The Size Structure of Theoretical Plant Populations: Spatial Patterns and Neighborhood Effects

The role of neighborhood competition in the formation of size hierarchies was investigated using an individual—plant, spatially explicit growth model of annual plant population dynamics. Neighborhood effects were varied by having plants grow individually and in random and hexagonal spatial patterns. Resources shared between two individuals were allocated symmetrically or asymmetrically. The effect of genetic variability was controlled by first assigning constant initial masses and growth rate coefficients for all individuals and then drawing these parameters from normal frequency distributions. Size hierarchies formed when relative growth rates were positively correlated with plant mass. Neighborhood effects were an important means of inducing this relationship. When plants competed for resources, individuals had the same relative values of available growing space, relative growth rate, and mass so that size differences were enhanced over time. However, genetic variation in growth rates also caused a positive correlation between plant mass and relative growth rate. In this case, regardless of the type of resource allocation, neighborhood competition increased the variability in growth rates beyond that expected from genetic variation. These theoretical results suggest the importance of neighborhood effects in the formation of size hierarchies and imply that size structure is in part an expression of the spatial distribution and availability of resources within a stand.