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.