The relationship between taxonomic and phylogenetic diversity remains underexplored. Our goal was to determine whether a causal link exists between species richness and phylogenetic diversity. We wanted to evaluate whether species richness determines the phylogenetic diversity in realized assemblages (taxonomic determinant hypothesis) or phylogenetic diversity determines the species richness (phylogenetic determinant hypothesis). We also hypothesize that this causal framework could shift in different bioclimatic regions. We sampled over 1700 plant assemblages in grasslands and shrublands across three bioclimatic regions in Navarra, Spain. Using non-recursive structural equation modelling, we found that species richness influences phylogenetic diversity, and that this causal relationship remains consistently negative and is unaffected by climate differences among regions. Specifically, greater plant richness leads to increased phylogenetic convergence, resulting in reduced phylogenetic diversity. This means that the incorporation of new species into assemblages involves adding closely related species in phylogenetic terms, regardless of the bioclimatic region.