Global distribution, climatic preferences and photosynthesis-related traits of C₄ eudicots and how they differ from those of C₄ grasses

C-4 is one of three known photosynthetic processes of carbon fixation in flowering plants. It evolved independently more than 61 times in multiple angiosperm lineages and consists of a series of anatomical and biochemical modifications to the ancestral C-3 pathway increasing plant productivity under warm and light-rich conditions. The C-4 lineages of eudicots belong to seven orders and 15 families, are phylogenetically less constrained than those of monocots and entail an enormous structural and ecological diversity. Eudicot C-4 lineages likely evolved the C-4 syndrome along different evolutionary paths. Therefore, a better understanding of this diversity is key to understanding the evolution of this complex trait as a whole. By compiling 1207 recognised C-4 eudicots species described in the literature and presenting trait data among these species, we identify global centres of species richness and of high phylogenetic diversity. Furthermore, we discuss climatic preferences in the context of plant functional traits. We identify two hotspots of C-4 eudicot diversity: arid regions of Mexico/Southern United States and Australia, which show a similarly high number of different C-4 eudicot genera but differ in the number of C-4 lineages that evolved in situ. Further eudicot C-4 hotspots with many different families and genera are in South Africa, West Africa, Patagonia, Central Asia and the Mediterranean. In general, C-4 eudicots are diverse in deserts and xeric shrublands, tropical and subtropical grasslands, savannas and shrublands. We found C-4 eudicots to occur in areas with less annual precipitation than C-4 grasses which can be explained by frequently associated adaptations to drought stress such as among others succulence and salt tolerance. The data indicate that C-4 eudicot lineages utilising the NAD-ME decarboxylating enzyme grow in drier areas than those using the NADP-ME decarboxylating enzyme indicating biochemical restrictions of the later system in higher temperatures. We conclude that in most eudicot lineages, C-4 evolved in ancestrally already drought-adapted clades and enabled these to further spread in these habitats and colonise even drier areas.