Sensitivity of stand transpiration to wind velocity in a mixed broadleaved deciduous forest

Wind velocity (U) within and above forest canopies can alter the coupling between the vapor-saturated sub-stomatal airspace and the drier atmosphere aloft, thereby influencing transpiration rates. In practice, however, the actual increase in transpiration with increasing U depends on the aerodynamic resistance (R-A) to vapor transfer compared to canopy resistance to water vapor flux out of leaves (R-C, dominated by stomatal resistance, R-stom), and the rate at which R-A decreases with increasing U. We investigated the effect of U on transpiration at the canopy scale using filtered meteorological data and sap flux measurements gathered from six diverse species of a mature broadleaved deciduous forest. Only under high light conditions, stand transpiration (E-C) increased slightly (6.5%) with increasing U ranging from 0.7 to similar to 4.7 ms(-1). Under other conditions, sap flux density (J(s)) and E-C responded weakly or did not change with U. R-A, estimated from Monin-Obukhov similarity theory, decreased with increasing U, but this decline was offset by increasing R-C, estimated from a rearranged Penman-Monteith equation, due to a concurrent increase in vapor pressure deficit (D). The increase of R-C with D over the observed range of U was consistent with increased R-stom by similar to 40% based on hydraulic theory. Except for very rare half-hourly values, the proportion of R-A to total resistance (R-T) remained <15% over the observed range of conditions. These results suggest that in similar forests and conditions, the direct effect of U reducing R-A and thus increasing transpiration is negligible. However, the observed U-D relationship and its effect on R-stom must be considered when modeling canopy photosynthesis. (C) 2013 Elsevier B.V. All rights reserved.