Plant life of western North American ultramafics

Abstract

Soils weathered from ultramafic rocks are either devoid of vegetation (barrens) or support sparse but often distinctive floras. Vegetation on ultramafic soils takes the form of distinctive variants of conifer or mixed conifer-hardwood forest, chaparral, or grassland. Three types of floristic elements can be found: 1) serpentine enedemics, 2) local or regional indicator species, and 3) bodenvag species, widespread taxa. The greatest concentration of species endemic to serpentine is in the Klamath-Siskiyou mountain complex of NW California and SW Oregon, with secondary concentrations in the North Coast and South Coast ranges and the Sierra Nevada of California. The evolution of a serpentine flora may involve a variety of speciation routes. The most probable sequence for diploid taxa could involve: 1) genetic preadaptation to serpentine within a non-serpentine species: 2) racial fixation of the preadapted genotype; 3) further morphological and physiological divergence yielding an infraspecific variant; 4) attaining species status by further genetic and ecological isolation. This sequence is illustrated by Streptanthus, a genus of western North American crucifers, with varying degrees of fidelity and narrow endemism. A more rapid mode of speciation on serpentine, saltational speciation by catastrophic selection, has been proposed. Adaptation to ultramafic soils is likely to involve both physiological and morphological modifications. Xerophytism, nanism, glaucescence, plagiotropism and color changes (anthyocyanic, chlorotic) are frequent attributes of serpentine species. A few species possess the ability to accumulate >1000 μg g-1 Ni in their foliar dry matter (hyperaccumulators). -from Author