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 C-4 species within the grass flora (C-4 relative species richness) in various regions throughout the world, but very few have examined the relative abundance of C-4 vs. C-3 grasses (C-4 relative abundance). We sought to determine the climatic factors that influence C-4 relative abundance throughout Australia. Location Australia (including Tasmania). Methods We measured C-4 relative abundance at 168 locations and measured delta C-13 (the abundance of C-13 relative to C-12) of the bone collagen of 779 kangaroos collected throughout Australia, as bone collagen delta C-13 was assumed to be proportional to the relative abundance of C-4 grasses in the diet. Results Both C-4 relative abundance and kangaroo bone collagen delta C-13 were found to have a strong positive relationship with seasonal water availability, i.e. the distribution of rainfall in the C-4 vs. C-3 growing seasons (76% and 69% of deviance explained, respectively). There was clear evidence that seasonal water availability was a better predictor of both C-4 relative abundance and bone collagen delta C-13 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 C-4 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 C-4 relative abundance and C-4 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 C-4 relative species richness to infer the relative biomass and productivity of C-4 grasses on a global scale.