Water availability and branch length determine delta C-13 in foliage of Pinus pinaster

The stable carbon isotope composition (delta(13)C) Of foliage integrates si gnals resulting from environmental and hydraulic constraints on water movem ent and photosynthesis. We used branch length as a simple predictor of hydr aulic constraints to water fluxes and determined the response of delta(13)C to varying water availability. Foliage up to 6 years old was taken from Pi nus pinaster Ait. trees growing at four sites differing in precipitation (P ; 414-984 mm year(-1)) and potential evaporation (ET; 1091-1750 mm year(-1) ). Branch length was the principal determinant of temporal trends in delta( 13)C. The strong relationship between delta(13)C and branch length was a fu nction of hydraulic conductance, which was negatively correlated with branc h length (r(2) = 0.84). Variation in P and ET among sites was reflected in delta(13)C, which was negatively correlated with P/ET (r(2) = 0.66). Howeve r, this analysis was confounded by differences in branch length. If the eff ects of branch length on delta(13)C were first removed, then the 'residual' delta(13)C was more closely related to PIET (r(2) = 0.99), highlighting th e importance of accounting for variation in hydraulic constraints to water flux between sites and years. For plant species that exhibit considerable p henotypic plasticity in response to changes in environment (e.g., variation in leaf area, branch length and number, or stem form), the environmental e ffects on delta(13)C in foliage can only be reliably assessed if deconvolut ed from hydraulic constraints.