Plant families exhibit unique geographic trends in C(4) richness and cover in Australia

Numerous studies have analysed the relationship between C(4) plant cover and climate. However, few have examined how different C(4) taxa vary in their response to climate, or how environmental factors alter C(4):C(3) abundance. Here we investigate (a) how proportional C(4) plant cover and richness (relative to C(3)) responds to changes in climate and local environmental factors, and (b) if this response is consistent among families. Proportional cover and richness of C(4) species were determined at 541 one-hectare plots across Australia for 14 families. C(4) cover and richness of the most common and abundant families were regressed against climate and local parameters. C(4) richness and cover in the monocot families Poaceae and Cyperaceae increased with latitude and were strongly positively correlated with January temperatures, however C(4) Cyperaceae occupied a more restricted temperature range. Seasonal rainfall, soil pH, soil texture, and tree cover modified proportional C(4) cover in both families. Eudicot families displayed considerable variation in C(4) distribution patterns. Proportional C(4) 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 C(4) Chenopodiaceae richness and cover were weakly correlated with climate and local environmental factors, including soil texture. However, the explanatory power of C(4) Chenopodiaceae models were poor, suggesting none of the factors considered in this study strongly influenced Chenopodiaceae distribution. Proportional C(4) richness and cover in Aizoaceae, Amaranthaceae, and Portulacaceae increased with latitude, suggesting C(4) 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 C(4) taxa and climate, and the significant modifying effects of environmental factors on C(4) distribution. Our work also revealed C(4) families will not exhibit similar responses to local perturbations or climate.