Responses of foliar δ13C, gas exchange and leaf morphology to reduced hydraulic conductivity in Pinus monticola branches

We tested the hypothesis that branch hydraulic conductivity partly controls foliar stable carbon isotope ratio (delta C-13) by its influence on stomatal conductance in Pinus monticola Dougl. Notching and phloem-girdling treatments were applied to reduce branch conductivity over the course of a growing season. Notching and phloem girdling reduced leaf-specific conductivity (LSC) by about 30 and 90%, respectively. The 90% reduction in LSC increased foliar delta C-13 by about 1 parts per thousand (P < 0.0001, n = 65), whereas the 30% reduction in LSC had no effect on foliar delta C-13 (P = 0.90, n = 65). Variation in the delta C-13 of dark respiration was similar to that of whole-tissues when compared among treatments. These isotopic measurements, in addition to instantaneous gas exchange measurements, suggested only minor adjustments in the ratio of intercellular to atmospheric CO2 partial pressures (c(i)/c(a)) in response to experimentally reduced hydraulic conductivity. A strong correlation was observed between stomatal conductance (g(s)) and photosynthetic demand over a tenfold range in g(s). Although c(i)/c(a) and delta C-13 appeared to be relatively homeostatic, current-year leaf area varied linearly as a function of branch hydraulic conductivity (r(2) = 0.69, P < 0.0001, n = 18). These results suggest that, for Pinus monticola, adjustment of leaf area is a more important response to reduced branch conductivity than adjustment of c(i)/c(a).