Games roots play: effects of soil volume and nutrients

1. A growing body of literature suggests that some plants may engage in a game of Tragedy of the Commons when competing for soil resources. Two annual plants sharing a whole space will produce more roots per individual and less reproductive yield per individual than one plant with half the space to itself. 2. Several papers have recently suggested that the increase in soil volume in going from a single plant to two competitor plants (as has been the case in empirical tests of the above prediction) may produce the increased root proliferation independent of the competitor. 3. We extend the Tragedy of the Commons model of Gersani et al. (2001) to explicitly consider the separate effects of volume per plant (V) and nutrient concentration per unit volume (N) on root proliferation and yield of plants that 'own' their space relative to plants that share their space with a competitor. We then use simulations resulting from the model to evaluate existing data and the new perspectives provided by recent challenges to the conceptual basis for the Tragedy of the Commons. 4. When N is held constant, increasing V should produce an almost linear increase in root proliferation and net nutrient profit for reproduction. The increase in roots should be more pronounced for two plants sharing their space, and the increase in yield should be more pronounced for plants with exclusive access to half the space. When V $ N is held constant, roots per plant should at first increase and then decline with increased volume. The difference between a plant that 'owns' its space vs. one that shares its space with a competitor should be less pronounced as V increases. 5. Synthesis. We use this model to help clarify some confusion surrounding the Tragedy of the Commons theory of below-ground competition by showing how issues raised by others are not incompatible with the approach used to generate the Tragedy of the Commons theory and highlight future opportunities for research on root growth strategies.