Effects of thinning on wind damage in Pinus thunbergii plantation ——Based on theoretlcal derlvation of risk—ratios for assessing wind damage

Based on paper of “Theoretical derivation of risk-ratios for assessing wind damage in coastal forest“,wind damage in the pine coasteal 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 meen height and diameter only and from the combination of wind and stand sructure 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 growthand 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 30.2m 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.     

海岸林风害危险率评价的理论推导

本文在对间伐和风害之间关系的讨论和以往有关研究结果的基础上,确定了一种林木风害危险率的估计方法,用于评价间伐对林木风害的影响。这种对单株黑松及整个黑松林分的风害危险率估计,是根据风速廓线、枝条分布和透光分层疏透度(OSP)系数推导出来的。结果表明:如果枝条分布模型中的参数β等于单株树冠中风速廓线模型的衰减系数αs,参数H/D1.33可用于比较和评价对单株树木的风害危险率。这一原理也同样适用于整个林分,即用林分内风速廓线衰减系数α和透光分层疏透度(OSP)的分布中的衰减系数ν,结合D1.33估计和比较林分风害危险率。文中对间伐与非间伐单株黑松风害危险率进行了估计,得出了间伐两年后可以降低单株黑松风害危险率的结论;同时给出了确定林分风害危险率估计的过程图。图3参45。     

间伐对黑松海岸林风害的影响——基于风害危险率评价的理论推导

本文在“海岸林风害危险率评价的理论推导”(Journal of Forestry Research, 2002, 13(4))一文的基础上,对1997年12月经四种间伐处理的黑松海岸林进行了连续四个生长季的风害调查,同时,对黑松海岸林内与林外风速廓线和林分透光分层疏透度的分布也分别进行了观测。应用在这些观测数据,依据“海岸林风害危险率评价的理论推导”文中获得的风害危险率评价方法,对单株树木及林分的风害风险率进行了评估,结果表明,单独依据平均直径与结合风速廓线和林分结构的风害风险率,构成了评价海岸森林风害的重要指标。不同间伐强度和风害发生率之间的关系表明,在间伐初期林分的稳定性有所下降,这可能是由于林冠粗糙度和风负载的增加及周围树木的保护效应的减少而引起的。然而,经过一段时间后,间伐处理能够促进林分生长,从而提高了林分的稳定性和对强风的抵抗力。在本试验观察过程中只发现了强风对林冠的损害,而即使是在间伐海岸林外风速达到30.2 m·s-1的条件下,也没有发现风倒或风折。本次研究中获得的结果表明,尽管在间伐之初风害的概率增加了,但是,间伐作为一种最为有效的林分管理方法,可以成为保持海岸林健康、稳定的一种长期经营管理策略。图3表4参19。