Substrate adaptation in rock outcrop plants: Eastern United States Talinum (Portulacaceae)

The shallow soil rock outcrop plant Talinum teretifolium Pursh (Portulacaceae) occurs on granite, sandstone, and serpentine outcrops and T mengesii Wolf occurs on granite and sandstone. Neither species occurs on limestone. In contrast, Talinum calcaricum Ware occurs only on limestone. We wished to determine whether soil type limited the distribution of the more restricted species, and whether the more broadly distributed species was ecotypically adapted to the various soil types. Plants from one or more populations of each species were tested for their ability to grow (measured by mean dry weight) on soils from outcrop types other than their native one. Growth of the limestone endemic T calcaricum was greatly inhibited by acid (sandstone) soil. Growth of the other two species was greatly inhibited on limestone and serpentine soil as compared with granite or sandstone soil, whether the plants came from granite, sandstone, or, in the case of T teretifoluum, serpentine. The serpentine population of T teretifolium showed little evidence of adaptation to serpentine soil; it grew as poorly on serpentine as plants from sandstone and granite, and as well on these latter two soils as plants native to them. However, there was none of the chlorosis on serpentine soil that developed on limestone, so that despite extremely slow growth, T teretifolium remains healthy and can persist in the competition-free shallow soil environment on serpentine.