Plant strategy theories: replies to Grime and Tilman

1 In response to my essay review that attempted to reconcile the plant strategy theories of Grime and Tilman, Grime rejects five tenets that I had identified in his theories that were incomplete, inconsistent, or incorrect. 2 Grime fails to adequately address three of these concerns. Regarding the other concerns, it is clear that the concept of relative importance needs to be developed further. For example, if both competition and the stress of low nutrient supplies can remove species from a given environment, then neither can be elevated over the other without being tautological. 3 With regards to Tilman’s response, it is clear that it will be important to be explicit about the relationship between data and models. At times, Tilman is neither evaluating individual models with a Popperian approach nor comparing contrasting models in a Bayesian fashion. As it stands, it is unclear what is required for Tilman to reject R* as the mechanism of competition, or whether it passes some undetermined threshold of acceptability. 4 Comparing concentration reduction to supply pre-emption as the mechanism of competition recommends supply pre-emption on many fronts. Supply pre-emption better represents the movement of nutrients in soil solution. Assuming well-mixed soil solutions can theoretically lead to improper prediction of competitive outcomes. Empirically, there are examples where R* does not predict outcomes for competition for nutrients, and supply pre-emption appears to generate metrics (root length density) that are equivalent if not better than concentration reduction (R*) at explaining these outcomes. 5 The next plant strategies paradigm will not be a result of choosing either Grime’s or Tilman’s theories, but represent a synthesis of the two sets of theories while also incorporating novel concepts and research. I agree with Tilman that more research is needed in understanding mechanisms of coexistence, but there still remains large gaps in our understanding of plant traits and growth that limit our understanding of competition for nutrients and will likely constrain our understanding of coexistence even more.