Size symmetry of competition alters biomass–density relationships

As crowded populations of plants develop, the growth of some plants is accompanied by the death of others, a process called density–dependent mortality or ‘self–thinning’. During the course of density–dependent mortality, the relationship between total population biomass (B) and surviving plant density (N) is allometric: B = aN^b. Essentially, increasing population biomass can be achieved only through decreasing population density. Variation in the allometric coefficient a among species has been recognized for many years and is important for management, assessment of productivity and carbon budgets, but the causes of this variation have not been elucidated. Individual–based models predict that size–dependent competition causes variation in the allometric coefficient. Using transgenic Arabidopsis with decreased plasticity, we provide experimental evidence that morphological plasticity of wild–type populations decreases the size asymmetry of competition for light and thereby decreases density–dependent mortality. This decrease in density–dependent mortality results in more biomass at a given density under size–symmetric compared with size–asymmetric competition.