北京地区侧柏人工林密度效应

密度是影响森林尤其是人工林生长的重要因素,林冠层是森林生态系统与其他系统进行能量和物质交换的重要场所,树木及树冠生长对林分密度的响应关系可以看作是生物对环境变化产生的适应性现象。林分密度效应是生态学和森林培育学的重要研究内容之一。以23块8种不同密度梯度的北京山区侧柏人工幼龄林林分为研究对象分析其树木生长及树冠生长对密度的响应关系,其中树冠指标使用了参照了美国林务局(USDA)的树冠调查指标。研究结果表明:(1)林分平均胸径、平均树高和平均冠幅生长均随密度增大而减小,林分密度大于3000株/hm2时各指标减小的趋势变缓,使用异速生长模型可以很好地拟合这种变化关系;(2)随密度增加,树冠水平方向和垂直方向生长均到显著地抑制作用,树冠外形表现出由饱满冠型向狭长冠型变化的适应性现象;(3)使用树冠二维、三维指标与密度进行相关性分析可知树冠长度、树冠率等指标与林分密度呈负相关关系,树冠圆满度及树冠生产效率与密度表现出极显著正相关关系;(4)采用枝解析的方法研究了树枝长度、材积的平均生长量、连年生长量与密度的关系,结果表明幼龄期各生长量差异不大;(5)在建立冠幅模型时考虑了自变量间的多重共线性问题,所建的胸径单自变量二次方模型能够很好地预测侧柏人工幼龄林冠幅生长过程,模型相关系数R2为0.961。

The density effect of Platycladus orientalis plantation in Beijing area

Density is one of the important factors that affects the growth of forests, especially plantations. Tree crowns are places where forest ecosystems exchange energy and materials with other systems. The response of the growth of trees and tree crowns to forest density can be considered as a phenomenon of adaptation by organisms to the environment. Studying the response of tree growth to density effect is an important topic in ecology and silviculture. The objective of this study was to investigate the relationship between tree diameter at breast height (DBH), tree height, crown width and the stand density in even-aged Platycladus orientalis plantation. In total 23 plots with 8 different densities were etablished in Beijing mountain area, China in 2009. The densities of stands varied between 755 and 4577 trees/hm2. Two type of plots with an area of 20 m×20 m or 15 m×15 m were created, Average ages of selected stands were between 24 and 40 years. We referred to the crown investigation indexes developed by USDA, Forest Service during the investigation. The crown growth indexes were grouped into two-dimensional indexes and three-dimensional indexes. Results of the investigation were shown as following: (1) Stand density affected the growth indexes significantly. The growth of the average DBH, average height, and average crown width all decreased accompany the increase of density. The effect was particularly obvious when the density was under 3000 trees/ hm2. When the density was above 3000 trees/ hm2, the decrease of the growth indexes slowed. We built alleometric models to predict DBH, tree height, and crown width from the density. The format of the model was y=axb, the correlation index (R2) were 0.990, 0.955 and 0.891 for three models. (2) The increase of density affected the shape of tree crowns greatly. The crowns were inhibited significantly in both horizontal and vertical directions. The shape of tree crowns appeared to change from plump to narrow. (3) Two-dimensional indexes and three-dimensional indexes were correlated to tree densities. The result showed that the crown length, crown ratio, crown production efficiency, crown density, composite crown surface area, composite crown volume were negatively correlated with the tree density, while the crown fullness ratio, crown production efficiency were positively correlated with the tree density. All those indicated that the increase of stand density could inhibit the growth of tree crowns significantly. (4) Branches were analyzed to study the effects of tree densities on the length of branch, average growth increment of branch volume, and consecutive annual growth increment of branch volume. The increment of length of branch was not significantly different from that of the branch volume in young stands. The increment of length of branch did not change significantly as the tree density has been changed. The growth of volume of branch responded negatively to the tree densities. This might be caused by the complex environment. (5) Crown width is the most easy-to-measure index of tree crowns during forest inventory and other indexes can be calculated from it. A model was constructed to predict the crown width from DBH, crown ratio, and stand density. After considering the multicollinearity of independent variables, the final model was only based on DBH. The DBH-CW model could predict the growth of crown width of young Platycladus orientalis plantations well (R2=0.96).