Institute of Agro-environmental and Forest Biology, National Research Council, Via Salaria km 29,300, 00015 Monterotondo Scalo, Roma, Italy.
J Theor Biol. 2011 Sep 21;285(1):46-57. doi: 10.1016/j.jtbi.2011.06.031. Epub 2011 Jul 3.
Forests that are composed of two or more tree species with similar ecological strategies appear to contradict the competitive exclusion principle. Beech-maple communities are a well-known example of such a system. On a local scale, a number of mechanisms have been proposed to explain the coexistence of these two species. These are reciprocal replacement, external factors that favour alternatively one or the other species and demographic stochasticity. This paper presents and analyses a simple mathematical model that shows that external factors are not an essential requirement for coexistence. Rather, coexistence requires interspecific differences in light transmissivity through the crowns of adult trees. However, all the three mechanisms mentioned above can be interpreted within the framework of the model. Furthermore, many models of forest dynamics make use of shade tolerance as a key feature in describing successional dynamics. Despite its importance, however, shade tolerance does not have a commonly accepted quantitative definition. Here, a simple scheme is proposed where the relationship between shade tolerance, individual traits (growth and survival) and successional status is defined. This might have important implications in understanding the overall dynamics. Theoretical results have been compared with a number of studies carried out in North American forests. In particular, coexistence in beech-maple communities and the relation between shade tolerance and successional status in a beech-hemlock-birch community have been discussed.
由生态策略相似的两种或多种树种组成的森林似乎与竞争排斥原理相矛盾。山毛榉-枫香群落就是这样一个系统的一个著名例子。在局部尺度上，已经提出了许多机制来解释这两个物种的共存。这些机制包括互惠替代、有利于一个或另一个物种的外部因素和种群随机波动。本文提出并分析了一个简单的数学模型，表明外部因素并不是共存的必要条件。相反，共存需要树冠透过率的种间差异。然而，上述三种机制都可以在模型的框架内进行解释。此外，许多森林动态模型利用耐荫性作为描述演替动态的关键特征。然而，尽管耐荫性很重要，但它没有一个被普遍接受的定量定义。在这里，提出了一个简单的方案，其中定义了耐荫性与个体特征（生长和存活）和演替地位之间的关系。这可能对理解整体动态有重要意义。理论结果与在北美的森林中进行的许多研究进行了比较。特别是讨论了山毛榉-枫香群落中的共存以及山毛榉-铁杉-桦木群落中耐荫性与演替地位的关系。
