How big is the influence of tree leaves on global windsystems?
Wind is largely driven by differences in atmospheric pressure.

The effect of trees and forests is to force the winds to flow over the forests. The loss of leaves in forests would result in winds having a larger impact at ground level. Also, due to the change in the surface roughness to air flow because of the loss of leaves the wind would lose less of its energy.

It has been suggested in this paper, though not yet fully accepted by many, that water vapour in the atmosphere due to evaporation and transpiration can contain sufficient latent heat to accelerate air and drive winds.