Questions
Ecological restoration has traditionally been evaluated with analyses focused on species identities and abundances. These analyses provide no ecological explanation to why certain species change in abundance. One solution may be a functional trait analysis. We asked whether shifts in functional traits could explain vegetation changes in fens restored through tree cutting and rewetting, and how the functional traits in the restored sites compare to those of the reference site?
Location
Three former rich fens in east‐central Sweden.
Methods
Tree cutting and rewetting were applied in a factorial design, and species and abundance data were recorded for 8 yrs. Abundance data and trait data of canopy height, specific leaf area (SLA) and diaspore mass were used to calculate functional richness (FRic), functional divergence (FDiv), functional dispersion (FDis) and community‐weighted mean (CWM) of functional traits. Data were analysed in a linear mixed effect model for vascular plants and bryophytes jointly, and for vascular plants separately. Results of restoration treatments were compared to data from a reference site.
Results
Among vascular plants, tree cutting caused a decrease in SLA, as shade‐sensitive species increased. In accordance with the change in SLA, FDis increased. In the joint analysis, tree cutting led to increased FDis, FDiv and FRic, indicating reduced filtering caused by the removal of the shading canopy, which allowed shade‐sensitive species to establish. The comparison to the reference site shows that even after 8 yrs, the restoration treatments have higher trait diversity than the reference site, indicating that the restoration sites have a too relaxed trait filter compared to conditions in an undisturbed fen. Our interpretation is that this is primarily caused by insufficient rewetting (and increased nutrient availability) that allow species of both natural and degraded fen conditions to co‐exist, and which failed to suppress the regrowth of trees.
Conclusions
Analysis of functional diversity improves our understanding of the ecological mechanisms affecting restoration results, and allows comparison among regions and communities with different species composition.
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.
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