MECHANISMS DETERMINING THE DEGREE OF SIZE ASYMMETRY IN COMPETITION AMONG PLANTS

When plants are competing, larger individuals often obtain a dispropor tionate share of the contested resources and suppress the growth of th eir smaller neighbors, a phenomenon called size-asymmetric competition . We review what is known about the mechanisms that give rise to and m odify the degree of size asymmetry in competition among plants, and at tempt to clarify some of the confusion in the literature on size asymm etry. We broadly distinguish between mechanisms determined primarily b y characteristics of contested resource from those that are influenced by the growth and behavior of the plants themselves. To generate size asymmetric resource competition, a resource must be ''pre-emptable.'' Because of its directionality, light is the primary, but perhaps not the only, example of a pre-emptable resource. The available data. sugg est that competition for mineral nutrients is often size symmetric (i. e., contested resources are divided in proportion to competitor sizes) , but the potential role of patchily and/or episodically supplied nutr ients in causing size asymmetry is largely unexplored. Virtually nothi ng is known about the size symmetry of competition for water. Plastici ty in morphology and physiology acts to reduce the degree of size asym metry in competition. We argue that an allometric perspective on growt h, allocation, resource uptake, and resource utilization can help us u nderstand and quantify the mechanisms through which plants compete.