Ecological selection pressures for C4 photosynthesis in the grasses

Grasses using the C₄ photosynthetic pathway dominate grasslands and savannahs of warm regions, and account for half of the species in this ecologically and economically important plant family. The C₄ pathway increases the potential for high rates of photosynthesis, particularly at high irradiance, and raises water-use efficiency compared with the C₃ type. It is therefore classically viewed as an adaptation to open, arid conditions. Here, we test this adaptive hypothesis using the comparative method, analysing habitat data for 117 genera of grasses, representing 15 C₄ lineages. The evidence from our three complementary analyses is consistent with the hypothesis that evolutionary selection for C₄ photosynthesis requires open environments, but we find an equal likelihood of C₄ evolutionary origins in mesic, arid and saline habitats. However, once the pathway has arisen, evolutionary transitions into arid habitats occur at higher rates in C₄ than C₃ clades. Extant C₄ genera therefore occupy a wider range of drier habitats than their C₃ counterparts because the C₄ pathway represents a pre-adaptation to arid conditions. Our analyses warn against evolutionary inferences based solely upon the high occurrence of extant C₄ species in dry habitats, and provide a novel interpretation of this classic ecological association.