Hydraulic resistance in Acer saccharum shoots and its influence on leaf water potential and transpiration

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Abstract
A new method is presented for measuring whole-shoot hydraulic conductance, KT (kg s−1 MPa−1). The method was also used to determine other conductance values in maple (Acer saccharum Marsh.) stem segments of differing diameter including: Kh (absolute conductance or conductance per unit pressure gradient, kg s−1 m MPa−1), Ks (specific conductance or Kh per unit wood area, kg s−1 m−1 MPa−1), and LSC (leaf specific conductance or Kh per unit leaf area, kg s−1 m−1 MPa−1). A regression of KT versus stem basal diameter, D (m), gave KT = 5.998 × 10−2D1.402 (R2 = 0.986 for D from 0.001 to 0.1 m) and a regression for leaf area, AL (m2), gave AL = 4.667 × 103D2.007 (R2 = 0.981 for D from 0.001 to 0.3 m). More than 50% of the resistance to water flow in large shoots (0.1 m in diameter and 8 to 10 m long) was contained in branches less than 0.012 m in diameter, i.e., in the distal 1.5 m of branches. We used the regressions to predict the steady state difference in pressure potential, P, between the base of a shoot of diameter D and the average pressure potential at the apices of the shoot; the relation is given by P = 7.781 × 104ED0.605, where E is the average evaporative flux density (kg s−1 m−2) in the leaves attached to the shoot. After comparing the predictions of this equation to field observations of E and leaf water potential and stomatal conductance, we concluded that the hydraulic conductance of large maple shoots is sufficiently low to prevent maximum stomatal conductance in maple leaves.

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