AbstractTo study the dispersion of scalar trace constituents such as heat, water vapour or CO2from extensive sources within vegetation canopies, a Lagrangian approach is essential because of the need to account for the influence of travel time (as a fraction of the Lagrangian time scale) on the dispersion properties of marked fluid particles. A simple, analytic Lagrangian theory is developed for a canopy in which the turbulence is homogeneous. For a given scalar source distribution, predictions are obtained for mean scalar concentration, vertical flux, terms in the balance equation for vertical flux, and vertical diffusivities at first, second and third order. the predictions agree remarkably well with measurements of dispersion from an elevated plane source within a model plant canopy in a wind tunnel. the theory also explains the countergradient heat and CO2fluxes observed in real forest canopies.