A Large Eddy Simulation (LES) has been performed of forest microclimate to investigate edge effects on the budget of a scalar. Outputs from the LES demonstrate that strong horizontal and vertical advections are of great importance at the transition from smooth terrain to forest. An upward tilt to the contours of horizontal advection can be seen in the region from the edge to about ten canopy heights, similar to that of the growth of an internal boundary layer (IBL). The scalar gradient and scalar source gradient between the upstream flow and adjusting flow causes relatively strong horizontal advection at the interface. The upward motion produced by the forest edge triggers vertical advection.

The contribution to the scalar budget from turbulent fluxes is smaller in the transition region than in the equilibrium region. This difference is associated to the suppression of turbulence in the transition region and re-establishment in the equilibrium region. The integration (from the ground to the canopy top) of horizontal advection balances out the scalar source right on the back of forest edge, while the upward flux at the canopy top is negligible. The former is about ten times larger than the latter. Flux transfer by turbulent motion gradually picks up as airflow adjusts to the new rough surface. Downwind of ten canopy heights in the streamwise direction, scalar flux (resolved scale plus sub-grid scale) at canopy top becomes dominant while the advection terms die out. One consequence of this finding is that a fetch of at least ten canopy heights is needed in eddy covariance flux measurement downwind of a forest edge.