Constraints and tradeoffs: toward a predictive theory of competition and succession

The development of mechanistic, predictive ecological theory will entail the explicit inclusion of organismal tradeoffs, of environmental constraints, and of the basic mechanisms of interspecific interaction. This approach was used to address the causes of species dominance and successional dynamics in sandplain vegetation in Minnesota. A series of field experiments performed over the last eight years have shown that the major constraints on plants were soil nitrogen and disturbance, with nitrogen competition being a major force. Nutrients other than nitrogen (P, K, Ca, Mg, S and trace metals), herbivory, and light were of minor importance. As predicted by theory, the superior nitrogen competitors were the species that, when growing in long-term monocultures in the field, lowered soil extractable N the most. These species had high root biomass and low tissue N levels. Seven alternative hypotheses of succession, each named after its underlying tradeoff, were proposed and tested. The colonization - nutrient competition hypothesis provided the best explanation for the initial dominance (years 0 to 40) of herbs, whereas the nutrient versus light competition hypothesis best explained the long-term dominance by woody plants. Hypotheses involving transient dynamics caused by differences in maximal growth rates were rejected. In total, the results demonstrate that the inclusion of simple mechanisms of interspecific interactions, and of allocationbased tradeoffs, can allow models to predict the composition and successional dynamics of vegetation.