Competition and Allometry in Three Species of Annual Plants

Abstract

Comparisons between competing and noncompeting populations of three annual plant species demonstrate that plant allometry is altered by competition. When plants are grown in isolation, relationships between stem diameter, height, and plant mass generally show simple allometry (i.e., the relationships are linear on log—log scale). When plants are competing, however, these relationships are curvilinear or discontinuous. We attempt to clarify the relationship between interindividual allometry (the usual source of allometric data) and allometric growth. When plants are not competing, the allometric relationships among individuals of different sizes at one point in time and the allometry of individual plants as they grow appear to be similar, but these two classes of allometric relationships are very different for competing plants. We present a simple model that explains both static and dynamic patterns of plant allometry in terms of (a) the allometric responses of individual plants to competition, and (b) the size dependence of growth after the onset of competition. Our results illustrate the importance of reciprocal interactions between competition and allometry, and emphasize the difficulty in making inferences from one size measure to another. We conclude that the commonly held assumption that plant size is a single entity, which can be reflected by any of several measures, may not be justified.