Nutrients, isolation and lack of grazing limit plant diversity in restored wetlands

Agricultural intensification has led to the loss of most natural wetlands from human-dominated landscapes but, in recent decades, wetland restoration has gained traction worldwide. Restoration of suitable habitat conditions is hampered by nutrient residues from decades of high-input farming, by continued input of nutrients from surrounding farmland, and by incomplete restoration of natural hydrology. We assessed the effect of wetland restoration and disturbance (grazing) on vascular plant and bryophyte diversity in restored wetlands in Denmark at plot and landscape scales. Compared to near-natural wetlands, we found a lower species richness and community uniqueness in restored wetlands, but we also found that grazing had a positive effect on vascular plant richness in restored wetlands. The size of the local species pool, the presence of near-natural habitat prior to restoration, and near-natural restored hydrology all had positive effects on plant diversity, while high soil iron and nitrogen had negative effects.

Synthesis and applications: Restoration of plant diversity in wetlands is challenging, but our results point to potential remedies: plan restoration areas near colonisation sources of the target biota, restore hydrology to near-natural conditions, discontinue nutrient loads from surrounding arable land and restore near-natural grazing regimes. Bryophytes may be particularly useful as indicators for successful restoration of wetlands.

Methods
We conducted this study on landscape (hectares) scale and plot (m2) scale. At the landscape scale, we used landscape-scale predictors to evaluate the plant biodiversity in wetlands that had been restored under The Action Plans on the Aquatic Environment and compared those sites with near-natural reference sites. At the plot scale, we evaluated the effects of grazing (no active management vs. grazing) and local environmental predictors on plant biodiversity in restored and near-natural wetlands. We define restoration success as similarity between restored and near-natural sites in terms of plant species richness, community uniqueness and community composition.