This Special Feature focuses on lowland fens and flood plains. In this introduction we discuss the most important mire-related terms, present status, threats and conservation and restoration attempts. Floodplains and especially lowland fens are rare and vulnerable ecosystems. They are highly threatened all over the world because of direct conversion to agricultural land and especially the lack of appropriate management and altered catchment hydrology. Finally we present a framework for the conservation and restoration of these ecosystems. This consists of (1) optimising abiotic conditions; (2) safeguarding propagule availability of the target species; (3) creating and maintaining conditions for (re)establishment of these species, and (4) appropriate management to keep the conditions suitable.

Questions: Various floodplain communities may differ in their relative abilities to influence water quality through nutrient retention and denitrification. Our main questions were: (1) what is the importance of sediment deposition and denitrification for plant productivity and nutrient retention in floodplains; (2) will rehabilitation of natural floodplain communities (semi-natural grassland, reedbed, woodland, pond) from agricultural grassland affect nutrient retention?

Location: Floodplains of two Rhine distributaries (rivers IJssel and Waal), The Netherlands.

Methods: Net sedimentation was measured using mats, denitrification in soil cores by acetylene inhibition and bio-mass production by clipping above-ground vegetation in winter and summer.

Results: Sediment deposition was a major source of N and P in all floodplain communities. Highest deposition rates were found where water velocity was reduced by vegetation structure (reedbeds) or by a drop in surface elevation (pond). Sediment deposition was not higher in woodlands than in grassland types. Denitrification rates were low in winter but significantly higher in summer. Highest denitrification rates were found in an agricultural grassland (winter and summer) and in the ponds (summer). Plant productivity and nutrient uptake were high in reedbeds, intermediate in agricultural grasslands, ponds and semi-natural grasslands and very low in woodlands (only understorey). All wetlands were N-limited, which could be explained by low N:P ratios in sediment.

Conclusions: Considering Rhine water quality: only substantial P-retention is expected because, relative to the annual nutrient loads in the river, the floodplains are important sinks for P, but much less for N. Rehabilitation of agricultural grasslands into ponds or reedbeds will probably be more beneficial for downstream water quality (lower P-concentrations) than into woodlands or semi-natural grasslands.

Question: Why do similar fen meadow communities occur in different landscapes? How does the hydrological system sustain base-rich fen mires and fen meadows?

Location: Interdunal wetlands and heathland pools in The Netherlands, percolation mires in Germany, Poland, and Siberia, and calcareous spring fens in the High Tatra, Slovakia.

Methods: This review presents an overview of the hydrological conditions of fen mires and fen meadows that are highly valued in nature conservation due to their high biodiversity and the occurrence of many Red List species. Fen types covered in this review include: (1) small hydrological systems in young calcareous dune areas, and (2) small hydrological systems in decalcified old cover sand areas in The Netherlands; (3) large hydrological systems in river valleys in Central-Europe and western-Siberia, and (4) large hydrological systems of small calcareous spring fens with active precipitation of travertine in mountain areas of Slovakia.

Results: Different landscape types can sustain similar nutrient poor and base-rich habitats required by endangered fen meadow species. The hydrological systems of these landscapes are very different in size, but their groundwater flow pattern is remarkably similar. Paleo-ecological research showed that travertine forming fen vegetation types persisted in German lowland percolation mires from 6000 to 3000 BP. Similar vegetation types can still be found in small mountain mires in the Slovak Republic. Small pools in such mires form a cascade of surface water bodies that stimulate travertine formation in various ways. Travertine deposition prevents acidification of the mire and sustains populations of basiphilous species that elsewhere in Europe are highly endangered.

Conclusion: Very different hydrological landscape settings can maintain a regular flow of groundwater through the top soil generating similar base-rich site conditions. This is why some fen species occur in very different landscape types, ranging from mineral interdunal wetlands to mountain mires.

Question: In fen meadows with Junco-Molinion plant communities, falling groundwater levels may not lead to a boosted above-ground biomass production if limitation of nutrients persists. Instead, depending on drainage intensity and microtopography, acidification may trigger a shift into drier and more nutrient-poor plant communities.

Location: Nature reserve, central Netherlands, 5 m a.s.l.

Methods: Long-term study (1988–1997) in a fen meadow along a gradient in drainage intensity at different scales.

Results: Above-ground biomass increased only slightly over ten years, despite a lower summer groundwater table. The accountable factors were probably a limited availability of nutrients (K in the higher well-drained plots, P in the intermediate plots and N in the lower hardly drained plots), plus removal of hay.

Junco-Molinion species increased in dry sites and Parvocaricetea species increased in wet sites, presumably primarily because of soil acidification occurring when rainwater becomes more influential than base-rich groundwater. The extent of the shift in species composition depends primarily on the drainage intensity and secondarily on microtopography. Local hydrological measures have largely failed to restore wetter and more basic-rich conditions.

Conclusions: Acidification and nutrient removal, leaching and immobilization resulted in the succession towards Junco-Molinion at the cost of Calthion palustris elements. Lower in the gradient this change was reduced by the presence of buffered groundwater in slightly drained sites. To conserve the typical plant communities of the Junco-Molinion to Calthion gradient in the long term, further acidification must be prevented, for example by inundation with base-rich surface water.

Nomenclature: van der Meijden (1990) for phanerogams and Schaminée et al. (1996) for syntaxa.

Questions: Were continued groundwater discharge and mowing regimes sufficient for vegetation preservation from 1944 to 1993? Which has a stronger effect on vegetation development; groundwater discharge or mowing? What is the role of surface water eutrophication as driver of vegetation change?

Location: Het Hol, The Netherlands (ca. 92 ha, 52°13′ N, 5°05′ E).

Methods: Hydrology was simulated for the late 1940s, early 1960s and 1987. Vegetation maps (1944, 1960, 1975 and 1993) were compared for biotope cover. Vegetation recordings in 1944 and 1987 were compared. Surface water quality was compared between 1950 and 1987. Which sites were mown was reconstructed from an interview. Effects of periodic mowing and groundwater discharge on vegetation development were tested for correlation.

Results: Biotope diversity reduced significantly through decrease of semi-aquatic and tall-herb biotopes, and expansion of forest. The quagfen terrestrialization sere nearly disappeared from 1987 recordings, while the reed sere did well concerning abundance and species richness. Several typical (rich) fen species disappeared from recordings, while new species were mostly field margin species. Periodic mowing and discharge combined are correlated with increasing species numbers. The P-concentration in surface water increased while N-concentration decreased.

Conclusions: Preservation of the reed sere was successful, whereas preservation of the quagfen sere was not. Periodic mowing and discharge stimulate species richness, discharge more so than periodic mowing. But slight eutrophication likely induced a shift from P-limitation to N-limitation, which stimulated the reed sere at the expense of the quagfen sere.

Nomenclature: Phanerogams: van der Meijden (1996); Bryophytes except Sphagna: Margadant & During (1982); Sphagna: Bouman (2002); Syntaxa: Schaminée et al. (1995–1999).

Question: Why is bryophyte succession in eutrophicated fens faster than in natural fens?

Location: Mineral-rich fens in The Netherlands and NW Europe.

Methods: Literature review on the ecology of four bryophyte species in various successional types as observed in Dutch fens.

Results: Bryophyte succession in eutrophicated fens from the brown moss Calliergonella cuspidata to Sphagnum squarrosum is much faster than in natural fens with species shifts from Scorpidium scorpioides to Sphagnum subnitens. Under P-poor conditions, the brown moss stage is stabilized as long as mineral-rich water is supplied. This is because S. scorpioides is tolerant of rainwater, is a strong competitor and can counteract acidification to some extent while S. subnitens is intolerant to groundwater and has low growth rates and low acidification capacity. In contrast, the Sphagnum stage is stable after rapid succession from rich-fen mosses under P-rich conditions. Calliergonella cuspidata has suboptimal growth in rainwater, possibly due to ammonium toxicity, while the high growth rates of S. squarrosum in nutrient-rich and highly acidic groundwater allow early establishment and rapid expansion.

Conclusions: If measures to improve fen base status occur in environments of increased nutrient (P) availability, the management may not lead to the desired restoration of brown moss stages, but instead to rapid acidification by S. squarrosum.

Nomenclature: Smith (1978) for bryophytes; van der Meijden (1996) for vascular plants.

Abbreviations: EC = Electrical conductivity, IR = Ionic ratio

Question: How do nitrogen and phosphorus budgets and balances differ between eutrophic fens and floodplains in western Europe and fens and floodplains in Poland, where we expect less eutrophication to occur?

Location: Wetlands along the rivers Dommel (The Netherlands), Zwarte Beek (Belgium) and Biebrza (NE Poland).

Methods: Assessment of external input and output fluxes as well as net N-mineralization rates. Annual N- and P-balances were estimated by the sum of all external input and output fluxes: atmospheric deposition, input of dissolved matter by flooding, input of sediment by flooding, input by groundwater, output by leaching, output by hay-making and for N also input by N₂-fixation. For N we also estimated net annual N-avail-ability for plant growth, i.e. the N-budget, which includes net mineralization in soil.

Results: The studied wetland sites had a negative balance, which means that nutrients are depleted but only if mown annually, except for the Dutch/Belgian fens which had an equilibrium N-balance and the Polish fen which had an equilibrium P-balance. For the N-budget it appeared that atmospheric deposition added significantly to the budget of Dutch/Belgian fens and N-mineralization added significantly to fen and floodplain budgets, except for the Polish fens. Mineralization dominates the N-budget of the western European floodplains. Haymaking is the most important output pathway, particularly if practised annually. It seems to diminish N-enrichment in the Dutch fens and floodplains.

Conclusions: We conclude that western European fens and floodplains as well as Polish floodplains have a significant positive N-budget indicating that there is a surplus of N for plant growth. In the Polish fens this is less due to low atmospheric deposition and lower N-mineralization rates. The latter is associated with less drying out of the studied Polish ecosystems in summer. Our approach, although an approximate quantification, is helpful for assessing priorities focused on nutrient management.

Question: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland?

Location: Clongawny Lake (53°10′ N, 07°53′ W), County Offaly, Ireland

Methods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a 38-month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio-assays were used to investigate temporal changes in nutrient limitation.

Results: Primary production in the new lake was phytoplankton-driven due to the scarcity of recolonizing macro-phytes. Phytoplankton growth was initially phosphorus-limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5-fold. Consequently, the bio-volume of phytoplankton increased 30-fold, and chlorophyll-a concentrations increased eightfold, reaching hyper-eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen-limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake.

Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.

Abbreviations: DIN = Dissolved inorganic nitrogen, TP = Total phosphorus

Question: What is the relative importance of competition and physical factors in restricting the occurrence of different fen species to certain zones of the riparian landscape?

Location: Biebrza National Park, NE Poland.

Methods: We carried out a two-season reciprocal transplantation experiment: sod-blocks were replaced between a low-productive groundwater-fed small-sedge fen and a high-productive fluviogenous tall-sedge fen. Five treatments were applied to the transplanted sods: (1) no manipulation; (2) fertilization with NPK to exclude the effect of nutrient limitation; (3) clipping vegetation around the transplant to exclude competition for light; (4) clipping fertilizing; (5) sods were also transplanted within their own environment.

Results: After two seasons, the small-sedge sods transplanted into the high-productive zone had a biomass two times lower than that of the control transplants, mainly due to a decrease in small sedges and bryophytes. Tall sedges of the high-productive zone did not respond significantly to transplantation in the low-productive zone. Removal of the surrounding vegetation largely enhanced the growth of small sedges and bryophytes and, to a lesser degree, of tall sedges. Fertilization, on the other hand, resulted in increased growth of tall sedges, grasses and non-graminoid herbs.

Conclusion: Species characteristic of low-productive fen communities are competitively excluded from the high-productive zone through light competition. In contrast, the performance of tall sedges in the low-productive zone is lowered by nutrient limitation. In the long run this may lead to a complete disappearance of these species from this zone. We did not find evidence that the physical stress of flooding has a direct effect on the performance and distribution of species. Results from the experiment suggest that productivity gradients and their influence on competition intensity are of primary importance for structuring vegetation patterns in lowland riparian fens.

Nomenclature: Names of vegetation types follow Palczyński (1984), species names follow Mirek et al. (1997).

Abbreviations: RGR = Relative Growth Rate, SRR = Shoot : Root Ratio

Questions: Does succession of rewetted species-poor fen grasslands display similar trends when different water levels, sites and regions are compared? Will restoration targets as peat growth and waterfowl diversity be reached?

Location: Valley fen of the river Peene (NE-Germany) and the Hanság fen (Lake Neusiedler See, NW-Hungary).

Methods: Analysis of permanent plot data and vegetation maps over a period of up to seven years of rewetting. The general relations between newly adjusted water levels and changes in dominance of helophytic key species during early succession are analysed considering four rewetting intensities (water level classes) and eight vegetation types (Phalaris arundinacea type, Carex type, Glyceria maxima type, Phragmites australis type, Typha type, aquatic vegetation type, open water type and miscellaneous type).

Results: The initial period of balancing the site conditions and vegetation is characterised by specific vegetation types and related horizontal vegetation structures. Most vegetation types displayed similar trends within the same water level class when different sites and regions were compared. A significant spread of potentially peat forming vegetation with dominance of Carex spp. or Phragmites as desired goal of restoration was predominantly restricted to long-term shallow inundated sites (water level median in winter: 0–30 cm above surface). Open water patches as bird habitats persisted mainly at permanent inundated sites (water level median in winter > 60 cm above surface).

Conclusions: Site hydrology appeared as a main force of secondary succession. Thus the rewetting intensity and restoration targets have to be balanced adequately.

Question: Is raising groundwater tables successful as a wetland restoration strategy?

Location: Kennemer dunes, The Netherlands; Moksloot dunes, The Netherlands and Bullock Creek fen, New Zealand.

Methods: Generalizations were made by analysing soil dynamics and the responsiveness of integrative plant traits on moisture, nutrient regime and seed dispersal in three case studies of rewetted vs. control wetlands with the same actual groundwater levels. Soil conditions included mineral (calcareous and non-calcareous) soils with no initial vegetation, mineral soils with established vegetation and organic soils with vegetation.

Results: The responsiveness of traits to raised groundwater tables was related to soil type and vegetation presence and depended on actual groundwater levels. In the moist-wet zone, oligotrophic species, ‘drier’ species with higher seed longevity occupied gaps created by vegetation dieback on rewetting. The other rewetted zones still reflected trait values of the vegetation prevalent prior to rewetting with fewer adaptations to wet conditions, increased nutrient richness and higher seed longevity. Moreover, ‘eutrophic’ and ‘drier’ species increased at rewetted sites, so that these restored sites became dissimilar to control wetlands.

Conclusions: The prevalent traits of the restored wetlands do not coincide with traits belonging to generally targeted plant species of wetland restoration. Long-term observations in restored and control wetlands with different groundwater regimes are needed to determine whether target plant species eventually revegetate restored wetlands.

Abbreviations: F = Mean moisture indicator values, lnRR = Natural log of response ratio, N = Nutrient indicator value, PCA = Principal Component Analysis, RDA = Redundancy analysis

Question: Do regional species pools, landscape isolation or on-site constraints cause plants from different guilds to vary in their ability to colonize restored wetlands?

Location: Iowa, Minnesota, and South Dakota, USA.

Methods: Floristic surveys of 41 restored wetlands were made three and 12 years after reflooding to determine changes in local species pools for eight plant guilds. The effect of landscape isolation on colonization efficiency was evaluated for each guild by plotting local species pools against distance to nearby natural wetlands, and the relative importance of dispersal vs. on-site constraints in limiting colonization was explored by comparing the local species pools of restored and natural wetlands within the region.

Results: Of the 517 wetland plant taxa occurring in the region, 50% have established within 12 years. The proportion of the regional species pool represented in local species pools differed among guilds, with sedge-meadow perennials, emergent perennials and floating/submersed aquatics least represented (33–36%) and annual guilds most represented (74–94%). Colonization-to-extinction ratios suggest that floating/submersed aquatics have already reached a species equilibrium while sedge-meadow and emergent perennials are still accumulating species. Increasing distance to nearest wetlands decreased the proportion of the regional species pool present in local pools for all guilds except native annuals and woody plants. The maximum proportion predicted, assuming no distance constraint, was comparable to the lowest-diversity natural wetlands for most perennial guilds, and also lower than what was achieved in a planted, weeded restoration.

Conclusions: Abiotic constraints seem to limit the colonization of floating/submersed aquatics into natural or restored wetlands, whereas all other guilds are potentially constrained by dispersal or biotic factors (i.e. competition from invasive species). Using species pools to evaluate restoration progress revealed that immigration potential varies considerably among guilds, that local species richness does not necessarily correspond to immigration limitations, and that some guilds (e.g. sedge-meadow perennials) will likely benefit more than others from being planted at restoration sites.

Nomenclature: Anon. (2004).

Questions: For wetland plants, dispersal by wind is often overlooked because dispersal by water is generally assumed to be the key dispersal process. This literature review addresses the role of seed dispersal by wind in wetlands. Why is wind dispersal relevant in wetlands? Which seeds are dispersed by wind and how far? And how can our understanding of wind dispersal be applied to wetland conservation and restoration?

Methods: Literature review.

Results and conclusions: Wind is a widely available seed dispersal vector in wetlands and can transport many seeds over long distances. Unlike water, wind can transport seeds in all directions and is therefore important for dispersal to upstream wetlands and to wetlands not connected by surface water flows. Wind dispersal transports seeds to a wider range of sites than water, and therefore reaches more sites but with lower seed densities.

Many wetland plant species have adaptations to facilitate wind dispersal. Dispersal distances increase with decreasing falling velocity of seeds, increasing seed release height and selective release mechanisms. Depending on the adaptations, seeds may be dispersed by wind over many km or only a few m. The frequency of long-distance wind dispersal events depends on these adaptations, the number of produced seeds, the structure of the surrounding vegetation, and the frequency of occurrence of suitable weather conditions.

Humans reduce the frequency of successful long-distance wind dispersal events in wetlands through wetland loss and fragmentation (which reduce the number and quality of seeds) and eutrophication (which changes the structure of the vegetation so that seed release into the wind flow becomes more difficult). This is yet another reason to focus on wetland conservation and restoration measures at increased population sizes, prevention of eutrophication, and the restoration of sites at short distances from seed sources.

Nomenclature: van der Meijden (1990) for taxa, Schaminée et al. (1995, 1996) and Stortelder et al. (1999) for syntaxa.

Question: How does seed dispersal reduce fen isolation and contribute to biodiversity?

Location: European and North American fens.

Methods: This paper reviews the literature on seed dispersal to fens.

Results: Landscape fragmentation may reduce dispersal opportunities thereby isolating fens and reducing genetic exchange. Species in fragmented wetlands may have lower reproductive success, which can lead to biodiversity loss. While fens may have always been relatively isolated from each other, they have become increasingly fragmented in modern times within agricultural and urban landscapes in both Europe and North America. Dispersal by water, animals and wind has been hampered by changes related to development in landscapes surrounding fens. Because the seeds of certain species are long-lived in the seed bank, frequent episodes of dispersal are not always necessary to maintain the biodiversity of fens. However, of particular concern to restoration is that some dominant species, such as the tussock sedge Carex stricta, may not disperse readily between fens.

Conclusions: Knowledge of seed dispersal can be used to maintain and restore the biodiversity of fens in fragmented landscapes. Given that development has fragmented landscapes and that this situation is not likely to change, the dispersal of seeds might be enhanced by moving hay or cattle from fens to damaged sites, or by reestablishing lost hydrological connections.

Nomenclature: Anon. (2004).

Question: What is the relative ability of four species of Sphagnum (S. fuscum, S. rubellum, S. magellanicum and S. angustifolium) to establish on bare peat substratum in the field when re-introduced as single or multi-species re-introductions and in relation to interannual variations in climate?

Location: Continental southeastern Canada.

Methods: Diaspores (fragments) of four Sphagnum species alone or in combination were re-introduced onto residual peat surfaces and were monitored to follow the development of the moss carpet over four growing seasons. In order to compare results under a variety of climatic conditions, this whole experimental setting was repeated four times (trials), with a four-year follow-up for each trial.

Conclusions: The establishment rate of the moss carpet varied among years, in response to climatic variations between growing seasons. The relative success of different moss species and combinations of species, however, did not vary within or between trials. Thus, the species and combinations of species resulting in the highest short-term or long-term establishment rates remained the same for all trials, independent of the climatic conditions at the time of re-introduction. Our results showed no link between the number of species in the diaspore mixture and successful establishment of the moss carpet. Yet successful regeneration was clearly influenced by the identity of species chosen for re-introduction. S. fuscum, alone or in combination, was the species found to lead to the most extensive development of the moss carpet under the current test conditions.

Nomenclature: Anderson (1990).

Abbreviations: LSD = Least significant difference, LSMEANS = Least Square Means

Question: How successful is restoration that is focused on a single dominant plant at enabling the reassembly of the whole vegetation assemblage and what factors affect the relative success of such restoration?

Location: Moorlands in England and Scotland, UK.

Methods: Vegetation composition was sampled in grass-dominated (degraded), restored and long established Calluna vulgaris-dominated (target) areas within eight moorland restoration sites. Additional soil and biogeographic data were collected. Data were analysed by Canonical Correspondence Analysis, which allowed the impact of moorland management to be examined.

Results: All sites showed good restoration success when the dominant managed species (Calluna vulgaris, Molinia caerulea and Nardus stricta) were considered. Restoration success of the remainder of the plant assemblage, disregarding these dominant species, was lower with restored samples at some sites differing little from their respective degraded samples. Moors restored solely by grazing exclusion were more similar to their respective targets than were those restored by more intensive mechanical means. The most important factors in explaining vegetation assemblages were management status (i.e. whether samples represented degraded, restored or target parts of the moor) and latitude.

Conclusions: The project demonstrates that, where possible, restoration should be attempted by grazing exclusion alone. Furthermore the importance of applying local restoration targets and of monitoring the whole plant assemblages when assessing the success of moorland restoration is highlighted.

Nomenclature: Species names follow Stace (1991) for higher plants and Smith (2004) for bryophytes.

Abbreviations: CCA = Canonical Correspondence Analysis, LOI = Loss on Ignition

Question: Can the biodiversity of fens in Europe and North America be maintained through the use of grazing (especially cattle grazing), fire, and/or cutting?

Location: European and North American fens.

Methods: This paper is a review of the literature on the effects of grazing, fire and cutting on fens, to explore the relationship between management and biodiversity in fens.

Results: A reduction of cattle grazing, mowing and burning in fens has led to a reduction in biodiversity in fens. The vegetation of abandoned fens shifts to trees and shrubs after 10–15 years, which shade the smaller and rarer species of these wetlands. While careful use of fire is used to manage fens in North America, it is not widely used in European fens, perhaps because the peat of drained fens may catch fire. Cattle grazing cannot be considered a natural disturbance in North America, since cattle did not evolve on that continent. In Europe, cattle do not generally graze in unaltered fens, but they do use slightly drained fen meadows.

Conclusions: Three approaches have been used to control the dominance of tall woody and herbaceous species in abandoned fens, including the re-introduction of cattle, mowing, and burning. Overgrazing results in a permanent reduction in biodiversity, therefore cattle re-introduction must be approached cautiously. In Europe, but not in North America, mowing has been an important management tool, and mowing has been successful in maintaining species richness, particularly in fens that have been mowed annually for centuries. Fire has been the most common and successful management tool in North America although it is not effective in removing shrubs that have become large. Because the problems and solutions are similar, the literature of both European and North American fen management can be analyzed to better assess the management of fens on both continents. Many management questions require further study and these are listed in the paper.

Nomenclature: Anon. (2004).