The role of clearcuts and spatial disturbances on the downwind canopy layer flow and turbulence

One of the drastic changes noticed at the landscape scale and leading to subsequent flow modifications arises from logging of a large patch of trees over a forested area several hundreds of acres wide. This paper illustrates the effects of logging on flow characteristics using turbulence measurements within and above the forest canopy.

Results suggest that during daytime, large temperature differences between the bare soil surface and the surrounding forest give rise to the presence of a pressure gradient favoring the development of local circulations between the clearcut and the surrounding forested area. This is demonstrated by examining the flow properties at the AmeriFlux site near Gainesville, Florida, by means of a directional flow analysis on the tower flux data. The presence of the upwind clearcut area enhances the wind velocity shear near treetop and in the upper canopy layer, along with an increased directional shear especially within the tree crown.

Results show that low-frequency eddies contribute relatively more to the flux when the wind flows over the clearcut than it does as it flows from the direction of the homogeneous forest. In addition, analyses show what appears to be a modulation of turbulence as the relative contribution of high-frequency eddies/low-frequency eddies from the clearcut differs from that of the direction of the undisturbed forest canopy.

Though located hundreds of meters upwind of an otherwise uniform and homogeneous pine stand, the clearcut appears to significantly increase the level of intermittency and gustiness. This enhancement in turbulent exchange suggests an additional forcing mechanism that arises from the development of the circulations and that is contributed by large scale eddies.