High nutrient loads hinder successful restoration of natural habitats in freshwater wetlands
Restoration of natural processes in ecosystems is key to halt the biodiversity crisis. Here, we evaluate 20 different stream-valley wetland restoration projects - mainly rewetting - in a large region in Denmark in terms of successful restoration of natural wetland habitats. We used quadratic discriminant analysis and generalized linear models to compare the projects 80 vegetation plots with >50.000 natural wetland-habitat reference plots and modelled the influence of time, grazing, rewetting and nutrient availability on the study plots probabilities of belonging to such natural habitats and their richness of high-quality habitat indicator species. In our study, the probability of a restored wetland being a natural wetland habitat - almost always an alkaline fen - was generally below 10 %. Also, we only found half as many indicator species in restored wetlands than in reference wetlands and we demonstrated that the number of characteristic alkaline fen species did not deviate from what could be expected under the prevailing nutrient conditions. We found a negative effect of nutrient availability on the number of high-quality habitat indicator species and the lowest probability of plots being natural wetlands in the most nutrient rich plots. The effect of grazing was only positive in the first years after restoration and only in the most nutrient rich plots, while the effect of rewetting sites to their historical hydrological conditions was generally negative. Our findings reveal that unnaturally high nutrient availability is probably the core limiting factor for successful restoration of natural wetlands and their associated plant diversity. Implications for practice- To successfully restore natural and characteristic freshwater wetland habitats focus on recreating natural processes and conditions is needed - Restoring natural hydrology and grazing is not enough, the soil and water must be naturally nutrient poor for successful restoration of these habitats - Restoration of stream-valley wetlands such as alkaline springs and fens is more likely to be successful in spring-dominated landscapes where clean groundwater diffusely exfiltrates the soil