Coordinating leaf functional traits with branch hydraulic conductivity: resource substitution and implications for carbon gain

We Studied relationships among branch hydraulic conductivity, xylem embolism, stomatal conductance (g(s)), foliar nitrogen (N) concentration and specific leaf area (SLA) of seven tree species growing at four temperate woodland sites spanning a 464-1350 mm rainfall gradient. Specifically, we examined the question: are g(s) and foliar N concentration coordinated with branch hydraulic conductivity and, if so, what are the implications for carbon assimilation? Area-based, light-saturated photosynthetic rate (A(a)) was uniquely and positively correlated with g(s) and foliar N concentration. Multiple regression analyses showed that, when variability in SLA was controlled for, the (positive) partial slope for each predictor remained significant. In contrast, there was a negative correlation between gs and foliar N concentration such that, for any given A(a), leaves with it high gs allocated less N to foliage than leaves with a low g(s). Foliar N concentration was negatively correlated with branch hydraulic conductivity, whereas g(s) was positively correlated with branch hydraulic conductivity. These relationships were also significant when variability in leaf area to sap-wood area ratio, g(s) and SLA were controlled for in a multiple regression, suggesting that the relationships were unique and independent of other confounding factors. Trees with low water transport capacity were able to Support a high A(a) by increasing investment in foliar N. Resource substitution occurred such that there was a trade-off between g(s) and foliar N in relation to branch hydraulic conductivity. High A(a) could be Sustained through either a high branch hydraulic conductivity and hence high g(s) and a low allocation to foliar N, or the effect of a low branch hydraulic conductivity and hence low g(s) could be offset by a high allocation to foliar N. The results are discussed in relation to mechanisms for minimizing the negative effects of limited water availability on carbon gain.