A stochastic Lagrangian trajectory model is used to estimate scalar flux footprints for canopies of varying leaf area distributions and leaf area indexes. An analytical second order closure model is used to predict mean wind speed, second moments and dissipation rate of turbulent kinetic energy within a forest canopy. In order to measure all the flux components, and thus the total flux, with a desired accuracy, sources are located at the forest floor in the footprint function estimation. The footprint functions are calculated for five observation levels above the canopy top. The canopy turbulence has a significant effect on footprint estimates. The effect of canopy cannot be analysed by using analytical tools only. We present the main results in easy-to-use plots, where the required extent of the homogeneous forest for flux determination with the required accuracy can be looked up. It is found that at low observation heights both canopy density and canopy structure affect the length of the fetch. The higher above the canopy top flux is measured the more pronounced is the effect of the vertical leaf area distribution. The forest fetch for flux measurements is strongly dependent on the required accuracy.