Wind and remnant tree sway in forest cutblocks.: I. Measured winds in experimental cutblocks

Abstract

New silvicultural techniques use unharvested forest strips to provide wind shelter in harvest cutblocks, and reduce windthrow for remnant trees. The objective of this paper was to characterise the winds across two such sheltered cutblocks: a narrow cutblock whose width ( X c) was 1.7 canopy heights ( h), and a wide cutblock with X c = 6.1 h. Our focus was on the winds affecting understory trees, when the wind direction was across the cutblock width. Propeller and cup anemometer measurements were made along transects across the cutblocks, at height z = 0.4 h. These data were normalised using windspeeds measured simultaneously in a much larger, nearby `reference' clearing. In both cutblocks the best wind shelter was near the upwind forest edge, where the average cup windspeed ( S) was reduced to approximately 20% of its value in the reference clearing, the average across-cutblock wind velocity ( U) was approximately 10% of its clearing value, and the turbulent kinetic energy (TKE) was approximately 20% of its clearing value. Both U and S increased slowly with downwind distance ( x) across the cutblocks. The pattern of turbulence was different, as TKE increased rapidly with x immediately downwind of the forest, then attained near-constancy beyond x = 3 h downwind of the forest edge (in the wide cutblock), at a value slightly above the clearing value. Based on these observations, we conclude that effective wind shelter in a cutblock occurs within three tree heights of the upwind forest edge (for understory trees of height z ≈ 0.5 h), where both the average wind velocity and the turbulence are reduced relative to their levels in larger clearings.