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 Austral ia are related to gradients in temperature and rainfall. To determine whether intrinsic water-use efficiency (as indexed by carbon isotope d iscrimination, 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 Wa les. Phreatophytes were clearly distinguished from populations that ar e totally dependent upon soil moisture derived from rainfall by having higher Delta at a particular level of rainfall. Among 12 non-phreatop hytic populations (11 species), Delta was positively correlated with m ean annual precipitation (r = 0.75, P = 0.005), December-March precipi tation (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 a nd potential evaporation during the summer months, but Delta was not c orrelated 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 lea f nitrogen (r = -0.79, P = 0.002). These complementary correlations am ong Delta, rho(l), and nitrogen per unit leaf area indicate that varia tion in Delta may stem largely from variation in photosynthetic capaci ty. The results provide strong evidence that variation in Delta and rh o(l) reflect genetic adaptations to native habitats.