Seasonal Changes in Stable Carbon Isotope Ratios within Annual Rings of Pinus radiata Reflect Environmental Regulation of Growth Processes

Abstract

Seasonal cycles in stable carbon isotope composition (δ¹³C) were measured within annual rings of Pinus radiata D. Don from trees in two plantation sites which differ markedly in annual water balance. The amplitudes of seasonal δ¹³C variation at the wet and dry sites were 1–2 and 4 respectively. Mean δ¹³C values from the wet site were 3 more ¹³C depleted than those from the dry site implying lower water-use efficiency (carbon assimilation per unit transpiration). A process-based, leaf-level model of stomatal conductance and CO₂ assimilation was combined with a water balance model to estimate the average daily intercellular CO₂ concentration (c_i). Over two growing seasons at each site there was generally good agreement between mean canopy-level c_i derived from the tree-ring δ¹³C data and modelled leaf-level c_i levels. Further, the ratio of annual CO₂ assimilation to transpiration estimated 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, was driven principally by the interaction of seasonally changing micrometeorological variables and soil water availability.