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 competition for water between trees and crops in agroforestry. Coupling between windbreak trees and the atmosphere was therefore assessed by determining values of the decoupling coefficient (Ω) for windbreaks during four periods of the year, using experimentally-derived aerodynamic and surface conductances for trees of Azadirachta inidica A. Juss in windbreaks at Sadoré, Niger. Aerodynamic conductances were determined by scaling-up measurements of leaf boundary layer conductances made using leaf replicas. Surface conductances for the trees were derived from measurements of whole-tree transpiration made using the heat-pulse technique. Values of Ω for the windbreaks were ≈0.3 at moderate wind speeds in the more humid seasons of the year, but fell to 0.1 during the driest period of the year. Thus, windbreaks are generally well-coupled to the atmosphere, so that transpiration is predominantly driven by the ambient vapour pressure deficit at leaf surfaces and the trees are able to exert close physiological control over water use. Management strategies to reduce the surface conductances of windbreak canopies should, consequently, effectively reduce demand for water by windbreaks and help to control competition for water between trees and crops.
Forest Ecology and Management (1998) 105 (1/3) 159-173 [doi:10.1016/S0378-1127(97)00292-2]