Adaptation to substrate--and lack of it--in rock outcrop plants: Sedum and Arenaria

Sedum pulchellum Michx. (Portulacaceae) and Arenaria patula Michx. (Caryophyllaceae) are winter annuals confined to and often dominating the very shallow soil of rock outcrops. Both occur only on limestone soils in the eastern (KY-TN) portions of their ranges, but on a variety of substrates in the Interior Highlands. Such broad substrate distribution is usually accomplished by either wide physiological tolerance to soil chemistry or by ecotypic adaptation to substrate. In greenhouse tests of soil type tolerance, A patula showed strong ecotypic adaptation, with limestone and sandstone populations each growing poorly on the other's native substrate. In contrast, all S pulchellum populations were strongly inhibited by nonlimestone soils, either acidic (sandstone, granite, shale) or basic (dolomite), even when the nonlimestone soil was its native habitat. Thus, this species shows neither broad ecological tolerance nor ecotypic adaptation. In the shallow soil habitat, S. pulchellum has virtually no competitors, so it is able to survive and reproduce on some nonlimestone substrates (e.g., sandstone) despite its very slow growth. Despite very similar habitats, ecological life histories, and geographical ranges, these species do not occupy their broad range of substrate types by the same means.