The effect of plant traits and resource supply characteristics on plant competition : a mechanistic model

Abstract

An individual-based, spatially explicit model of herbaceous plants is presented in an attempt to investigate some of the predictions made by the CSR model (Grime 1979) and the Resource Ratio and R* hypotheses (Tilman 1982, 1988). The model simulates early growth of herbaceous individuals and competition between these individuals for light and soil nutrients (nitrogen and phosphorus), along a nutrient gradient. Various model plant species are constructed to investigate the effect of plant traits on competition. High allocation to root is predicted to confer a slight advantage in habitats with low nutrient availability, and conversely high allocation to shoots is predicted to confer a competitive advantage in habitats with high nutrient availability. A plastic response to the availability of resources in the allocation of growth between root and shoot is predicted to confer a competitive advantage in all habitats, though the bias of the plasticity {e.g. consistently greater allocation to root than shoot would be a root bia.sed allocation pattern) may affect this. Growth uncoupled from resource acquisition is predicted to be advantageous in nutrient poor habitats, while growth coupled to resource acquisition is predicted to be advantageous in nutrient rich habitats. Above- and below-ground inter-specific competition along nutrient gradients is examined for these .species. Below-ground competition intensity for a .soil re.source in the absence of light competition is predicted to be higher for a highly mobile resource than for a relatively immobile resource, but competition for light is predicted to be greater for the more mobile resource. Competition intensity for soil nutrients is predicted to be maximal at low nutrient availability, and the intensity of light competition is predicted to be greatest in nutrient rich habitats. The implications for current plant competition theories are discussed.