A generic model of forest canopy conductance dependent on climate, soil water availability and leaf area index

This paper analyses the variation in tree canopy conductance for water vapo ur (g(c)) in order to derive a general expression, including the effects of solar radiation (R), vapour pressure deficit (D), leaf area index (LAI) an d extractable soil water. Canopy conductance was calculated from transpirat ion measured in 21 broadleaved and coniferous forest stands, under differen t climates: temperate, mountain, tropical and boreal. Common features in th e dependence of g(c) on climate and on soil water content were exhibited. W hen soil water was not limiting, ge was shown to increase linearly with LAI in the range 0 to 6 m(2) m(-2) and reach a plateau value. Besides the posi tive effect of increasing R and the negative effect of increasing D on g(c) , it was surprisingly shown that a decrease in extractable soil water induc ed a similar reduction in ge in various tree species, equally in coniferous and in broadleaved. Based on these findings, a general canopy conductance function is proposed.