Aerodynamic conductances of trees in windbreaks
A scheme for scaling leaf boundary layer conductances (bb) to tree crowns was developed for Azadirachta indica A. Juss. trees in windbreaks in the Sahel. This scheme was derived from measurements of gb, made with heated leaf-replica pairs mounted in the crowns of windbreak trees, and values of bulk aerodynamic conductances for whole trees (ga) 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 gb 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 (gbt), calculated by summing gb over the total leaf area of each tree as conductances in parallel, were proportional to uz, where z varied between 0.5 and 0.8, the values expected for laminar and turbulent boundary layers, respectively. In contrast, ga was proportional to u1.1t and was much smaller than gbttb,, even if effects of differences in leaf areas among trees were accounted for.
The differences between measured values of ga and gbt were used to derive an empirical model of gac, 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 ga from wind speed, so that ga can be estimated by summing, as conductances in series, gac and values of gbt determined from in situ measurements of gb. Thus, at sites where trees are sparsely or non-homogeneously distributed and the cutting of more than a few trees to measure ga is not practicable, heated leaf-replica pairs and the hanging-tree technique can be used together to develop a scheme for scaling gb 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.
Agricultural and Forest Meteorology (1996) 86 (1-2) 17-31 [doi:10.1016/S0168-1923(96)02415-X]