Variation in bulk-leaf C discrimination, leaf traits and water-use efficiency-trait relationships along a continental-scale climate gradient in Australia.

Large spatial and temporal gradients in rainfall and temperature occur across Australia. This heterogeneity drives ecological differentiation in vegetation structure and ecophysiology. We examined multiple leaf-scale traits, including foliar C isotope discrimination (Δ C), rates of photosynthesis and foliar N concentration and their relationships with multiple climate variables. Fifty-five species across 27 families were examined across eight sites spanning contrasting biomes. Key questions addressed include: (i) Does Δ C and intrinsic water-use efficiency (WUE ) vary with climate at a continental scale? (ii) What are the seasonal and spatial patterns in Δ C/WUE across biomes and species? (iii) To what extent does Δ C reflect variation in leaf structural, functional and nutrient traits across climate gradients? and (iv) Does the relative importance of assimilation and stomatal conductance in driving variation in Δ C differ across seasons? We found that MAP, temperature seasonality, isothermality and annual temperature range exerted independent effects on foliar Δ C/WUE . Temperature-related variables exerted larger effects than rainfall-related variables. The relative importance of photosynthesis and stomatal conductance (g ) in determining Δ C differed across seasons: Δ C was more strongly regulated by g during the dry-season and by photosynthetic capacity during the wet-season. Δ C was most strongly correlated, inversely, with leaf mass area ratio among all leaf attributes considered. Leaf N was significantly and positively correlated with MAP during dry- and wet-seasons and with moisture index (MI) during the wet-season but was not correlated with Δ C. Leaf P showed significant positive relationship with MAP and Δ C only during the dry-season. For all leaf nutrient-related traits, the relationships obtained for Δ C with MAP or MI indicated that Δ C at the species level reliably reflects the water status at the site level. Temperature and water availability, not foliar nutrient content, are the principal factors influencing Δ C across Australia.