Underlying effects of spatial aggregation (clumping) in relationships between plant diversity and resource uptake

In the debate on the influence of declining plant diversity on ecosystem functioning, spatial aggregation has received little attention, in spite of the fact that the local distribution of species determines interaction. Using a simple conceptual model, we visualize the effect that clumping has on below ground resource uptake (RU). Two general conclusions can be drawn from the model: (1) clumping results in a loss of RU, and does so to a larger extent when species differ more in rooting depth and when plant density increases; (2) the current view on complementarity may be flawed, as we find that neighborhood species richness only promotes spatial complementarity directly at the transition from monocultures to higher species richness levels, but not at transitions between higher richness levels. The number of species in a community does, however, affect the probability of the occurrence of clumping. In randomly assembled communities, clumps are statistically larger when the number of species is lower, leading to reduced overall below ground complementarity. Complementarity, and thus also RU, will therefore increase indirectly with increasing species richness. As a result of our findings, we propose that clumping could have a number of 'hidden' consequences, for example on invasibility of a community and on competition between species. Clumping could also prove to be a new and important explanatory factor in diversity studies, as it extends the definition of complementarity effects to include the effect of spatial heterogeneity of plant positions within the community.