The R* rule states that the species that can survive at the lowest level of a limiting resource, Min(R*), will be the best competitor for that resource and will exclude all other species.  This rule has received an impressive amount of empirical support.  Nearly all of this support comes from studies of microbial systems, primarily bacteria, phytoplankton, and zooplankton.  Simple models support the R* rule and suggest that it trumps other generalizations that have been proposed to predict system behavior, such as the conjectured maximum energy flux (or power) principle, where the species with the highest energy intake, G, is assumed to have the greatest fitness.  However, these results are from simple models that may only be appropriate for microbial autotrophs.  The situation may be different for vascular plants, which are able to vary their energy allocation between foliage, roots and trunk. We used a model for competing trees in which the tree can allocate energy between foliage, roots, and wood.  The model trees with different allocation strategies were allowed to compete for nutrients and light under various environmental conditions, such as nutrient input rates.

Results/Conclusions

First, we found that strategy that minimized the nutrient level at equilibrium, R*, was not the same as that which maximized G, though the strategy for Min(R*) was proven to be mathematically identical with one that maximized energy allocated to roots.  Second, surprisingly, the allocation strategies of model plants that were able to out-compete the other plants differed from strategies that produced either Min(R*) or Max(G). Mathematical analysis of the model suggests reasons for the strategies of the successful competitors.  We conclude that vascular plants may not conform to the concepts that have been applied to competition theory in microbial systems, and we suggest possible empirical studies.