Global Distribution Patterns and Climatic Drivers of Plant Diversity in Rubiaceae.

The Rubiaceae family, the world's fourth-largest angiosperm group, exhibits exceptional species and morphological diversity across its global distribution. Understanding its biogeographic patterns and their environmental drivers is critical for elucidating the evolution, ecological adaptation, and conservation priorities of this family. Here, we integrated global-scale distribution data with growth-form classifications (herbaceous vs. woody) to map the species richness patterns of Rubiaceae. We then related these patterns to four types of environmental variables (water and energy, climate seasonality, historical climate change, human influence) to identify key drivers of richness variations across growth forms. Our results indicated that the species richness pattern of Rubiaceae species exhibits significant spatial heterogeneity at the global scale. Total and woody species richness generally exhibit distinct latitudinal distribution patterns, peaking in tropical regions, whereas herbaceous plants predominate in mid-to-high latitudes. The drivers of these patterns also differ among growth forms. Actual evapotranspiration (AET) is the strongest predictor of species richness for the family as a whole and for different growth forms. Among the four types of environmental variables, water and energy are key factors driving the diversity pattern of total species of Rubiaceae. Environmental factors, especially water and energy, have a much stronger impact on the species richness patterns of woody plants than on those of herbaceous plants. These findings highlight the unique biogeographic patterns and potential environmental driving mechanisms of the diversity of Rubiaceae species, providing a theoretical basis for the diversity of global conservation in Rubiaceae.