Assessment of Effects of Thinning on Wind Damage in Pinus thunbergii Plantations

A method for estimating risk ratios of wind damage for Pinus thunbergii trees and stands was developed based on stem bending theory and coefficients characterizing wind profiles (attenuation coefficients wtr, wst for single tree and stand respectively), distribution of branches (constant ac) and optical stratification porosity (OSP) (extinction coefficient ec). For individual trees, if acs equals wtr in value, the parameter H/D1.3 (H: tree height, D1.3: diameter at breast height) can be used to compare and evaluate the risk-ratio of wind damage. The same method can be applied to stands using the coefficients of wst, ec and H/D1.3. In order to test the effectiveness of the method, wind damage in a P. thunbergii plantation, which was thinned at four levels, was investigated for four successive growing seasons. Meanwhile, the wind profiles outside and inside the plantation, tree and stand features, and the distributions of OSP were observed. According to these data, risk-ratios of wind damage for both individual trees and stands were estimated using the developed method. 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, both 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, but 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-minute wind speed outside the plantation attained 30.2ms. The results indicate that thinning is the most effective silvicultural strategy available for managing coastal plantation despite the increased probability of wind damage soon after thinning.