Abstract Since tall vegetation canopies have large roughness lengths, it is in practice often necessary to apply the Monin-Obukhov theory to the “roughness sublayer”, which is influenced by the properties of the canopy. In this sublayer above tall vegetation canopies, the gradients of temperature and other scalar quantities are smaller than observed above low vegetation canopies at “normal” observation heights. For the forest site at Castelporziano a displacement height of d = 9 m = 0.75 h C and a roughness length of z 0 = 0.47 m = 0.039 h C was estimated, with h C being the mean canopy height. The deviation between the usual universal functions for smooth surfaces averages to a correcting factor γ θ = 2.2 . To understand the processes leading to this deviation, two cases are investigated in detail.