Vertical profiles of boundary layer conductance and wind speed in a cotton canopy measured with heated brass surrogate leaves

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Knowledge of single leaf boundary layer conductance ( g b) within extensive canopies is required to scale stomatally regulated gas exchange from leaf to canopy. Fluxes of terpenes and heat, formation of dew, and other processes occurring at the leaf surface may be controlled by g b. We develop, test, and deploy at five canopy heights, a set of paired, heated and unheated brass surrogate leaves, with realistic 3-dimensional shape and characteristic dimension ( d) for cotton ( Gossypium hirsutum L.) in California. An equation describing heat transfer from a flat plate, enhanced by a factor of 1.15 for complex leaf shape, adequately described g b of the surrogate leaves. In the field, values of g b measured with the surrogate leaves near the top of the canopy were closely related to wind speed, as expected from theory. The dependence of g b on leaf size was apparent at all heights in the canopy using surrogate leaves differing in d. The profile of within-canopy wind speed ( u) was determined using the profile of g b and d of the surrogate leaves. This profile of effective u in the leaf environment was well described by a conventional exponential decay model with an empirically derived wind extinction coefficient ( K = 1.5). This profile of u was combined with the directly measured profile of d of biological leaves to determine the actual profile of g b in this canopy. The increasing leaf size toward the middle of the canopy caused actual g b to decline more steeply than predicted from constant d. With depth in the canopy sunflecks increasingly contributed to unequal radiation interception between paired surrogate leaves and therefore to anomalies in calculated g b. A simple data filtering protocol, rejecting data outside the range observed at the top of the canopy, is introduced. Data rejection increased appropriately with increasing sunfleck frequency and depth in the canopy. These results extend the applicability of the heated, paired surrogate leaf technique to sheltered positions in dense canopies, and to large-leafed species, allowing direct measurement of an important canopy parameter ( g b) that has typically been estimated or calculated as a residual.