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.