Vortrag, Jahrestagung der Gesellschaft für Ökologie, Gießen: 13.09.2004 - 17.09.2004
Monocultures and pair-wise competition plots are still most widely used to assess competitive interactions within plant communities. This includes studies about changing competitive hierarchies due to so called intransitive loops. Intransitivity is supposed to be a key factor increasing coexistence in natural communities but so far has only been demonstrated in aquatic systems. In two controlled, field-like competition experiments on sand, the competitive interactions of early successional species of dry acidic grasslands were investigated. These studies included single growing controls and monocultures of all studied species, as well as two, three and four species mixtures in a target-neighbour design. We used Hieracium pilosella and Corynephorus canescens in both studies together with Deschampsia flexuosa and Carex arenaria in the first experiment and Festuca ovina ssp. psammophila and Jasione montana in the second one. Total above and belowground biomass of all plants was measured as well as various shoot parameters and root length and area. The results of the pair-wise plots revealed strictly transitive competitive hierarchies in both studies with F. ovina and C. canescens as superior competitors and C. arenaria and J. montana as inferior competitors while H. pilosella and D. flexuosa took the intermediate positions. Intransitivity or intransitive loops could not be detected in this system. In multi-species plots, however, several cases of ‘competitive release’ could be detected, indicating species-specific changes of competitive interactions in multi-species communities. J. montana, for example, strongly reduced the competitive effect of F. ovina and H. pilosella, while it had no effect whatsoever on C. canescens. Overall, two-species mixtures are insufficient to describe competitive interactions in plant communities, and additional multi-species approaches are necessary if we want to establish a better understanding of the complex nature of competition and coexistence in natural systems.