Kull Olevi, Tulva Ingmar
Institute of Botany and Ecology, University of Tartu, Lai 40, 51005 Tartu, Estonia.
Tree Physiol. 2002 Nov;22(15-16):1167-75. doi: 10.1093/treephys/22.15-16.1167.
We investigated shoot growth patterns and their relationship to the canopy radiation environment and the distribution of leaf photosynthetic production in a 27-m-tall stand of light-demanding Populus tremula L. and shade-tolerant Tilia cordata Mill. The species formed two distinct layers in the leaf canopy and showed different responses in branch architecture to the canopy light gradient. In P. tremula, shoot bifurcation decreased rapidly with decreasing light, and leaf display allowed capture of multidirectional light. In contrast, leaf display in T. cordata was limited to efficient interception of unidirectional light, and shoot growth and branching pattern facilitated relatively rapid expansion into potentially unoccupied space even in the low light of the lower canopy. At the canopy level, T. cordata had higher photosynthetic light-use efficiency than P. tremula, whereas P. tremula had higher nitrogen-use efficiency than T. cordata. However, at the individual leaf level, both species had similar efficiencies under comparable light conditions. Production of new leaf area in the canopy followed the pattern of photosynthetic production. However, the species differed substantially in extension growth and space-filling strategy. Light-demanding P. tremula expanded into new space with a few long shoots, with shoot length strongly dependent on photosynthetic photon flux density (PPFD). Production of new leaf area and extension growth were largely uncoupled in this species because short shoots, which do not contribute to extension growth, produced many new leaves. Thus, in P. tremula, the growth pattern was strongly directed toward the top of the canopy. In contrast, in shade-tolerant T. cordata, shoot growth was weakly related to PPFD and more was invested in long shoot growth on a leaf area basis compared with P. tremula. However, this extension growth was not directed and may serve as a passive means of avoiding self-shading. This study supports the hypothesis that, for a particular species, allocation patterns and crown architecture contribute as much to shade tolerance as leaf-level photosynthetic acclimation.
我们研究了喜光的欧洲山杨（Populus tremula L.）和耐荫的心叶椴（Tilia cordata Mill.）组成的27米高林分中枝条的生长模式及其与冠层辐射环境和叶片光合产物分布的关系。这两个树种在叶冠层形成了两个不同的层次，并且在枝条结构上对冠层光照梯度表现出不同的响应。在欧洲山杨中，随着光照减弱，枝条分叉迅速减少，叶片展示方式有利于捕获多方向的光。相比之下，心叶椴的叶片展示仅限于有效拦截单向光，并且枝条生长和分枝模式有利于其在较低冠层的弱光条件下相对快速地扩展到潜在的未被占据空间。在冠层水平上，心叶椴的光合光利用效率高于欧洲山杨，而欧洲山杨的氮利用效率高于心叶椴。然而，在单叶水平上，在可比的光照条件下，两个树种的效率相似。冠层中新叶面积的产生遵循光合产物的模式。然而，这两个树种在延伸生长和空间填充策略上有很大差异。喜光的欧洲山杨通过少数长枝条扩展到新空间，枝条长度强烈依赖于光合光子通量密度（PPFD）。在这个树种中，新叶面积的产生和延伸生长在很大程度上是不相关的，因为对延伸生长没有贡献的短枝条产生了许多新叶。因此，在欧洲山杨中，生长模式强烈指向冠层顶部。相比之下，在耐荫的心叶椴中，枝条生长与PPFD的关系较弱，与欧洲山杨相比，基于叶面积在心叶椴上更多地投入到长枝条生长上。然而，这种延伸生长不是定向的，可能是一种避免自我遮荫的被动方式。本研究支持这样一种假设，即对于特定树种而言，分配模式和树冠结构对耐荫性的贡献与叶水平的光合适应一样大。
