Correlations between carbon isotope discrimination and climate of native habitats for diverse eucalypt taxa growing in a common garden

Distributions of common species of Eucalyptus in south-eastern Australia are related to gradients in temperature and rainfall. To determine whether intrinsic water-use efficiency (as indexed by carbon isotope discrimination, Delta) or other leaf attributes were related to climate of native habitats, we sampled 17 populations representing 14 species of Eucalyptus growing in a common garden in southcentral New South Wales. Phreatophytes were clearly distinguished from populations that are totally dependent upon soil moisture derived from rainfall by having higher Delta at a particular level of rainfall. Among 12 non-phreatophytic populations (11 species), Delta was positively correlated with mean annual precipitation (r = 0.75, P = 0.005), December-March precipitation (r = 0.79, P = 0.002), an index of annual soil moisture (r = 0.81, P = 0.001) and seasonality of precipitation (r = 0.85, P < 0.001). There were similarly strong but negative correlations between Delta and potential evaporation during the summer months, but Delta was not correlated with annual potential evaporation of the source sites. Leaf mass per unit area (rho(l)) was negatively correlated with indices of water availability, positively correlated with nitrogen per unit leaf area (r = 0.90, P < 0.001), and negatively correlated with Delta (r = -0.73, P = 0.007). Delta was negatively correlated with area-based leaf nitrogen (r = -0.79, P = 0.002). These complementary correlations among Delta, rho(l), and nitrogen per unit leaf area indicate that variation in Delta may stem largely from variation in photosynthetic capacity. The results provide strong evidence that variation in Delta and rho(l) reflect genetic adaptations to native habitats.