Seed Availability and Biotic Interactions in Granite Outcrop Plant Communities

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Abstract
Factors controlling the structure of plant communities occupying soil—filled depressions on granite outcrops in southeastern United States were investigated by adding and/or removing species. The effects of the treatment on vascular plant density and cover, and on moss and lichen cover, bare ground, and some soil characteristics (depth, bulk density, pH, and organic matter content) were followed over 2 yr. Addition experiments performed in shallower soil island communities revealed that shallower soil species often significantly increased the establishment of deeper soil species. This positive effect might have resulted from indirect interactions mediated by mosses and lichens. Extreme abiotic factors (i.e., drought) seemed to have overridden this initial facilitative effect, however. Species removals performed in the tension zones of island communities indicated that, although physiological tolerances, and not biotic interactions, appeared to determine the lower limit of some species (annuals) on the depth gradient, for others (perennials), biotic interactions with shallower soil species seemed to be significant in restricting the populations to the deeper section of the gradient. Few soil characteristics responded to our experimental manipulations over the 2—yr study period. Succession occurs in the soil—filled depressions on granite outcrops, and because the spatial patterns studied here appear to be good analogs of the temporal patterns, our results suggest that mechanisms of facilitation followed by tolerance and inhibition are operative in the sere.