Model computations on wind flow and turning moment by wind for Scots pines along the margins of clear-cut areas

Abstract

The risk of windthrow for Scots pines ( Pinus sylvestris L.) was estimated along the downwind margins of clear-cut areas in terms of turning moment arising from the dynamic wind load. In the model computations, a mechanistic model for the mechanism of windthrow (Peltola and Kellomäki, 1993) was linked to the model for steady-state turbulent flow across a forest clearing (Li et al., 1990). The stands along the margins of clear-cut areas varied in stand density and tree height, and clearing size increased from 0.04 ha to 4.0 ha (equilateral square). The total forest area was taken to be 16 ha (flat area) and the clearing was placed at the midpoint of the forested area, surrounded by identical homogenous Scots pine stands. Model computations showed that the windspeeds in the centre of large clearings were somewhat higher than in the centre of smaller clearings. However, the windspeed was more reduced along the downwind margins than in the centre of the clearing. There was also little difference in windspeed at crown height along the margins between clearings of various sizes because of the edge effect of the dense margin. Thus, there were also small differences in turning moment for clear-felled areas of different sizes but with identical stand heights and densities. Along the margins of clear-felled areas, stand density and height were the major controlling factors acting on the turning moment of trees. It decreased as the density increased, but it increased with increasing height. The turning moment also diminished substantially with increasing distance from the edge. This decrease was at its maximum near the dense margin; i.e. within a distance of one tree height from the edge. Trees with height equal to 24 m seemed to be at much greater risk of being uprooted than trees 20 m in height but with the same taper. Trees 16 m in height were the least likely to be uprooted.