Multiple origins of serpentine-soil endemism explained by preexisting tolerance of open habitats

Abstract

Plant specialization on soils derived from unusual parent materials is an important contributor to regional biodiversity. These stressful substrates include serpentine, gabbro, and other ultramafic rocks rich in heavy metals. The effect of substrate on plant diversity is illustrated by serpentine soils in California: they comprise less than 1% of the surface of the state (1), but serpentine endemics (species restricted to serpentine soils) make up about 10% of the flora (2). How such “edaphic endemics” (plants restricted to stressful soils) evolve is a long-standing question that remains largely unresolved. For example, plant tolerance of serpentine soils may often involve tradeoffs in competitive ability, and restriction to serpentine soil may reflect poor competitive ability on less stressful soils rather than obligate association (3⇓–5), although possible counter-examples exist (6). A common feature of plant communities on stressful soils is the wide spacing of plants and openness of the habitat. Openness may itself be stressful for a variety of reasons (detailed below) (7⇓–9). In PNAS, Cacho and Strauss (10) use a novel comparative experimental approach to explicitly assess the role of openness vs. soil chemistry as factors in the evolution of plant tolerance of, and endemism to, serpentine soils.