One of the prominent features of natural and managed plant populations is that their members are often highly variable in size. Since many physiological and morphological properties of individuals depend on plant size, the size structure of a population can affect the way it responds to environmental conditions.

Size variation is caused by many factors, often simply by differences in age or microsite characteristics. However, competition can amplify initiall small size differences, especially when plants compete for light. When taller plants shade and suppress the growth of shorter neighbors, size differences can become extreme and suppressed plants can even die. This type of competition, where larger plants suppress smaller plants but smaller plants have almost no impact on the growth of larger plants, is called "size-asymmetric" competition.

Competition for light is almost always size-asymmetric, while competition for soil resources is often size-symmetric and does not amplify differences in the size of competitors. However, the effects of competition on population size structure does not only depend on the nature of the limiting resource, but also on phenotypic plasticity, i.e. the way in which plants adjust to the presence of competitors. For example, shaded seedlings often grow taller faster, which helps them to escape severe competitive supression.

The form of competition, i.e. whether competition is size-symmetric or -asymmetric, can affect the success of invaders into plant communities. When competition is size-asymmetric, invaders with larger seedling sizes than the seedlings of the resident community stand a greater chance to succeed than invaders with smaller seedling sizes. When competition is predominantly symmetric in a plant community, seedling sizes are less important.

Further Reading (download list as pdf file or Endnote file)

Schwinning, S. & Weiner, J. 1998. Mechanisms determining the degree of size-asymmetry in competition among plants, Oecologia 113, 447-455.pdf file

Schwinning, S. 1994. Effects of competitive symmetry on populations of annual plants. PhD Thesis, University of Arizona.

Schwinning, S. 1996. Decomposition analysis of competitive symmetry and size structure dynamics. Annals of Botany 72: 47-57..pdf file

Schwinning, S. & Fox, G.A. 1994. Population dynamic consequences of competitive symmetry in annual plants. Oikos 72: 422-432.

Stoll, P., Weiner, J., Muller-Landau, H., Muller, E., & Hara, T. (2002) Size symmetry of competition alters biomass-density relationships. Proceedings of the Royal Society of London Series B-Biological Sciences, 269, 2191-2195.

Weiner, J., Stoll, P., Muller-Landau, H., & Jasentuliyana, A. (2001) The effects of density, spatial pattern, and competitive symmetry on size variation in simulated plant populations. American Naturalist, 158, 438-450.

Weiner, J. 1990. Asymmetric competition in plant populations. Trends in Ecology and Evolution: 5: 360-364.