Evolutionary strategies for reproduction and dispersal in African Restionaceae

Abstract

The wide variation of diaspore types in African Restionaceae reflects two different evolutionary strategies for utilisation of resources for reproduction and propagation. The interplay between fecundity and seedling survivorship has apparently resulted in reproductive strategies ranging from the production of a few large dispersal units with comparatively high drought resistance, to production of numerous, smaller dispersal units which experience high levels of drought mortality. A sample of Restionaceae representing all the major African lineages in the family was examined to investigate the influence of diaspore size on diaspore production, seedling growth and seedling survival in the fynbos vegetation of the Western Cape. Correlates of diaspore size were investigated by using phylogenetically independent contrasts to assess which ecological factors may have influenced the evolution of different dispersal mechanisms. Results indicate that diaspore size strongly influences the size to which a seedling can grow in its first few months, which in turn influences the plant's ability to survive summer drought. Larger seedlings, produced by larger diaspores, are less likely to succumb to drought-induced mortality than the smaller seedlings produced by smaller diaspores. The evolution of large, drought-resistant diaspores coincides with a hypothesised drying of the Western Cape climate between 5 and 7 million years ago. This occurrence of seasonal drought may have opened up a new ecological opportunity for plants producing drought-resistant propagules, which, under wetter conditions, would be overwhelmed by less robust seedlings through sheer weight of numbers produced.