Most plant species endemic to a rock outcrop system have high constancy to one substrate type. However, the complex geology of the Ozark region of Arkansas and Missouri has promoted a diversity of adaptive strategies and challenged the traditional classifications of edaphic adaptation. For example, the small aromatic mint Satureja arkansana ( = Calamintha arkansana) has been considered an obligate calciphile, growing abundantly on calcareous rock outcrop communities in the Ozarks and occurring on sandstone outcrops only when plants receive alkaline drainage from nearby calcareous formations. However, we observed in Arkansas that this technically perennial, but functionally winter annual, plant regularly occurs on sandstone outcrops not receiving calcareous effluent. To determine whether this occurrence represented ecotypic specialization or substrate indifference, we compared growth parameters of plants collected from non-alkaline sandstone and calcareous limestone outcrops on their own and the other's native soil. Plants from sandstone outcrops produced significantly greater biomass on native sandstone than on non-native limestone soil. Surprisingly, plants from limestone outcrops also grew better on the non-native sandstone than on their native soil. Other differences in plant growth parameters (e.g., numbers of branches and leaves) between the soil types occurred in both populations and were consistent with the biomass results, although high variances meant that not all parameters were statistically significant. Our results reveal that S. arkansana is not an obligate calciphile, and actually has better growth on sandstone soil, but has a “broad substrate tolerance” that allows success in rock outcrop communities on both substrates.
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Vol. 136 • No. 3