Variation in bulk-leaf ¹³ C discrimination, leaf traits and water-use efficiency-trait relationships along a...

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 ¹³ C isotope discrimination (Δ¹³ 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 Δ¹³ C and intrinsic water-use efficiency (WUE_i ) vary with climate at a continental scale? (ii) What are the seasonal and spatial patterns in Δ¹³ C/WUE_i across biomes and species? (iii) To what extent does Δ¹³ 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 Δ¹³ C differ across seasons? We found that MAP, temperature seasonality, isothermality and annual temperature range exerted independent effects on foliar Δ¹³ C/WUE_i . Temperature-related variables exerted larger effects than rainfall-related variables. The relative importance of photosynthesis and stomatal conductance (g_s ) in determining Δ¹³ C differed across seasons: Δ¹³ C was more strongly regulated by g_s during the dry-season and by photosynthetic capacity during the wet-season. Δ¹³ C was most strongly correlated, inversely, with leaf mass area ratio among all leaf attributes considered. Leaf N_mass 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 Δ¹³ C. Leaf P_mass showed significant positive relationship with MAP and Δ¹³ C only during the dry-season. For all leaf nutrient-related traits, the relationships obtained for Δ¹³ C with MAP or MI indicated that Δ¹³ 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 Δ¹³ C across Australia.