Leaf habit, maximum height, and wood density of tropical woody flora in Africa: Phylogenetic constraints, covariation, and responses to seasonal drought

How woody floras have evolved to cope with seasonal drought is still
poorly known, especially in tropical Africa, which experiences a drier and
more seasonal climate compared to other tropical regions. Here, we
characterized the phylogenetic and climatic distribution of three key
traits associated with desiccation avoidance or tolerance, focusing on
self-supporting woody species of tropical African forests and savannas. We
assembled a large database with newly compiled data on species leaf habit
(evergreen vs deciduous, 1,255 species) and maximum height (1,281
species), along with new data on oven-dry wood density (1,363 species),
climatic niche, and phylogenetic relatedness. Bayesian phylogenetic mixed
models were used to assess the phylogenetic signal in niches and trait
attributes, to explore individual trait responses to climate and to
investigate trait covariations, both in general and for each
characterisation of the climatic niche. To date, this study represents the
first quantitative assessment of the phylogenetic signal in the species
leaf habit for the woody flora of tropical Africa, though drought
deciduousness is widespread in seasonal forests and savannas. We
identified a strong phylogenetic signal, notably for species biome
affinity and leaf habit, but also major evolutionary changes. Relying
solely on climate was found to be ineffective for predicting species wood
density and insufficient for accurately predicting species leaf habit or
maximum height. Among forests, the effect of climate on leaf habit became
evident when considering covariations with maximum height or wood density.
Small understory species are more likely to be evergreen with dense wood,
while canopy species are more likely to be deciduous with light wood. This
general pattern varies with climate as both evergreen and deciduous
species coexist in the canopy of the wettest sites.