SEASONAL-CHANGES IN STABLE CARBON-ISOTOPE RATIOS WITHIN ANNUAL RINGS OF PINUS-RADIATA REFLECT ENVIRONMENTAL-REGULATION OF GROWTH-PROCESSES

Seasonal cycles in stable carbon isotope composition (delta(13)C) were measured within annual rings of Pinus radiata D. Don from trees in tw o plantation sites which differ markedly in annual water balance. The amplitudes of seasonal delta(13)C variation at the wet and dry sites w ere 1-2 parts per thousand and 4 parts per thousand respectively. Mean delta(13)C values from the wet site were 3 parts per thousand more C- 13 depleted than those from the dry site implying lower water-use effi ciency (carbon assimilation per unit transpiration). A process-based, leaf-level model of stomatal conductance and CO2 assimilation was comb ined with a water balance model to estimate the average daily intercel lular CO2 concentration (c(i)). Over two growing seasons at each site there was generally good agreement between mean canopy-level c(i) deri ved from the tree-ring delta(13)C data and modelled leaf-level c(i) le vels. Further, the ratio of annual CO2 assimilation to transpiration e stimated by the model for each site correlated with the differences in water-use efficiency between the sites. Carbon isotope discrimination of leaves in the canopy was thus reflected directly in the stem wood, and its dynamic variation within a site, as well as between sites, wa s driven principally by the interaction of seasonally changing microme teorological variables and soil water availability.