Coexistence and differentiation of ‘flowering stones’: the role of local adaptation to soil microenvironment

Abstract

Summary 1 In most documented examples of adaptive radiation, the processes driving divergence of phenotypes and, ultimately, speciation remain speculative. The remarkable radiation of the Aizoaceae in the southern African winter-rainfall deserts is no exception. 2 We examined the role of specialization to soil microenvironments in the coexistence and differentiation of the aizoaceous genus Argyroderma, which is confined to quartz-lag gravel habitat in southern Africa. 3 We examined patterns of edaphic habitat use and morphological divergence between three sympatrically occurring species (one habitat generalist and two quartz specialists) and used transplant experiments to investigate the extent of local adaptation to quartz microenvironments. 4 Soils utilized by populations of the generalist, A. fissum, and one of the specialists, A. pearsonii, differ significantly in terms of stone content and quartz cover. The soils utilized by the two specialist taxa (A. pearsonii and A. delaetii) do not differ significantly in terms of the variables measured, although A. pearsonii consistently uses habitats with higher quartz cover at all contact zones investigated. 5 The three species are clearly separated morphologically. The morphological trend between Argyroderma species occupying sparse (A. fissum) and dense (A. pearsonii and A. delaetii) quartz habitats is towards reduction in plant size and height (dwarfism), increased levels of submergence, decreased branching, spherical leaf shapes, increased leaf thickness and increased enclosure of the leaf by the old leaf sheaths. The two specialist taxa differ in terms of leaf sheath enclosure, plant submergence and fruit traits. 6 Transplant experiments demonstrate home-site survival advantage in transplants between species occupying sparse and dense quartz habitats as well as between microenvironments within the dense quartz habitat. 7 The results suggest that divergence in potentially functional morphological traits between Argyroderma species occupying different edaphic microenvironments probably results from local adaptation, with coexistence facilitated by response to fine-scale habitat variation. In addition, the results suggest a possible role for edaphic specialization in the evolutionary divergence of Argyroderma and perhaps in the broader radiation of the Aizoaceae in southern Africa.