Ecological Modelling 189 (2005) 447–453 Symmetry of belowground competition in a spatially explicit model of nutrient competition Xavier Raynaud ∗ , Paul W. Leadley Laboratoire ´ Ecologie, Syst´ ematique et ´ Evolution, UMR 8079 Universit´ e Paris-Sud XI, Bˆ at. 362 ORSAY, F-91405 Cedex, France Received 3 September 2004; received in revised form 18 February 2005; accepted 29 March 2005 Available online 23 May 2005 Abstract We studied the symmetry of belowground competition at the level of species using a model of plant competition at the scale of individual roots. This model predicts the nutrient uptake of plants with varying root densities and uptake kinetics, competing for the same resource, at equilibrium between resource supply and plant uptake. A symmetry parameter is derived from this model. The analysis of the model behaviour suggests that the symmetry of belowground competition depends on the diffusive supply of the nutrient (buffer capacity × effective diffusion coefficient) for which competition occurs. As the diffusive supply is determined to a greater extent by soil water content and soil type rather than by nutrient type, this means that the symmetry of belowground competition should vary with soil water content and soil type. In particular, our model suggests that competition for nutrients that are primarily supplied by diffusion through the soil is (1) symmetrical at high diffusive supply when root uptake kinetics are identical or at low diffusive supply when roots are randomly distributed in the soil whatever their uptake kinetics are, (2) size-asymmetrical at high diffusive supply when root uptake kinetics are different, (3) partially symmetric or size-asymmetric at low diffusive supply when root are not randomly distributed in the soil. © 2005 Elsevier B.V. All rights reserved. Keywords: Belowground competition; Symmetry; Nutrient uptake model 1. Introduction The symmetry or asymmetry of plant competition is thought to play a key role in determining competi- tive outcomes in natural ecosystems (Schwinning and Weiner, 1998), in part, because competitive displace- ∗ Corresponding author. Tel.: +33 169155701. E-mail addresses: xavier.raynaud@ese.u-psud.fr (X. Raynaud), paul.leadley@ese.u-psud.fr (P.W. Leadley). ment may be much more rapid under size-asymmetric competition than under size-symmetric competition. Symmetry and asymmetry of competition have generally been defined by studying the relationships between plant size (usually measured as plant height or plant biomass) and plant resource acquisition (usually measured as plant growth rates or nutrient uptake) (Schwinning and Fox, 1995). When resources are taken up in proportion to plant size, competition is defined as being size-symmetric. When larger plants acquire 0304-3800/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2005.03.008