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

C₄ 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 'C₃-C₄', which have been widely studied to understand C₄ 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 C₃-C₄ lineages, as well as their close C₃ and C₄ relatives. While C₃-C₄ taxa tend to occur in warm climates, their abiotic niches are spread along other dimensions, making it impossible to define a universal C₃-C₄ niche. Phylogeny-based comparisons suggest that, despite shifts associated with photosynthetic types, the precipitation component of the C₃-C₄ niche is particularly lineage specific, being highly correlated with that of closely related C₃ and C₄ taxa. Our large-scale analyses suggest that C₃-C₄ lineages converged toward warm habitats, which may have facilitated the transition to C₄ photosynthesis, effectively bridging the ecological gap between C₃ and C₄ plants. The intermediates retained some precipitation aspects of their C₃ ancestors' habitat, and likely transmitted them to their C₄ descendants, contributing to the diversity among C₄ lineages seen today.