太白红杉顶芽与分枝格局的适应性分析

Adaptive analysis of apical buds and branching pattern in a woody plant Larix chinensis

野外调查发现太白红杉 (L arix chinensis)枝条顶芽死亡比例较高 ,顶芽死亡对分枝格局产生较大影响 ,可形成 3种分枝类型 : 型、 型和 型。对 3种分枝类型枝条的芽数量、计盒维数以及植冠不同部位的分枝类型比例、顶芽死亡比例、主侧枝平均枝长和主侧枝总数量分别进行了统计分析。结果显示 ,芽数量 : 型 (115 .3) < 型 (15 4 .8) < 型 (2 0 9.9) ;计盒维数 : 型(1.30 5 ) < 型 (1.4 0 0 ) < 型 (1.5 37) ;顶芽死亡比例由树冠上层至下层逐渐提高 ,而冠层东南西北 4个方向的顶芽死亡比例无显著差异 ;主侧枝平均枝长由树冠上层至下层逐渐增加 ,而主侧枝总数量则逐渐降低 ;由于风、光照、坡度和坡向的影响 ,冠层4个方向间的主侧枝平均枝长和总数量均存在显著差异 ; 型分枝使植冠半径扩大 , 型分枝快速扩展植冠的横向空间 , 型分枝在扩展空间的基础上并实现对空间的有效占据。研究表明太白红杉枝条中一定比例的顶芽死亡增加了分枝形态的多样性 ,表现出顶芽和分枝格局的环境适应性 。

Because plants cannot move, they must be able to tolerate environmental variation, which provides a plausible explanation of why woody plants exhibit so great phenotypic plasticity in canopy growth. The growth of woody plants is a process of the accumulation of modules, and hence knowledge of module dynamics is essential to our understanding of the mechanisms of plant canopy plasticity. Larix chinensis is distributed in the alpine regions (altitude 2600～3600m) of Qingling Mountain in Shannxi Province, China, and it is the only upper-limit tree species at the region. As a national second-grade protected plant, it has an important function in conserving water, fixing sand or rock, and adjusting local environment. Because of longer life span and harsh growing conditions, a large number of apical buds die regularly in branches of L. chinensis. The death of apical buds visibly affects the branching pattern, leading to 3 types: type-I is typical of monopodial branching, type-Ⅱis very similar to false dichotomous branching, and type-Ⅲ is an admixture of type-Ⅰand type-Ⅱ. To study the plasticity of apical buds and branching and its ecological significance to canopy formation, the bud number and Box-counting dimension within 3 branching types are calculated in this paper. The mortality of apical buds, the proportion of branching types, the average length of main branching and the total number of main branching in 12 parts of canopy are also statistically analyzed. The results show that the branching pattern of L. chinensis is a kind of fractal structure, satisfying self-similarity laws. Among the three types of branching, there are obvious differences in bud number and Box-counting dimension. The bud number shows a pattern of type-Ⅱ (115.3) < type-Ⅰ(154.8) < type-Ⅲ (209.9), and for Box-counting dimension, type-Ⅱ (1.305) < type-Ⅰ (1.400) < type-Ⅲ (1.537). Type-Ⅰbranching extends the canopy radius, while type-Ⅱbranching expands rapidly the crosswise interspaces, and type-Ⅲ branching not only expands the interspaces but also takes up the interspaces effectively. The death of apical buds can reduce, but sometimes may also improve the capability of a plant to take up space. Thus the death of apical buds may increase the branching morphologic diversity and the adaptability to environment, favorable to the development of plant canopy. In different parts of the plant canopy, there are obvious differences in apical buds' mortality, average length of main branching and total number of main branchings. Apical buds' mortality increases from upper layer to lower layer (upper layer 0.2897<middle layer 0.8131<lower layer 0.9626), while no significant difference exists among different canopy orientations. Average length of main branching increases form upper layer to lower layer (upper layer 30.9533cm<middle layer 127.3178cm<lower layer 166.6075cm), while the total number of main branching decreases (lower layer 480<middle layer 676<upper layer 688). In different orientations of plant canopy, affected by wind, light illumination, slope gradient and slope orientation, there is significant difference in both the average length of main branching and the total number of main branching (average branching length:south part 88.2278cm < west part 92.7407cm < east part 117.0864cm < north part 134.9500cm,total branching number:south part 356 < east part 454 < north part 472 < west part 562). These results show that the canopy of L. chinensis is affected both by genetic factor and by environmental conditions.