Mode of Competition and Size‐structure Dynamics in Plant Communities

Abstract

Abstract Competition between individual plants is defined as a mathematical function for the mean growth rate of individuals in a vertically mono‐layered crowded stand as compared with growth of an isolated individual of the same size. The degree of competitive asymmetry can be described mathematically by using this definition. A canopy photosynthesis model for individual growth in a mono‐layered stand shows that physiological and environmental variations have little effects on the size‐structure dynamics of plant populations undergoing strongly asymmetric competition. These variations affect greatly the size‐structure dynamics under symmetric competition. It has also been shown that one‐sided competition (the most asymmetric competition) is never realised in a mono‐layered stand. A spatial competition model shows that variation in the spatial distribution pattern of individuals greatly affects the size‐structure dynamics of populations undergoing symmetric competition, whilst it has little effect on populations undergoing strongly asymmetric competition. Therefore, it can be concluded that a population undergoing strongly asymmetric competition is a stable system little affected by variations in various factors and that a population undergoing symmetric competition is an unstable system greatly affected by these variations.Based on these theoretical results, the relationships between the mode of competition, species diversity and community stability are discussed and a hypothesis on these processes and mechanisms is proposed for grassland and forest plant communities. One‐sided competition is associated with the stability of community structure, and this type of competition works as a structuring force for plant communities. Symmetric competition is associated with species diversity, and this type of competition cannot be a structuring force of plant communities, i.e. symmetric competition is irrelevant to the deterministic community dynamics. Distinctly multi‐layered forests show “apparent” one‐sided competition between layers (i.e. little direct interactions between canopies in different layers, but higher‐layer trees suppress lower‐layer trees one‐sidedly), bringing about structural stability, but trees in each layer undergo symmetric competition, bringing about species diversity. Mono‐layered grassland shows symmetric competition, bringing about small‐scale spatio‐temporal changes in species' abundance and hence species diversity. The real world lies between the two extreme types of competition, one‐sided and completely symmetric, and each plant community has its own position in this continuous spectrum of the competition mode.