Nutritional soil heterogeneity and mycorrhiza as determinants of plant species diversity

Patterns in nutrient availability often vary both in space and time (e.g. Pegtel 1987; Stark 1994; Marschner 1995) and small differences can lead to large consequences in the ecophysiological responses and competitive abilities of plant species (Fitter 1982; Wedin & Tilman 1990; Grime 1994). Nevertheless the spatial scale and the degree of spatial heterogeneity and how this might differ among communities are poorly understood. Most plant individuals experience nutrient availability through their fungal partner. Different mycorrhiza types have some important ecological and physiological differences that may have important consequences for the competitive abilities of the associated plants. The impact of mycorrhiza networks relative to other mechanisms on the interaction between plant species remains unclear, but several experiments suggest a considerable influence, at least in some terrestrial ecosystems. In this review, we will discuss causes and consequences of soil heterogeneity, at spatial scales ranging from individual plants to the level of plant communities, in view of their impact on competition and coexistence. A similar approach will be applied to the issue of effects of mycorrhizal fungi, asking the question whether they intensify interspecific competition or facilitate coexistence.