Effects of evolutionary history on assembly of flowering plants in regions across Africa

The global cooling beginning in the Eocene has caused the coverage of tropical forests to contract from covering nearly all of Africa during the early Tertiary to about one tenth of Africa at the present time. Accompanying the climate cooling and drying was the evolution of traits tolerating cold and drought, particularly extreme drought in the Sahara. Here, we analyze a comprehensive data set of angiosperms to determine the effects of evolutionary history on assembly of angiosperms in regional floras in Africa. In particular, using phylogenetic metrics accounting for different depths of evolutionary history, we assess whether drought has caused stronger phylogenetic clustering relative to cold. We divided Africa into 27 regions, and collated species lists of angiosperms for each region. We used different phylogenetic metrics (tip- vs basal-weighted) reflecting different evolutionary depths to quantify phylogenetic structure, and related phylogenetic structure metrics to climatic variables representing average, extreme and seasonality conditions of climate. Variations of phylogenetic structure across Africa were well explained by climate, with R-2 values being similar to 0.84 for each phylogenetic metric when all climatic variables used in this study were considered. Angiosperm assemblages with the strongest phylogenetic clustering are located in temperate regions, rather than in tropical desert regions of Africa. Minimum temperature explained much more variation in phylogenetic metrics than did minimum precipitation, suggesting that evolution of traits conferring cold tolerance is more phylogenetically constrained compared to those conferring drought tolerance. Temperature seasonality also explained much more variation in phylogenetic metrics than did precipitation seasonality. When mean annual temperature and precipitation are considered or when all temperature-related variables and all precipitation-related variables are considered, temperature variables are more important than precipitation variables in driving tip-weighted phylogenetic structure of angiosperms in Africa but the pattern is reversed when basal-weighted phylogenetic structure is considered.