Taxonomic diversity masks leaf vein–climate relationships: lessons from herbarium collections across a latitudinal rainfall gradient in West Africa

Cross-biome data suggest that leaf venation density increases with decreasing moisture availability. If leaf venation traits were related to climatic conditions in such a predictable manner, these traits could serve, for example, as proxies for the reconstruction of past climates from fossil leaf remnants. However, our knowledge on vein–climate relationships is still scarce and relationships are poorly understood, particularly concerning broad-scale patterns in tropical regions. In this study, based on phylogenetic generalized least squares regression, we analysed leaf venation traits together with climatic niche data from 238 herbarium specimens to infer trait–trait and trait–climate relationships along a strong north–south rainfall gradient crossing West African savannas. Variation in leaf venation traits was strong along the climate gradient, yet vein–climate relationships were weak overall. This is in contrast to our expectation from the strong adaptive forces operating in drought-adapted vegetation covered by the gradient. On the other hand, several common patterns including intervein and vein–leaf size relationships were confirmed. A key finding was that while trait–climate relationships were mostly weak, some of these relationships became stronger when breaking down the entire taxon assemblage into smaller family-level units. This suggests that vein traits underlie partially opposing constraints in different clades, a pattern that is likely to become stronger with increasing higher-rank taxonomic diversity of species assemblages.