Ecological and physiological processes in mixed versus monospecific stands

Abstract

Tree growth depends on the resource availability, the proportion of resources acquired, and the efficiency with which those resources are used. Each of these variables can be influenced by species interactions. These interactions are dynamic and change spatially and temporally as resource availability and climatic conditions change. It is important to understand these processes when designing and managing mixed-species stands and also when modelling these processes. These interactions and their dynamics are the focus of this chapter. To begin with, the production ecology equation is described because it provides a useful framework to quantify the types of processes that influence the growth of forests and how these are influenced by species interactions. This equation describes growth as a function of resource availability, resource acquisition, and resource-use efficiency. Then, while referring to this equation, some of the main types of processes are described in terms of how they influence these variables and hence the productivity of mixtures. This is done for nutrients, then light, and thenwater. The influence of a given type of interaction on growth is not static. Instead, it changes with spatial and temporal variability in resource availability and climatic conditions and as a stand develops. Therefore, the next section describes a framework that explains these spatial and temporal dynamics and indicates when different types of interactions are important. Finally, stand density can influence the effect of these interactions. As stand density increases, interactions may become more favourable or less favourable, depending on how, and which, resources are influenced by the change in density. The final section therefore shows why stand density needs to be taken into account when examining how species interact.