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Restoration of severely degraded fens: ecological feasibility, opportunities and constraints.
Abstract and Figures
The majority of the valuable habitats of fens in Europe have disappeared or become degraded. Intensive drainage, causing irreversible changes in the soil, together with high nutrient availability resulted in a reduction of the biodiversity. In this thesis, I consider the possibilities to restore fens and fen meadows on severely degraded peatlands. The main questions of this research are: how and to what extent can we restore fen or fen meadow systems on severely degraded peatlands? What are the major biotic constraints limiting successful restoration? Possibilities to restore species-rich fen meadows by topsoil removal, hay transfer and rewetting In degraded fens, a classic conservation or nature management will not result in restoring the high biodiversity of fen meadows. Techniques such as rewetting, topsoil removal, diaspore transfer or combinations of these were proposed for fen meadow restoration. However, the degree of their success varies strongly. It was found that wet meadow restoration with these techniques had a limited success, with an increase in species richness below 10 % of the regional species pool (Chapter 2). Plant communities developing after restoration differ from reference semi-natural fen meadows and contained on average 30 % of the regional species pool. It seems that rewetting alone is not enough to restore severely degraded fens (Chapter 2). Possibly, this is caused by phosphorus mobilisation and consequent eutrophication. However, improving the hydrological system is necessary in any fen meadow restoration. Topsoil removal was found to be an effective technique for nutrient impoverishment on severely degraded peatlands. Application of this method results also in the removal of competitive non-target species together with their soil seed bank. On severely degraded fens, deep topsoil removal (30-50 cm) was found to be more effective than shallow sod cutting (Chapters 2 & 5). To overcome the seed availability limitation, methods of seed addition can be used, i.e. seeds can be transferred with hay from local provenance donor meadows. However, it should be noted that seed transfer alone may result in very variable outcomes and is, as such, not always successful (Chapter 2). A combination of topsoil removal and diaspore transfer appeared to be the most effective (Chapter 2). This was confirmed in the experimental study on a severely degraded fen (Chapter 5). A combination of deep soil removal, hay transfer and the exclusion of large animals resulted in vegetation development that was most similar to the reference meadow (Chapter 5). This combination of techniques provided abiotic conditions favorable for fen meadows and eliminated the non-target species and their seed banks. Besides, it supplied the seeds of target species, provided a suitable germination and establishment environment, and limited the colonization by non-target species. After hay transfer, it is often assumed that a community similar to a donor meadow will develop. However, if the similarity between donor vegetation composition and the transferred seed composition is low, the restored vegetation will differ from the desired one. The same is true if the number of transferred seeds is small, compared to the influx of non-target seeds. It was found that the composition of the seeds in hay partly represented the donor meadow composition and only half of the species would be transferred to a new site. The number of seeds transferred with hay was much lower than the estimated seed production in the donor meadow (Chapters 4 & 5). In fens or fen meadows, the majority of seeds is produced by a few species. These species might not be abundant in the donor vegetation or might be non-target species (Chapter 4). Also, if the hay is harvested during the regular mowing season of wet meadows, mainly the seed of the dominant, perennial grasses are introduced with hay (Chapter 5). Therefore, to ensure high species richness, repeated hay transfer from carefully selected donor sites and from various mowing dates is recommended. Biotic constraints related to past vegetation development, the soil seed bank and seed production Ecological processes, such as fen degradation influence species selection, causing the selection of plant with particular functional traits. This has consequences for restoration prospects. The trait combinations favored in the aboveground vegetation and in the soil seed bank composition during fen degradation were explored using a chrono-sequence of fen, fen meadow, degraded meadow and using multivariate statistics (Chapter 3). The expected shift in dominating life strategy from a stress tolerance strategy in fens, to a competitive strategy in fen meadows, and to a ruderal strategy in degraded meadows was confirmed (Chapter 3). The vegetative growth was replaced by generative reproduction during fen degradation. An increase in the abundance of species with strong regeneration traits suggested that the disturbance is strongly influencing community assembly during fen degradation (Chapter 3). The abundance of plants with a long flowering period, regenerating from seeds and accumulating persistent soil seed bank was increasing. Plants with strong re-sprouting abilities were promoted during degradation. This is likely an adaptation to an increasingly unpredictable and harsh environment, allowing for regenerating after damage and producing seeds in favorable conditions. The same trait combinations influenced the aboveground vegetation and the soil seed bank composition. The soil seed bank on the degraded fens does not provide propagules for the redevelopment of the species-rich fen meadows and instead is a source of persistent seeds of non-target species (Chapters 3 & 5). The density of the short-term persistent seeds in the soil increases in fen meadows and the density of the long-term persistent seeds increases in the degraded meadows. Data on the seed production in plant communities of fens, fen meadows and degraded meadows is scarce. The size and composition of the seed production in 13 vegetation types was estimated. The seed production of the plant communities is large, variable, and increasing with the disturbance intensity. The estimated annual seed production was c. 25*103 seeds m-2 in fens, 170*103 seeds m-2 in fen meadows and 560*103 seeds m-2 in degraded meadows. Seed production may have consequences for the invasibility of a community and, therefore, also for the likelihood to develop species-rich fen meadows. High seed pressure on degraded meadows and from them to a restored site is likely to cause a high immigration resistance against encroachment by target species. This resistance is not only related to the actual seed production, but in the case of persistent seeds also to the past situation. The future development of plant communities on degraded meadows can be influenced by the legacy of a large number of persistent seeds of non-target species in the soil. A legacy of a large seed production on degraded meadows combined with long-term persistent seed banks may hamper the establishment probability of newly arriving target species and delay the development of target communities. The restorability of severely degraded fens In severely degraded peatlands, major changes have occurred in the hydrological systems. The analysis of the model system showed that under the present circumstances, a restoration of the original fen system on degraded fens is not possible (Chapter 6). The restoration prospect of a degraded fen depends on the landscape settings of the peatland and its surroundings and the possibility to secure stable hydrological regimes. The potential to restore the system supporting the existence of fens or fen meadows, depends on groundwater recharge and discharge characteristics. Various elements, natural or constructed ones (fishponds, roads, etc.) can function as drainage structures when connected to the drainage channels. Even if a discharge of mineral-rich seepage still occurs, but groundwater is quickly drained, the water levels would be too low, compared with the typical fens or fen meadows (Chapter 6). When the entire hydrological system was changed, the small-scale rewetting measures have a very limited effect. On the regional scale, the groundwater recharge characteristics might change due to land use changes in the infiltration areas e.g. forestation, causing increased evapotranspiration. Also an increasing water abstraction, forest drainage or climatic changes may contribute to less groundwater discharge. During restoration process, the early stages of vegetation development are important for later pathways of succession. This was shown by the divergent development of vegetation after the application of different restoration methods (Chapter 5). Hay transfer accelerated the re-establishment of the target vegetation, but most likely, also some priority effects were introduced by the addition of seeds and litter in the beginning of the restoration. In a restored community, ‘priority’ is given to target species that arrive first in sufficient numbers (e.g. with hay) to the site and are able to establish under the protective cover of litter, while other, non-target species (from soil seed bank or colonisation) are inhibited from germination or establishment (Chapter 5). The litter layer or young matrix species probably act as a nursing environment and provide better conditions for seedling survival. Moderately ambitious restoration goals can be achieved on severely degraded fens. Restoration of semi-natural fen meadows is a feasible alternative (Chapter 5). The restored fen meadows will not resemble the historical reference, they will host only a limited number of target species and instead, relatively common species of eutrophic fen meadows will re-establish there (Chapter 2). The fen meadow restoration can be achieved with a set of increasingly effective actions, where also the timing of actions is important. The application of appropriate measures in the beginning of the restoration, without a time lag, might be crucial for the outcomes (Chapters 5 & 7). The tasks for ecological restoration on severely degraded fens consist of returning a degraded system into a less degraded state. To achieve this, the methods of topsoil removal and seed transfer can be applied (Chapters 2 & 5). The challenge for restoration ecology lies in a better understanding of the biotic processes and abiotic-biotic feedback during the restoration process and in improving restoration techniques.
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