Adaptation to Substrate in Rock Outcrop Plants: Interior Highlands Talinum (Portulacaceae)

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Abstract
Talinum calycinum Engelm. (Portulacaceae) and T. parviflorum Nutt. are small succulent-leaved perennial herbs of very shallow soils of rock outcrops in the Interior Highlands (Ozarks and Quachitas), United States. The first species has been considered substrate indifferent, occurring on a variety of outcrops but only rarely on limestone; the second also occurs on a variety of outcrops, but not on limestone. Populations of T. parviflorum from sandstone outcrops grown experimentally on native and alien soils grew as well on shale outcrop soils as on their own native sandstone soils, but when grown on calcareous outcrop soil showned abnormal coloration (yellow-green, with much red pigment), high mortality, and extremely poor total growth. In T. calycinum also, all populations, regardless of origin (sandstone, shale, granite, syenite, limestone), grew well on all noncalcareous soils, but grown with extremely poor on calcareous soils. Talinum calycinum thus is not substrate indifferent but, in fact, its growth is greatly inhibited by calcareous soils. However, while populations of T. calycinum from the first four outcrop types showed abnormal coloration and brown spotting on leaves when on the limestone outcrop soil, the limestone outcrop population had normal coloration. The population had normal coloration. The population from limestone is not a limestone outcrop ecotype in the usual sense that it grows well there, but some ecotypic adaptation has occurred in this population. Natural selection has not shifted its adaptive peak away from noncalcareous soil types, but has only extended the extreme limit of its tolerance curve to include calcareous soils. Thus, T. calycinum does not accomplish its broad substrate distribution by broad ecological tolerance or classical ecotypic races with divergent adaptive peaks. Instead, a local population has a slightly broadened tolerance that allows it to remain healthy and grow, albeit very slowly, in an environment even more inhospitable to the more common population type.

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