Seasonal water availability predicts the relative abundance of C3 and C4 grasses in Australia

Aim: Numerous studies have examined the climatic factors that influence the abundance of C4 species within the grass flora (C4 relative species richness) in various regions throughout the world, but very few have examined the relative abundance of C4 vs. C3 grasses (C4 relative abundance). We sought to determine the climatic factors that influence C4 relative abundance throughout Australia. Location: Australia (including Tasmania). Methods: We measured C4 relative abundance at 168 locations and measured δ13C (the abundance of 13C relative to 12C) of the bone collagen of 779 kangaroos collected throughout Australia, as bone collagen δ13C was assumed to be proportional to the relative abundance of C 4 grasses in the diet. Results: Both C4 relative abundance and kangaroo bone collagen δ13C were found to have a strong positive relationship with seasonal water availability, i.e. the distribution of rainfall in the C4 vs. C3 growing seasons (76% and 69% of deviance explained, respectively). There was clear evidence that seasonal water availability was a better predictor of both C4 relative abundance and bone collagen δ13C than other climate variables such as mean annual temperature and January daily minimum temperature. However, seasonal water availability appeared to be a relatively poor predictor of C4 relative species richness, which was most closely related to January daily minimum temperature (90% of deviance explained). Main conclusions: Our results highlight the relatively poor relationship between C4 relative abundance and C4 relative species richness, and suggest that these two variables may be related to different climatic factors. They also suggest that caution is required when using C4 relative species richness to infer the relative biomass and productivity of C4 grasses on a global scale. © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Ltd.