Roughness length
From Wikireedia
The roughness length is used in numerical models to express the roughness of the surface. It affects the intensity of mechanical turbulence and the fluxes of varies quantitities above the surface. Trees have a high roughness length compared to tree-free suburban areas. Tree planting therefore reduces the wind speed. Another example is the clearing of the Amazon rainforest to make way for pastures and crops. This would reduce the roughness length.