采用逐步扩大范围的方法确定影响对象木 (Objectivetree) 的最佳竞争范围, 利用单木竞争指数的改进模型对太白红杉 (Larixchinensis) 种内和种间竞争强度进行了定量分析, 并讨论了不同竞争强度下太白红杉的形态变化。结果表明 :随对象木胸径的增大, 由于太白红杉种群自然稀疏过程中密度调节作用, 植株距离增加, 种内竞争强度降低 ;太白红杉主要分布于亚高山地段, 群落内其它物种较少, 个体普遍较小, 结果种间竞争相对较弱, 种内与种间竞争关系顺序为 :太白红杉 太白红杉 >巴山冷杉 (Abiesfargesii) 太白红杉 >牛皮桦 (Betulaplatyphylla) 太白红杉 >其它树种 太白红杉 ;竞争强度和对象木胸径的关系服从幂函数关系 (CI =AD-B), 当太白红杉胸径达到 35cm以上时, 竞争强度几乎没有变化, 所得的预测模型能很好地预测太白红杉种内和种间的竞争强度 ;不同竞争强度下, 太白红杉主茎各层的分枝角度、总分枝数、当年生枝条长、平均枝长和活枝数均表现出显著的差异。表明采用逐步扩大范围的方法能有效地确定竞争木范围, 较好地反应太白红杉种内和种间的竞争关系。同时, 太白红杉通过自身形态变化, 提高了对光的截获能力和对不同竞争强度的适应能力。

Ever since Darwin, competition has been considered to be one of the major forces that shape the morphology and life history of plants and affect the structure and dynamics of plant communities. Competition among plants occurs when resource availability falls below the sum requirement of the community needed for optimal growth. Competition includes intraspecific and interspecific competition for both aboveground and belowground resources and takes place over different spatial and temporal scales. Although many researchers have determined the zone of competition according to their past experiences, it is important to define the zone of influence by using a competition index (CI) that includes all competitors and sources of competition for scarce resources. Phenotypic plasticity is thought to be an adaptive trait evolved in plants growing in spatially or temporally heterogeneous environments. Plasticity of growth and morphology in response to different levels of light, nutrient and water availability in herb plant species has been extensively documented. However, very few studies have addressed phenotypic responses to neighborhood competition in hardwood plants, especially concerning morphology. Larix chinensis is an endemic species in China distributed only in the alpine and sub-alpine belt of the Qinling Mountains. It is also an endangered species listed in the Chinese Plant Red Data Book. It is a dominant species at altitudes of 2 560-3 500 m with special functions that maintains the ecological balance at timberline. In order to define the zone of competition, intraspecific and interspecific competition in L. chinensis communities were investigated using a method that gradually increases the zone of interaction and Zhang's competition index model for individual trees. Data for this study were collected in the Taibai Mountain Natural Reserve of Shaanxi Province in China. Ten 40 m 2-plots were investigated, and every plot was stem-mapped, and species, DBH, total height, and crown length were recorded for each tree that had a diameter of at least 5 cm at breast height. In order to study neighborhood competition effects, morphological characteristics, such as the branching angle, total branch-number, current-year branch length, average branch length, and living branch number of the main stem in different strata were investigated for L. chinensis under different levels of neighborhood competition intensity. The results showed that as a tree increased in size and the distance between plants increased that intraspecific competition intensity decreased due to self-thinning. L. chinensis is the dominant tree in the community, and the few other species tree species in the forest community are all much smaller than L. chinensis. As a result, intraspecific competition was more intense than interspecific competition. The order of competition intensity was: L. chinensis-L. chinensis >Abies fargesii-L. chinensis > Betula platyphylla-L. chinensis > other species-L. chinensis. The relationship between competition intensity and growth of the tree of interest closely followed the following equation: CI×AD -B. The change in competition intensity is very small when the diameter of the objective tree reaches 35 cm. The model can simulate and predict intraspecific and interspecific competition efficiently. The morphology of L. chinensis, such as the branching angle, total branch-number, current-year branch length, average branch length, and living branch number of main stem, was modified in response to different intensities of neighborhood competition. The study provides a new approach to ascertain the zone of competition among trees for studying plant competition. L. chinensis can improve its capacity to intercept light and adapt to different levels of neighborhood competition through morphological changes.