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.