Mechanisms that stabilize the dynamics of plant species in grassland communities

Background/Question/Methods

To date all biodiversity experiments in grasslands showed positive diversity-productivity relationships that could be attributed to complementarity effects. This insight has helped to understand the possible consequences of diversity loss for important ecosystem services. But even more important is that such complementarity effects might have significant stabilizing impacts on plant species abundances in stochastically fluctuating environments. Unravelling the mechanisms behind these complementarity effects may provide an important step forward in our understanding of the processes that stabilize or destabilize the dynamics of plant communities. Two hypothesized mechanisms are: (1) plant-species-specific impacts of root pathogens and (2) vertical root stratification. We performed an 11-year garden plot experiment to analyse the effects of species diversity on plant productivity. At the end of the experiment we analysed the vertical distribution of roots in plots with different diversity levels using new molecular methods that enabled us to distinguish the roots of different species in mixed root samples. In addition, we collected soil from each treatment, which was added to sterile soils in a greenhouse pot experiment to test the effects of the different soil communities that had built up during 11 years on the growth of target plant species in monoculture and 8-species mixtures.

Results/Conclusions

During 11 year the average yield in the monocultures of the field experiment strongly declined, while it did not in the diverse mixtures, suggesting that the development of specific soil pathogen communities was responsible for the observed complementarity effects. The results of our greenhouse inoculation experiment confirmed this hypothesis. The growth of plant species in monocultures was negatively affected by the frequency of that species in the former garden plots from which soil was collected for inoculation. These negative effects were rather small when the  target plants were grown in monoculture (-15%), but much stronger in 8-species mixtures (-57%). There were also asymmetric reciprocal effects between species belonging to the same family (Asteraceae, Poaceae) which might result in complicated interaction networks within communities. The root studies revealed that all species were able to exploit the whole soil column in monocultures without striking differences among species. But in mixtures we observed clear spatial separation between plant species with different rooting patterns (especially between Poaceae and Asteraceae). Although the observed vertical stratification of root growth does not lead to increased resource absorption and complementarity, we found that it can have strong stabilizing impacts on the dynamics of competing species in stochastic environments.