Improving our understanding of environmental controls on the distribution of C3 and C4 grasses

A number of studies have demonstrated the ecological sorting of C-3 and C-4 grasses along temperature and moisture gradients. However, previous studies of C-3 and C-4 grass biogeography have often inadvertently compared species in different and relatively unrelated lineages, which are associated with different environmental settings and distinct adaptive traits. Such confounded comparisons of C-3 and C-4 grasses may bias our understanding of ecological sorting imposed strictly by photosynthetic pathway. Here, we used MaxEnt species distribution modeling in combination with satellite data to understand the functional diversity of C-3 and C-4 grasses by comparing both large clades and closely related sister taxa. Similar to previous work, we found that C-4 grasses showed a preference for regions with higher temperatures and lower precipitation compared with grasses using the C-3 pathway. However, air temperature differences were smaller (2 degrees C vs. 4 degrees C) and precipitation and % tree cover differences were larger (1783 mm vs. 755 mm, 21.3% vs. 7.7%, respectively) when comparing C-3 and C-4 grasses within the same clade vs. comparing all C-4 and all C-3 grasses (i.e., ignoring phylogenetic structure). These results were due to important differences in the environmental preferences of C-3 BEP and PACMAD clades (the two main grass clades). Winter precipitation was found to be more important for understanding the distribution and environmental niche of C-3 PACMADs in comparison with both C-3 BEPs and C-4 taxa, for which temperature was much more important. Results comparing closely related C3C4 sister taxa supported the patterns derived from our modeling of the larger clade groupings. Our findings, which are novel in comparing the distribution and niches of clades, demonstrate that the evolutionary history of taxa is important for understanding the functional diversity of C-3 and C-4 grasses, and should have implications for how grasslands will respond to global change.