Wind and its effects on (Canterbury) forests (1984)
AuthorsPapesch, A. J. G.show all
The importance of man-made coniferous forests to the New Zealand economy makes the protection of this renewable resource essential. Forests on the Canterbury Plains are subject to high winds and are susceptible to internal wind damage when they are thinned, particularly near forest fronts. This has prompted research on wind and its effects on flat Canterbury-type forests. Tree response is studied in relation to wind changes at forest borders. A brief review of open country boundary layers establishes a reference wind structure upwind of forests. Changes in wind structure occur as infinite length forests are thinned. The responses of trees within conifer-type forests and of agricultural crops are compared. A study is then made of the forest leading edge. The wind structure is further modified by the abrupt change process, by the density of the roughness and by the response of the individual trees. So that tree response can be studied analytically in such a modified wind structure, simple and complex mathematical equations are formulated using a modified form of Solari's along-wind response calculations. Two full scale and two model experiments contribute to the assessment of the modified wind structure and the tree response from the leading edge inwards. After making a number of comparisons, both model and full scale, of mean wind speed profiles, mean turbulent intensity profiles, spectra, and tree response both by calculation and by photography, there is enough correspondence in results to state that premature wind damage to forests will occur just in from forest leading edges unless optimum spacing is used. Both model tests indicate that this spacing is about 1/3 of mean tree-top height. It is also evident that the wind spectra above tall flexible crops have more energy than is normal in the inertial region, especially near the leading edge. They have steeper gradients than that given by the Kolmogorov Laws. Also, these spectra sometimes possess two or more peaks probably resulting from frequency shifts in wind turbulence from the interaction of discrete vortices and turbulence generated by shear distortion coupling with the waving motion of the tree tops, causing coherent tree sway or 'Honami'.