Effects of water status and soil fertility on the C-isotope signature in Pinus radiata

The efficiency with which trees use water is a major determinant of growth under water-limited conditions. We investigated whether increased access to water and nutrients alters water-use efficiency in Pinus radiata D. Don. Intrinsic transpiration efficiency, defined here as the ratio of CO₂ assimilated and water transpired at a given vapor pressure deficit, is determined by the difference between ambient atmospheric CO₂ concentration (C(a)) and leaf intercellular CO₂ concentration (C(i)). The mean value of c(i)/c(a) can be inferred from an analysis of carbon isotope discrimination (Δ) in wood samples. A total of 117 trees, growing at sites with widely varying soil and climatic conditions in Australia and New Zealand, were cored and distinct annual rings were analyzed for their carbon isotope ratio, and correlated with rainfall during the July-June growing season in the year in which the wood was grown. Where possible, carbon isotope ratios were compared for different years within the same trees. The c(i)/c(a) ratio decreased with decreasing water availability, suggesting that intrinsic transpiration efficiency increased with decreasing water availability. An increase in growing season rainfall of 900 mm resulted in an increase in Δ of about 2.0‰, corresponding to a decrease in intrinsic transpiration efficiency of approximately 24%. A stronger relationship was obtained when carbon isotope discrimination was expressed as a function of the ratio of rainfall to potential transpiration. Carbon isotope discrimination was also negatively correlated with mean annual vapor pressure deficit at different sites. In contrast, nutrient availability had no significant effect on carbon isotope discrimination.