Shrub removal facilitates recovery of wetland species in a rewetted fen

Drainage and shrub expansion are the main threats to the biodiversity of fens and fen meadows, whereas rewetting and the removal of shrubby species are frequently applied restoration measures. We examine whether removal of shrubs enhances recovery of target species in a degraded fen subjected to moderate rewetting. The study was located in the drained fen Całowanie (central Poland), where remnants of open fen communities and willow-invaded fens exist in former turf-pits, surrounded by degraded meadows on dried peat. All these three habitat types were included in a monitoring grid, which covered an area of 2.2 ha. Within 55 quadrats of 20 m × 20 m we monitored occurrence of 52 species, i.e. two groups of target species (fen indicators and wet meadow indicators) and indicators of two failure scenarios (degraded fen indicators and eutrophic wetland indicators), during six years following shrub removal, rewetting and re-application of conservational mowing, using a 3-step ordinal abundance scale. NMDS ordination revealed a gradual convergence of shrub removal plots and reference plots. We noticed significant effects of year and habitat type on all indicator groups, but only fen indicators have shown a clear (increasing) trend within shrub removal plots. Degraded fen indicators (ruderal and opportunistic species) initially expanded on shrub removal plots, but this effect disappeared in the following years. We conclude that shrub removal enhances establishment of target species in a moderately drained and then rewetted fen and attribute this effect to lowered competition for light. However, given high costs of this method and long-lasting problems with shrub resprouts, we recommend applying shrub removal only to recently overgrown sites, which still retained high botanical diversity. Heavily degraded fen meadows did not react on the increase of moisture, which indicates that more advanced restoration measures, such as top soil removal are needed there.