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, I ntrinsic transpiration efficiency, defined here as the ratio of CO2 assimil ated 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-a) The mean value of c(i)/c(a), can be inferred from an analysis of carbon isotope discrimination (Delta) in w ood samples. A total of 117 trees, growing at sites with widely varying soil and climati c conditions in Australia and New Zealand, were cored and distinct annual r ings were analyzed for their carbon isotope ratio, and correlated with rain fall during the July-June growing season in the year in which the wood was grown. Where possible, carbon isotope ratios were compared for different ye ars within the same trees. The c(i)/c(a) ratio decreased with decreasing water availability, suggestin g that intrinsic transpiration efficiency increased with decreasing water a vailability. An increase in growing season rainfall of 900 mm resulted in a n increase in Delta of about 2.0 parts per thousand, corresponding to a dec rease in intrinsic transpiration efficiency of approximately 24%. A stronge r relationship was obtained when carbon isotope discrimination was expresse d as a function of the ratio of rainfall to potential transpiration. Carbon isotope discrimination was also negatively correlated with mean annual vap or pressure deficit at different sites. In contrast, nutrient availability had no significant effect on carbon isotope discrimination.