C4 plants likely will respond quite differently than C3 plants to global change, and thus a better understanding of the controls on their respective distributions is needed. C4 grasses are abundant in tropical and subtropical grasslands and savannas but are conspicuously absent from colder climate grasslands where C3 grasses dominate. Physiological aspects of photosynthetic pathway variation have been invoked to explain the biogeography of C3 and C4 grasses, although recent research has questioned the role of physiology in determining these patterns. We studied the distribution of grass species across the Hawaiian Islands to assess how species with each photosynthetic pathway (and, for C4 species, the biochemical subtypes) were distributed along steep climate gradients. We also examined how two main grass clades, BEP and PACCMAD, were distributed along these climate gradients, as C4 species are restricted to the PACCMAD clade, and BEP contains only C3 species. We utilized herbarium collection data (which we geo-referenced) and GIS climate layers. We then tested three approaches for predicting C3/C4 variations. One approach relies on a mechanistically based prediction (the crossover temperature approach), a second uses a regression from previous grassland studies in the US Great Plains, and the third employs ecological niche modeling

Results/Conclusions
Our results show the expected sorting of C3 and C4 grass species along temperature gradients, with reduced C4 species occurrence at higher elevations (and corresponding increases of C3 species). The mean annual temperature at which 50% of C4 observations occur is approximately 22°C, whereas for the 50th percentile of C3 species occurrence, the mean annual temperature is 17°C. However, these trends overlap strongly with the distribution of grasses in the PACCMAD and BEP clades, thus questioning a physiological basis for the distribution of C3 and C4 grasses along temperature gradients. By contrast, there is a significant sorting of C3 and C4 species along precipitation gradients, even when phylogenetic differences are accounted for. 90% of C4 observations occur below a mean annual precipitation of 2,500 mm, compared to over 3,000 mm for C3 grass species. The three approaches for predicting C3 and C4 occurrence generally agreed at lower elevations and hotter temperatures (i.e., predicted C4 dominance), but only the ecological niche modeling approach accurately predicted the (less common) occurrence of C4 grasses at higher elevations and colder temperatures. This approach also better predicted the sorting of C3 and C4 grasses along precipitation gradients