Wind speed and leaf boundary layer conductance variation within tree crown - Consequences on leaf-to-atmosphere coupling and tree functions

in situ measurements and model simulations were used to analyse the spatial variations of wind speed (U) and leaf boundary layer conductance (g(b)(H)), and their effects on leaf transpiration and photosynthesis within the crown of a 20-year-old walnut tree (Juglans regia L.). Wind speed attenuation within the tree crown was strongly correlated to the cumulative leaf area along the wind path into the crown, as deduced from the 3D distribution of the tree foliage, obtained by the combined use of digitizing and allometric relationships. Simulations by the RATP model, i.e., a 3D numerical model of radiation transfer, leaf transpiration and photosynthesis (Sinoquet et al., 1999), were used (i) to study the spatial variations of transpiration, photosynthesis and leaf-to-atmosphere coupling within the tree crown; and (ii) to analyse coupling at tree level, in order to evaluate the sensitivity of total transpiration to climate variations. The main results of the simulations were: (i) spatial variations of stomatal conductance were mainly responsible for the important spatial variation of the leaf-to-air decoupling factor (Omega) within the tree crown; (ii) high coupling was observed at tree level, so that whole tree transpiration was primarily controlled by air VPD and secondarily by radiation; (iii) wind velocity and direction influenced only weakly the local transpiration, and had no effect on photosynthetic rates. (C) 1999 Elsevier Science B.V. All rights reserved.