Aerodynamic conductances of trees in windbreaks

Abstract

A scheme for scaling leaf boundary layer conductances ( b b) to tree crowns was developed for Azadirachta indica A. Juss. trees in windbreaks in the Sahel. This scheme was derived from measurements of g b, made with heated leaf-replica pairs mounted in the crowns of windbreak trees, and values of bulk aerodynamic conductances for whole trees ( g a) in windbreaks, which were determined from the rate of evaporation from two artificially wetted, excised trees, using the hanging-tree technique. Heated pairs of leaf replicas were constructed and tested in a wind tunnel before being deployed in the field. The effects of wind speed ( u) on g b in the wind tunnel agreed with expectations based on observations by others of the aerodynamic properties of leaves. Similar responses to wind speed were found when the heated leaf-replica pairs were used in situ; aggregated leaf boundary layer conductances ( g bt), calculated by summing g b over the total leaf area of each tree as conductances in parallel, were proportional to u z , where z varied between 0.5 and 0.8, the values expected for laminar and turbulent boundary layers, respectively. In contrast, g a was proportional to u 1.1t and was much smaller than g bttb,, even if effects of differences in leaf areas among trees were accounted for. The differences between measured values of ga and g bt were used to derive an empirical model of g ac, the conductance for scalar transfer from the limits of the leaf boundary layers to the reference position outside the tree crown. This model can be used to estimate g a from wind speed, so that g a can be estimated by summing, as conductances in series, g ac and values of g bt determined from in situ measurements of g b. Thus, at sites where trees are sparsely or non-homogeneously distributed and the cutting of more than a few trees to measure g a is not practicable, heated leaf-replica pairs and the hanging-tree technique can be used together to develop a scheme for scaling g b to the whole tree. This approach should be particularly useful in studies examining energy budgets in agroforestry, horticulture or other settings where trees are isolated or do not form a closed canopy.