A simplified second-moment turbulence closure model, which has been reasonably well tested in various geophysical problems, is used to simulate effects of a tall tree canopy on air circulations in the atmospheric boundary layer. Qualitative simulation of a canopy flow, with nearly constant and low wind speeds in a canopy, but large wind shears near a treetop, and unstable (stable) temperature layers within a canopy during the night (day) are all satisfactory. Strong couplings between the mean and turbulence variables are obvious when simulations performed with and without a canopy in the model are compared with one another.