All but four of 833 native and 292 naturalised Australian grasses (Poaceae) have been assigned as having the C or C photosynthetic pathway. In conjunction with comprehensive species composition data for 75 geographic subdivisions Australiawide, this has permitted the construction of distribution maps for C and C grasses. C and C grass distributions have been considered (i) independently, using subdivisional native species numbers; and (ii) relatively, using 'subdivisional % C'. C species are most numerous in the Southern Tablelands (New South Wales), the QZ subdivision (Victoria), and Tasmania; C species in the northern Northern Territory and northern Queensland, including the Cook subdivision where 54% of Australia's native C grasses can be found. C 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. C 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. C species number correlates most highly with January average maximum temperature (-ve) and spring rainfall (+ve); C species number with October average minimum temperature (+ve) and February median rainfall (+ve); '%C' with January average minimum temperature (+ve). Predictive multiple linear regression equations were generated using climatic variables. In general, C grass species, like C species, increase in number with increasing rainfall, in their preferred temperature regime. C species are most numerous where the summer is hot and wet; C species where the spring is cool and wet. C species numbers decline with decreasing temperature and/or decreasing summer rainfall; C 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.

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Statement of authorship: MRL, PAC, and CPO designed the study. MRL generated and analysed data. EGE and RCL performed the PEPC quantifications. MRL, PAC, BSR, GB, and CPO contributed plant material. MRL, CL, PWH, and RPE contributed accession collection locations and isotope data.MRL, PAC, and CPO wrote the paper, with the help of all the authors.

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