Harnessing facilitation: Why successful re-introduction of Stratiotes aloides requires high densities under high nitrogen loading

Dense mats of floating vegetation form complex structures that support high biodiversity in wetlands. Since the 1960s, however, high-density stands of Stratiotes aloides have shown strong declines throughout Europe and restoration efforts have often failed, both of which have been linked to high nitrogen (N) input rates. The low success rate of reintroduction is striking, since seemingly healthy, high-density stands are observed to grow under similar environmental conditions. We therefore hypothesise that N tolerance of S. aloides increases with density due to joint N uptake and detoxification. To test our hypothesis, we set up a controlled full factorial mesocosm experiment in which we manipulated N loading and S. aloides density, and investigated interacting effects on habitat biogeochemistry and plant performance. High-density S. aloides stands strongly lowered ammonium availability through shared uptake, even at extremely high N loads. Furthermore, dense stands strongly reduced water layer oxygen concentrations, which stimulated sediment phosphorus mobilisation, and increased carbon dioxide concentrations, which enhanced underwater photosynthesis. High-density stands thus not only detoxify ammonium, but also facilitate population growth through habitat manipulation. Combined, these mechanisms enabled S. aloides to remain viable at high loads of 800 kg N ha− 1 y− 1, whereas low-density stands already collapsed at 200 kg N ha− 1 y− 1. Overall, our results show that under similar environmental conditions, high-density stands can thrive, while restoration or natural recruitment by a low number of individuals may be impossible. We therefore conclude that acknowledging and harnessing intraspecific facilitation can be vital for successful conservation and restoration.