PHYSIOLOGICAL AND ENVIRONMENTAL-CONTROL OF TRANSPIRATION BY TREES IN WINDBREAKS

Knowledge of the environmental and physiological variables controlling water use by trees in windbreaks should assist efforts to manage comp etition for water between trees and crops in agroforestry. coupling be tween windbreak trees and the atmosphere was therefore assessed by det ermining values of the decoupling coefficient (Omega) for windbreaks d uring four periods of the year, using experimentally-derived aerodynam ic and surface conductances for trees of Azadirachta inidica A. Juss i n windbreaks at Sadore, Niger. Aerodynamic conductances were determine d by scaling-up measurements of leaf boundary layer conductances made using leaf replicas. Surface conductances for the trees were derived f rom measurements of whole-tree transpiration made using the heat-pulse technique. Values of Omega for the windbreaks were approximate to 0.3 at moderate wind speeds in the more humid seasons of the year, but fe ll to 0.1 during the driest period of the year. Thus, windbreaks are g enerally well-coupled to the atmosphere, so that transpiration is pred ominantly driven by the ambient vapour pressure deficit at leaf surfac es and the trees are able to exert close physiological control over wa ter use. Management strategies to reduce the surface conductances of w indbreak canopies should, consequently, effectively reduce demand for water by windbreaks and help to control competition for water between trees and crops. (C) 1998 Elsevier Science B.V.