Plant functional diversity, species diversity and climate in arid and semi-arid southern Africa

This paper analyses data on growth form (functional) diversity, species richness and climate from nine Nama-Karoo/Desert sites (predominantly summer rain) and 10 Succulent Karoo sites (predominantly winter rain) along comparable gradients of total rainfall in arid and semi-arid southern Africa. Sites from the latter region were richer in total species and succulent species, but had lower growth form diversity. Contrary to the predictions of niche theory, the more heterogeneous rocky habitats did not have significantly more species or higher growth form diversity than adjacent loamy flats at the community level (25 m2 plot). However, significantly more species were recorded in rocky than flat habitats at the site scale (10,000 m2), probably reflecting the longer resource axes and higher internal beta diversity in the former habitat. Species richness showed a significant positive relationship with growth form diversity. This relationship was much tighter for the Nama-Karoo/Desert than Succulent Karoo sites indicating a higher incidence of functional redundancy (coexistence of species with similar growth forms) in the latter region. An index of climatic heterogeneity (reflecting the diversity of conditions suitable for plant growth) and measures of rainfall evenness throughout the year, emerged as the strongest predictors of species richness and growth form diversity. These results are consistent with predictions of classical niche theory. Sites with a high percentage winter rain as well as those with evenly distributed rain (with a predictable winter component) had the highest cover of succulents and Mesembryanthema. In Succulent Karoo communities functional redundancy is associated with a high number of relatively short-lived leaf succulent shrub species, belonging largely to the Mesembryanthema. However, owing to their differential response to rare but catastrophic drought-induced mortality, the diversity within this functional type ensures long-term stability of community-level growth form mix or functional diversity.