Decomposition Analysis of Competitive Symmetry and Size Structure Dynamics

An analysis is introduced, based on the decomposition of relative growth rates, to examine the mode of competition (i.e. whether competition is symmetric or asymmetric), the size-dependence of growth, and their interdependence. In particular, the basis for two commonly held views is examined: (1) that the type of resource limitation determines the mode of competition, and (2) that asymmetric competition always leads to size-divergence between unequal competitors. It is shown that in field-grown millet plants, competition for light was symmetric at low density and asymmetric at high density. However, size variation at low density decreased during growth, because small plants had greater relative growth rates than larger plants. Size variation stayed constant at high density, since plants of all sizes had equal average relative growth rates. Based on these results and a general discussion, it is proposed that the type of resource limitation does not determine the mode of competition. Competition for light can be symmetric, and foraging for heterogeneously distributed soil resources can produce asymmetric competition below-ground. Furthermore, the mode of competition alone does not determine size structure dynamics. Size-dependence of resource conversion efficiency and allocation can modify the effects of resource uptake on growth. # 1996 Annals of Botany Company