors interact with micro‐scale factors to produce variation in vegetation within outcrops. To provide a quantitatively‐based classification of outcrop vegetation we used a TWINSPAN analysis of 154 100‐m2plots. We recognized nine communities that primarily correspond to different combinations of elevation, bedrock type, geography, and moisture. Within outcrops of a single bedrock type, vegetation composition of 100‐m2plots was consistently correlated with elevation and solar radiation, but relationships to soil nutrients varied with bedrock type. Both site‐scale (100 m2) factors (e.g. elevation, slope, aspect, and bedrock type) and plot‐scale (1‐m2) microsite factors (e.g. soil depth, vegetation height, soil nutrients) were strongly correlated with species composition at the 1‐m2level. Environment can be used to predict composition more effectively for 100‐m2plots on a single bedrock type than either across bedrock types or at a 1‐m2scale. Composition‐environment relationships resemble those described for outcrop systems from other regions with pronounced topographic relief more than they do those described for the nearby but flatter and lower‐elevation outcrops of the Southeastern Piedmont. There is strong spatial autocorrelation in this community, perhaps owing to dispersal limitation. Consequently, a comprehensive conservation strategy must include reservation of both a range of geologic types and a range of geographic locations.