Stand overstorey processes

Abstract

Typical turbulent eddies in a plant canopy are coherent over much of the canopy depth, making gradient-diffusion theory inappropriate for describing the vertical transfer of scalars and momentum. A better description of scalar transfer is obtained by regarding the canopy as an assembly of source elements (individual leaves), each releasing a plume of scalar material into the turbulent flow. The contrasting behaviours of the plume from each leaf in the near and far fields (or small and large travel times, in comparison with the turbulence time scale) account naturally for observed phenomena such as counter-gradient fluxes within the canopy. With two hypotheses, this paradigm yields a simple, analytic expression for the scalar concentration profile corresponding to a given source density in the canopy, effectively replacing gradient-diffusion theory.