The stability of different silvicultural systems: a wind tunnel investigation.

We report on a wind-tunnel study with 1 : 75 scale model trees designed to examine the influence of canopy structure on the formation of turbulent gusts above forests. This was to test the hypothesis that more irregular canopy structures produce less intense gusts because the change in wind speed with height at canopy top is less severe. Measurements were made of wind speeds and turbulence within and above the model forests and of the wind loading on model trees in four different silvicultural systems. The systems tested were even-aged, single-tree selection, shelterwood/group selection and strip felling. The measurements showed that the profiles of different mean wind and turbulence characteristics above the forests are remarkably similar when vertical heights are normalized by the height of the tallest tree but differences do exist within the canopy. The wind loading measurements indicated no difference between the systems in terms of stability except possibly for the shelterwood/group selection. In the shelterwood/group selection system the presence of smaller sub-canopy trees appears to reduce the loading on the main canopy trees either by providing support and increasing damping or by absorption of energy from the canopy-penetrating gusts