兴安落叶松天然林自然整枝特征及其影响因子

  目的  分析天然林自然整枝规律，确定表征自然整枝指标及其主要影响因子，为森林抚育、优化林分结构、促进自然整枝提供依据。  方法  以中幼龄兴安落叶松Larix gmelinii天然林为对象，利用32块样地共1 279株立木实测数据，以活枝下高、活枝下高占树高比例、死枝下高、死枝下高占树高比例、活枝下高与死枝下高差作为自然整枝指标同林木和林分因子进行相关分析，探讨不同结构林分自然整枝规律。在此基础上，进行逐步回归分析确定影响自然整枝的主要因子。  结果  兴安落叶松天然林的活枝下高、活枝下高占树高比例、死枝下高、死枝下高占树高比例、活枝下高与死枝下高差平均值分别为4.8 m、46.4%、2.8 m、25.2%、1.4 m。不同密度和聚集系数的林分自然整枝差异极显著(P＜0.01)。相关性分析表明:林木胸径、林木树高、林木冠幅、林木年龄、林分平均胸径、林分平均树高、林分密度、聚集系数等因子影响自然整枝。除死枝下高占树高比例以外，其他4个自然整枝指标均受多个因子的影响，但各因子所影响的指标不完全一致。其中，林分密度和聚集系数同死枝下高、死枝下高占树高比例、活枝下高与死枝下高差等3个指标呈显著负相关(P＜0.05)，其他因子同自然整枝指标呈极显著(P＜0.01)或显著(P＜0.05)正相关。逐步回归分析表明:林木树高、林木年龄、林分平均树高和林分平均胸径是影响自然整枝的主要因子。其中，活枝下高主要受林木树高和年龄影响，活枝下高占树高比例主要受林木年龄影响，死枝下高主要受林分平均树高影响，活枝下高与死枝下高差主要受林分平均胸径和林木树高影响。  结论  林木树高和年龄、林分平均树高和平均胸径等4个因子为自然整枝的主要影响因子。活枝下高、活枝下高占树高比例、死枝下高、活枝下高与死枝下高差等4个指标能较好地表征自然整枝。其中，活枝下高、活枝下高占树高比例主要受林木因子影响，活枝下高与死枝下高差、死枝下高主要受林分因子影响。林分生长越好，越能促进自然整枝，但对死枝脱落速度无促进作用。林分密度和聚集系数的增加，不仅促进自然整枝，而且能够加速死枝的脱落。表5参26

Characteristics and impact factors of self-pruning in natural Larix gmelinii forest

  Objective  This paper, with an analysis of the natural pruning laws of natural forests, is aimed to clarify the indicators and main influencing factors of natural pruning so as to provide a basis for forest tending, optimizing stand structure and promoting natural pruning.   Method  First, with measured data collected of a total of 1 279 standing trees in 32 plots in the middle and young Xing’an larch (Larix gmelinii) natural forest, a correlation analysis with forest trees and stand factors was conducted to explore the natural pruning laws of forest stand with different structures using the height under live branches, the proportion of the height under live branches in tree height, the height under dead branches, the proportion of the height under dead branches in tree height, the difference between the height under live branches and the height under dead branches as natural pruning indicators. Then, on this basis, the stepwise regression analysis was carried out to determine the main factors that were likely to affect the natural pruning.   Result  The average value for the height under live branches, the proportion of the height under live branches in tree height, the height under dead branches, the proportion of the height under dead branches in tree height, and the difference between the height under live branches and the height under dead branches in Xing’an larch natural forest were: 4.8 m , 46.4%, 2.8 m, 25.2%, 1.4 m. The natural pruning indicators of the forest stands with different densities and aggregation coefficients were extremely significantly different (P＜0.01). The factors affecting natural pruning include tree diameter at breast height (DBH), tree height, tree crown width, tree age, stand average DBH, stand average tree height, stand density, and aggregation coefficient. Except for the proportion of the height under dead branches in tree height, the other four natural pruning indicators were all affected by several factors, without a significant consistency in the indicators affected by each factor and among them, the forest stand density and aggregation coefficient, which were significantly and negatively correlated with the three indicators of the height under dead branches, the proportion of the height under dead branches in tree height, and the difference between the height under live branches and the height under dead branches, whereas the other factors were extremely positively correlated with the natural pruning indicators, showing extremely significant (P＜0.01) or significant (P＜0.05) positive correlation. Stepwise regression analysis indicated that tree height, tree age, stand average tree height and stand average DBH were the main affecting factors in natural pruning of which, the height under live branches was mainly affected by the tree height and age, the proportion of the height under live branches in tree height was mainly affected by the tree age, while the height under dead branches was mainly affected by the stand average tree height. The difference between the height under live branches and the height under dead branches were mainly affected by the stand average DBH and the tree height.   Conclusion  The four factors, including the tree height and age, stand average tree height and stand average DBH, are the main influencing factors in natural pruning. The four indicators such as the height under live branches, the proportion of the height under live branches in tree height, the height under dead branches, and the difference between the height under live branches and the height under dead branches can be used to better characterize natural pruning. Among them, the height under live branches and the proportion of the height under live branches in tree height are mainly affected by forest tree factors, and the height under dead branches, the difference between the height under dead branches and the height under live branches are mainly affected by forest stand factors. If the forest stand growth is better, it can promote natural pruning, but has no effect on the shedding rate of dead branches. The increase of stand density and aggregation coefficient not only promotes natural pruning, but also accelerates the shedding of dead branches. Ch, 5 tab. 26 ref.]