Game theoretic models of resource usage commonly predict evolution toward extreme individual strategies that result in a tragedy of the commons. In plants, these models predict extreme investment in roots in competition for water. Rootier species can always invade less rooty species. They are able to obtain a larger proportion of the water available, while only having a negligible effect on overall water availability. Yet, once these rootier species have taken over each plant experiences lower water availability while paying the high cost of large root systems. This process is predicted to continue until plants are no longer able to support themselves. As this is clearly not the case in natural systems, we assert there must be widespread mechanisms missing from these simple game theoretic models.

Theoretical and experimental work has shown that individual plants exhibit greater constraint in rooting strategies if they share soil with fewer competitors (Gersani et al. 2001. Journal of Ecology 89: 660, Zea-Cabrera et al. 2006 Water Resources Research 42: W06D02). However, we know that the roots of trees extend well beyond drip line and overlap considerably. Results/Conclusions

We present two additional solutions to the tragedy of the commons for water usage that assume a complete sharing of water. We have found that variation in precipitation and separately constraints on the individual plasticity of root:shoot ratios for plants can lead to competitive dominance of intermediate rooting strategies, and an end result of realistic growth rates for plants. These two mechanisms have the commonality that plants derive fitness from discrete periods of light and water limitation. In the case of variable precipitation, these periods occur over the seasons. In the case of constrained root:shoot ratios, these periods occur over the light stages of the plant: light limited in the understory, and water limited as a canopy tree.

The tragedy of the commons for resource use has been a paradox for evolutionary ecologists as it predicts a perverse result from simple evolutionary models. This paradox may be resolved by incorporating the mechanisms of 1) spatial segregation by roots 2) changing water availability and 3) a constraint on the plasticity of root:shoot ratios as plants move through light and water limitations.