Co-dominance and succession in forest dynamics: The role of interspecific differences in crown transmissivity

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.