Effects of thinning on wind damage in Pinus thunbergii plantation ―Based on theoretical derivation of risk-ratios for assessing wind damage

摘要

Based on paper of "Theoretical derivation of risk-ratios for assessing wind damage in coastal forest", wind damage in the pine coastal forest, which was thinned at four levels in December of 1997, was investigated for four successive growing seasons. Besides wind damage, the wind profiles outside and inside the coastal forest stand and the distributions of optical stratification porosity (OSP) were also observed. Based on these data, risk-ratios of wind damage for both individual trees and stands were estimated according to the methods developed in "Theoretical derivation of risk-ratios for assessing wind damage in a coastal forest". The results showed that risk-ratios of wind damage, which were calculated from the mean height and diameter only and from the combination of wind and stand structure profiles, accurately predicted wind damage in the plantation. Relationships between different thinning ratios and incidence of wind damage showed that stand stability decreased soon after the thinning. This was due to the immediate effects of thinning on increasing the canopy roughness and wind load, and on decreasing the sheltering effects from surrounding trees. However, thinning strategies could improve the stability by long-term effects on growth and development of trees against extreme wind. Only canopy damage was recorded during the experimental period, no stem damage was found, even though the maximum 10-min wind speed outside the coastal forest attained 30.2 m s~(-1). The results obtained in this study indicate that thinning is the most effective silvicultural strategy available for managing coastal forest despite the increased probability of wind damage soon after thinning.