Despite phylogenetic effects, C3-C4 lineages bridge the ecological gap to C4 photosynthesis

C-4 photosynthesis is a physiological innovation involving several anatomical and biochemical components that emerged recurrently in flowering plants. This complex trait evolved via a series of physiological intermediates, broadly termed 'C-3-C-4', which have been widely studied to understand C-4 origins. While this research program has focused on biochemistry, physiology, and anatomy, the ecology of these intermediates remains largely unexplored. Here, we use global occurrence data and local habitat descriptions to characterize the niches of multiple C-3-C-4 lineages, as well as their close C-3 and C-4 relatives. While C-3-C-4 taxa tend to occur in warm climates, their abiotic niches are spread along other dimensions, making it impossible to define a universal C-3-C-4 niche. Phylogeny-based comparisons suggest that, despite shifts associated with photosynthetic types, the precipitation component of the C-3-C-4 niche is particularly lineage specific, being highly correlated with that of closely related C-3 and C-4 taxa. Our large-scale analyses suggest that C-3-C-4 lineages converged toward warm habitats, which may have facilitated the transition to C-4 photosynthesis, effectively bridging the ecological gap between C-3 and C-4 plants. The intermediates retained some precipitation aspects of their C-3 ancestors' habitat, and likely transmitted them to their C-4 descendants, contributing to the diversity among C-4 lineages seen today.