Plant families exhibit unique geographic trends in C₄ richness and cover in Australia

Numerous studies have analysed the relationship between C₄ plant cover and climate. However, few have examined how different C₄ taxa vary in their response to climate, or how environmental factors alter C₄:C₃ abundance. Here we investigate (a) how proportional C₄ plant cover and richness (relative to C₃) responds to changes in climate and local environmental factors, and (b) if this response is consistent among families. Proportional cover and richness of C₄ species were determined at 541 one-hectare plots across Australia for 14 families. C₄ cover and richness of the most common and abundant families were regressed against climate and local parameters. C₄ richness and cover in the monocot families Poaceae and Cyperaceae increased with latitude and were strongly positively correlated with January temperatures, however C₄ Cyperaceae occupied a more restricted temperature range. Seasonal rainfall, soil pH, soil texture, and tree cover modified proportional C₄ cover in both families. Eudicot families displayed considerable variation in C₄ distribution patterns. Proportional C₄ Euphorbiaceae richness and cover were negatively correlated with increased moisture availability (i.e. high rainfall and low aridity), indicating they were more common in dry environments. Proportional C₄ Chenopodiaceae richness and cover were weakly correlated with climate and local environmental factors, including soil texture. However, the explanatory power of C₄ Chenopodiaceae models were poor, suggesting none of the factors considered in this study strongly influenced Chenopodiaceae distribution. Proportional C₄ richness and cover in Aizoaceae, Amaranthaceae, and Portulacaceae increased with latitude, suggesting C₄ cover and richness in these families increased with temperature and summer rainfall, but sample size was insufficient for regression analysis. Results demonstrate the unique relationships between different C₄ taxa and climate, and the significant modifying effects of environmental factors on C₄ distribution. Our work also revealed C₄ families will not exhibit similar responses to local perturbations or climate.