Are we restoring functional fens? – The outcomes of restoration projects in fens re-analysed with plant functional traits

Abstract

In peatland restoration we often lack an information whether re-established ecosystems are functionally similar to non-degraded ones. We re-analysed the long-term outcomes of restoration on vegetation and plant functional traits in 38 European fens restored by rewetting (18 sites) and topsoil removal (20 sites). We used traits related to nutrient acquisition strategies, competitiveness, seed traits, and used single- and multi-trait metrics. A separate set of vegetation records from near-natural fens with diverse plant communities was used to generate reference values to aid the comparisons. We found that both restoration methods enhanced the similarity of species composition to non-degraded systems but trait analysis revealed differences between the two approaches. Traits linked to nutrient acquisition strategies indicated that topsoil removal was more effective than rewetting. After topsoil removal competitive species in plant communities had decreased, while stress-tolerant species had increased. A substantial reduction in nutrient availability ruled out the effect of initial disturbance. An ability to survive and grow in anoxic conditions was enhanced after restoration, but the reference values were not achieved. Rewetting was more effective than topsoil removal in restricting variation in traits values permitted in re-developing vegetation. We found no indication of a shift towards reference in seed traits, which suggested that dispersal constraint and colonization deficit can be a widespread phenomena. Two functional diversity indices: functional richness and functional dispersion showed response to restoration and shifted values towards reference mires and away from the degraded systems. We concluded that targeting only one type of environmental stressor does not lead to a recovery of fens, as it provides insufficient level of stress to restore a functional ecosystem. In general, restoration efforts do not ensure the re-establishment and long-term persistence of fens. Restoration efforts result in recovery of fen ecosystems, confirmed with our functional trait analysis, although more rigid actions are needed for restoring fully functional mires, by achieving high and constant levels of anoxia and nutrient stresses.

Rewetting does not return drained fen peatlands to their old selves

“…It is commonly assumed that the stabilization of the water table close to the surface is a premise for successful restoration of peatlands 3 because too low water tables lead to high carbon emissions and non-wetland plants remaining dominant while too high water tables create shallow lakes rather than peatlands. While closest similarity to near-natural counterparts with respect to biodiversity, ecosystem functioning and land cover characteristics indeed occurred for rewetted sites with annual median water tables close to the surface, we also observed sites with water tables close to the surface being strongly dissimilar to their near-natural counterparts.…”

“…4 ). Duration and intensity of drainage might likely matter here 3 as these determine nutrient availability and irreversible changes in peat hydraulic variables (Fig. 4b ).…”

“…Furthermore, nutrient availability after rewetting is considerably higher than in natural peatlands, mainly because of peat mineralization and fertilization while the peatland was drained 4 , 26 , and because of mobilization of phosphorus upon rewetting 27 . Consequently, microbial 4 and plant 3 communities show prompt recovery towards their pre-drainage status only in previously weakly disturbed ecosystems.…”

Potentially peat‐forming biomass of fen sedges increases with increasing nutrient levels

“…Rewetting leads to the establishment of peat‐forming plants, including sedge communities, with some variation in community composition depending on the conditions before and after rewetting and the time since rewetting (Klimkowska et al., 2019; Timmermann et al., 2006; Zerbe et al., 2013). It remains to be investigated which factors promote this development.…”

The Power of Nature‐Based Solutions: How Peatlands Can Help Us to Achieve Key EU Sustainability Objectives

“…[ 16 ] Recent studies from European fens, the peatland type most dramatically affected by drainage for agriculture, point at quick recovery of key microbial communities and plant functional types. [ 22,23 ]…”