The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G(Si)) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G(Si) decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G(Si) decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q(o)). Under conditions of D = 1 kPa and saturating 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 with A(S):A(L). Thus, in F. sylvatica, adjustments in A(S):A(L) partially compensate for the negative effect of increased flow-path length on leaf conductance. 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. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.