Wind and remnant tree sway in forest cutblocks. II. Relating measured tree sway to wind statistics

Abstract

New silvicultural designs use unharvested forest strips to provide wind shelter in harvest cutblocks, and reduce windthrow of remnant trees. In this study we attempted to quantify wind shelter in terms of tree sway. We investigated the relationship between wind statistics and tree sway in two cutblocks in northern Alberta, Canada: a narrow cutblock (1.7 canopy heights in width, X c = 1.7 h), and a wide cutblock ( X c = 6.1 h). We focused on the case of winds oriented across the cutblock width. The instantaneous across- and along-cutblock wind `forces' acting on an understory white spruce ( Picea glauca) were considered as proportional to u| u| and v| v| , where u and v are the across- and along-cutblock wind velocities at height z = 0.4 h. A simple mass–spring–damper displacement model, calibrated to tree sway measurements, was used to diagnose the sway response of a `characteristic' remnant spruce (damping coefficient ζ = 0.11, natural frequency ω n = 0.4 Hz). We determined that in the cutblocks this characteristic tree: (i) had a small average angular displacement 〈 θ〉 relative to its maximum displacement θ max; (ii) exhibited `resonant sway,' where the interaction of tree dynamics and turbulence increased sway by 10–35% over that expected if displacement was proportional to the instantaneous wind force; and (iii) had sway statistics which correlated well with the standard deviation of wind force σ u|u| . Modelled tree sway was used to infer the degree of windthrow protection in the sheltered cutblocks. Our approach was to predict a threshold average wind velocity in the open ( U w, essentially a weather station windspeed) which correlates with the occurrence of windthrow of remnant spruce in the cutblock. Our assumption was that an average velocity of 10 m s −1 causes windthrow of unprotected trees in the open. Larger windspeeds ( U w) are needed to cause windthrow in the cutblocks. In the wide cutblock U w varied from 13 m s −1 near the downwind forest edge to 25 m s −1 near the upwind edge. In the narrow cutblock U w varied from 19 m s −1 at the downwind edge to 30 m s −1 at the upwind edge. The most effective wind shelter, as given by the highest U w, was within three tree heights of the upwind forest. In designing harvest cutblocks to reduce windthrow, we suggest that cutblocks not exceed three tree heights in width.