Stomatal and Boundary-Layer Conductances in an Amazonian terra Firme Rain Forest

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Abstract
(1) Diurnal measurements of stomatal conductance (gs) and boundary layer conductance (ga) were made through the canopy at a primary rain forest site in the Amazon Basin, Brazil, in three experimental seasons and over a range of soil water conditions. (2) Maximum values of gs were observed near the top of the 40 m deep canopy and they declined towards the forest floor. Observed values of ga also decreased through the canopy, but a slight maximum occurred 20 m from the ground. (3) In the upper part of the tree canopy the diurnal maximum in gs occurred in the mid-morning and the value declined for the remainder of the day. This peak was less marked lower in the canopy, and was not present near the forest floor. (4) gs was correlated negatively with specific humidity deficit (D) and positively with solar radiation (S). The response of gs to D increased with increasing S at all but the highest value of S. (5) The lowest values of gs occurred at the same time as the lowest soil water potentials, although these corresponded to only moderate soil water deficits.