Climatic niche lability of the African woody flora

On the African continent and elsewhere, despite the prevalence of niche conservatism in plants, many clades have transcended major ecological barriers, such as the forest-savanna boundary, but the direction, timing, functional trait association of such shifts have not been characterized at the scale of a whole flora. To address this, we characterized the climatic niche of >4000 tropical African woody species, distinguishing two broad bioclimatic groups (forest and savanna biomes) and six subgroups. We quantified niche conservatism versus lability at the genus level with a simple taxonomic approach i.e dissecting the current distribution of clades in bioclimatic (sub)groups. Then, a genus-level phylogeny was used to infer the timing and origin of the woody flora in forest and savanna biomes. We found that, although niche stasis at speciation is prevalent, numerous clades individually cover vast climatic spaces suggesting a general ease in transcending ecological limits, especially across bioclimatic subgroups. The forest biome was the main source of diversity, providing many lineages to savanna, but reverse shifts also occurred. We identified clades that have been particularly successful in diversifying in savanna, such as the Fabaceae family. In addition, we showed that forest-savanna shifts were not consistently associated with a change in growth form (i.e. climber, shrub, tree), with clades mostly retaining their ancestral form, though we found evolutionarily labile clades whose presence in forest or savanna is associated respectively with climbing or shrubby species diversification.