The distribution of C₃ and C₄ grasses in Australia in relation to climate

All but four of 833 native and 292 naturalised Australian grasses (Poaceae) have been assigned as having the C₄ or C₄ photosynthetic pathway. In conjunction with comprehensive species composition data for 75 geographic subdivisions Australiawide, this has permitted the construction of distribution maps for C₃ and C₄ grasses. C₃ and C₄ grass distributions have been considered (i) independently, using subdivisional native species numbers; and (ii) relatively, using 'subdivisional % C₄'. C₃ species are most numerous in the Southern Tablelands (New South Wales), the QZ subdivision (Victoria), and Tasmania; C₄ species in the northern Northern Territory and northern Queensland, including the Cook subdivision where 54% of Australia's native C₄ grasses can be found. C₃ species predominate only in south-west Western Australia, parts of southern South Australia, the Tablelands, central and south coast, and south western slopes of New South Wales, Victoria and Tasmania. C₄ species predominate over 80-85% of the continental area. For 16 temperature and rainfall variables, subdivisional area-weighted means were calculated, and correlation analyses performed. C₃ species number correlates most highly with January average maximum temperature (-ve) and spring rainfall (+ve); C₄ species number with October average minimum temperature (+ve) and February median rainfall (+ve); '%C₄' with January average minimum temperature (+ve). Predictive multiple linear regression equations were generated using climatic variables. In general, C₄ grass species, like C₃ species, increase in number with increasing rainfall, in their preferred temperature regime. C₄ species are most numerous where the summer is hot and wet; C₃ species where the spring is cool and wet. C₄ species numbers decline with decreasing temperature and/or decreasing summer rainfall; C₃ species numbers decline with increasing temperature and/or decreasing spring rainfall. Results are also considered in relation to the taxonomic and physiological heterogeneity of grasses and to Australia's geobotanic history.