Analyzing the relationship between the size symmetry of competition and size inequality of two plants with simple, individual-based models

Background/Question/Methods
The concept of size asymmetry in resource competition among plants, in which larger individuals obtain a disproportionate share of contested resources, appears to be very straightforward, but the effects of size asymmetry on growth and size variation among individuals has proved controversial. It has often been assumed that competition among individual plants in a population has to be size asymmetric to result in higher size inequality than in the absence of competition, but this inference has been questioned. Using very simple, individual-based models, we investigate how the size symmetry of competition relates to size inequality. We demonstrate the usefulness of our approach by applying it to data from a greenhouse experiment investigating the size symmetry of belowground competition between pairs of Triticum aestivum (wheat) plants.

Results/Conclusions
We show that increased size inequality due to competition is not always strong evidence for size-asymmetric competition. Even completely size-symmetric competition, in which all plants receive the same amount of resources, irrespective of their size, can, under some assumptions, result in higher size inequality than when competition is absent. The effects of size symmetry / asymmetry on size inequality are dependent on (1) the individual plant growth model, (2) the parameters of the growth model that are affected by competition and (3) the initial sizes and growth rates. Across a range of reasonable assumptions, very general patterns that have been considered evidence for or against size-asymmetric competition do not always hold.  Our results emphasize the need for explicit growth models for making inferences about the effects of competition on plant growth and size inequality.