Ecological processes resulting from global change, such as more frequent extreme summer drought or increased propagule pressure of exotic species, often affect plant communities gradually at time scales of years or decades and may therefore not often be distinctly recognized. The effects of potential interactions of such processes are largely unknown. Interactions may partly compensate single effects and contribute to constancy of communities and ecosystems. However, ecologists are increasingly concerned about potentially adverse interactions that may stimulate cascades of species extinctions.
Over the last decade sowing experiments have found reliable evidence for seed-availability related effects on the local occurrence of species, the size of populations and the richness and productivity of communities. This implies that processes influencing the production and dispersal of seeds and/or the establishment of seedlings are important for the functioning of ecosystems. However, most of these experiments have applied seed additions at one single level of density. With such an approach, the degree of seed limitation cannot be quantified and the relationship between seed rain and number of successfully establishing species has remained obscure. Yet, the natural seed rain as also the production of seeds (community fecundity) have not been measured in the context of seed-addition studies and we still lack the information on how many species and/or individuals may be added to a saturation of the community and how many propagules are needed to reach species saturation of the community.
Although major cases of adverse invasions have not occurred so far, we suspect that species-rich meadows that show a high seedling turnover are potentially susceptible to invasion. Current invasibility theory predicts increased invasion during temporary resource pulses that could be associated with increased variability of rainfall. Successful invasion may primarily depend on the propagule pressures of local resident and potential invader species. A new multi-factorial seed-addition experiment will investigate hypotheses concerning the main and interactive effects of productivity, summer drought, origin of species, and propagule pressure. The experiment will be started in 2010 in meadows covering a wide productivity gradient in different Swiss regions. The knowledge gained from this experiment will be used to predict grassland invasibility and help to adapt the management of Swiss meadows to climate change.
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