不同密度下小陇山蒙古栎次生混交林树冠结构与密集度的关系

Relationships between Crown Structure and Crowding in Quercus mongolica Forest of Xiaolong Mountains under Different Stand Densities

[目的]林木树冠结构与密集度具有一定关联性。密度效应一直是森林经营关注的问题,本研究通过探究蒙古栎次生林林分密度与树冠结构、密集度的关系,以期深入掌握蒙古栎次生林结构特征,为蒙古栎林可持续经营提供充分的科学依据。[方法]于2020年8月在小陇山蒙古栎次生混交林中选取立地条件均一、不同密度的典型样地进行调查,采用最小显著性差异法、Pearson相关系数和最优结构方程模型,分析不同密度下树冠结构与密集度的关系。[结果]不同密度蒙古栎林中树冠体积的变异最强,其次为冠长与树冠面积,冠幅、冠长率和冠形率的变异最弱,树冠结构特征的变化趋势随林分密度的改变呈单峰型分布。随着林分密度递减,冠幅、树冠面积、树冠体积分别与胸径的相关性趋势近似于“L”型,而冠长和胸径的相关性趋势与林分密度的变化保持了一致,林分密度对冠长率、冠形率与胸径相关性的影响不大。蒙古栎林密集度处于中等密集至很密集状态,但不同密度林分内林木树冠的挤压与重叠程度有差异,其中优势树种蒙古栎、白桦、华山松和椴树起了决定性作用。蒙古栎林中林分密度对树冠结构与密集度关系的影响包含两种路径:林木竞争—树冠结构共同介导的间接影响;林木竞争—胸径—树冠结构共同介导的间接影响。[结论]不同密度蒙古栎林树冠结构有差异,树冠结构特征与林木胸径有关,林分密度影响了蒙古栎林树冠结构与密集度的关系。要实现蒙古栎林的可持续经营,不仅要考虑林木胸径大小,更要关注林木树冠结构特征,通过调整林分密度可优化树冠结构,改善林冠层挤压状况,以促进森林更新与增加物种多样性。

[Objective]This study explores the relationships between stand density,crown structure and crowding,in order to deeply clarify the structural characteristics of Quercus mongolica secondary forest for providing sufficient scientific basis for sustainable forest management in the future.[Method]In August 2020,typical plots with similar site conditions and different stand densities were selected from the secondary mixed forest of Quercus mongolicus in Xiaolong Mountains to analyze the relationship between crown structure and crowding under different stand densities by using Pearson correlation analysis and structural equation model.[Result]There were differences in the crown structure characteristics of Quercus mongolicus forest with different stand densities.The variation in crown volume was the strongest,followed by crown length and crown area.The variation in crown width,crown length ratio and crown shape ratio was the weakest.The variation trend in crown structure characteristics showed a unimodal distribution with changes in stand density.With the decrease of stand density,the correlation trends of crown width,crown area and crown volume with DBH were similar to"L"pattern,while the correlation between crown length and DBH were consistent with the change in stand density,and the effects of stand density on the correlation between crown length rate,crown shape rate and DBH was not significant.The crowding of the Quercus mongolica forest ranged from medium dense to very dense,but the crowding in different density stands was different.The dominant species of Quercus mongolica,Betula platyphylla,Pinus armandii and Tilia tuan dominated the crowding.The influence of stand density on the relationship between crown structure and crowding in Quercus mongolicus forest could be divided into two ways:forest competition-indirect effects mediated by crown structure;The indirect impact mediated by forest competition,diameter at breast height,and crown structure.[Conclusion]The crown structures of Quercus mongolica forest vary with different stand densities.Crown structure is related to the DBH.The relationship between crown structure and crowding of Quercus mongolicus forest is finally affected by stand density.In order to achieve sustainable management of the Quercus mongolicus secondary forest,we should not only consider the DBH,but also pay attention to the characteristics of the crown structure.By adjusting stand density,we can optimize the crown structure and improve the crowding state to promote forest regeneration and increase species diversity.