Contrasting soil-texture niches facilitate coexistence of two congeneric plants that differ in competitive ability

Abstract

Whether close evolutionary relatives can coexist is expected to depend on evolutionary divergence in niches relative to divergence in competitive abilities. We investigated how plant species' responses to soil texture might affect coexistence by analysing distributions, seedling emergence and performance, and competitive abilities of the winter annuals Clarkia speciosa ssp. polyantha and C. xantiana ssp. xantiana. A landscape survey showed that the species have distinct associations with soil texture, C. speciosa presence correlating with fine soil and C. xantiana correlating with coarse soil. At the scale of population presences, the species co-occur less often than would be expected at random. On small scales within sites where they do co-occur, each species was negatively associated with the other. Clarkia xantiana presence and/or density also correlated positively with coarse soil texture and steep, poleward slopes, suggesting limitation by water availability. Lab experiments that varied substrate texture and imposed drought revealed contrasting species' fundamental niches at the seed and seedling stages. In coarse substrates, C. xantiana seedlings emerged at several-fold higher rates than C. speciosa, and, unlike C. speciosa, emerged when seeds were buried 0.5 cm. Clarkia speciosa seedlings had superior drought tolerance, independent of substrate. Competition coefficients estimated in a response surface experiment in artificial substrates predicted competitive exclusion of C. speciosa by C. xantiana in coarse substrate, with possible founder control of competitive outcome in fine substrate. Species' differences in responses to soil texture generate spatial segregation that likely facilitates coexistence, despite competitive ability differences that oppose it.