Responses of foliar 13C, gas exchange and leaf morphology to reduced hydraulic conductivity in Pinus monticola branches

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Abstract
We tested the hypothesis that branch hydraulic conductivity partly controls foliar stable carbon isotope ratio (δ13C) 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 δ13C by about 1‰ (P < 0.0001, n = 65), whereas the 30% reduction in LSC had no effect on foliar δ13C (P = 0.90, n = 65). Variation in the δ13C 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 (ci/ca) in response to experimentally reduced hydraulic conductivity. A strong correlation was observed between stomatal conductance (gs) and photosynthetic demand over a tenfold range in gs. Although ci/ca and δ13C appeared to be relatively homeostatic, current-year leaf area varied linearly as a function of branch hydraulic conductivity (r2 = 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 ci/ca.

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