The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to env ironmental conditions. However, tree height may also contribute to the vari ability in mean stomatal conductance. Mean canopy stomatal conductance of i ndividual tree crowns (G(Si)) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G(Si) decreases with tree heigh t was tested. Over 13 d of the growing season during which soil moisture wa s not limiting, G(Si) decreased linearly with the natural logarithm of vapo ur pressure deficit (D), and increased exponentially to saturation with pho tosynthetic photon flux density (Q(o)). Under conditions of D = 1 kPa and s aturating Q(o), G(Si) decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood-to-leaf area ratio (A(S ):A(L)) doubled. A simple hydraulic model explained the variation in G(Si) based on an inverse relationship with height, and a linear relationship wit h A(S):A(L). Thus, in F. sylvatica, adjustments in A(S):A(L) partially comp ensate for the negative effect of increased flow-path length on leaf conduc tance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylv atica reduces carbon uptake through a corresponding decrease in stomatal co nductance.