Effects of restoration measures on plant communities of wet heathland ecosystems

Question: Which are the success and failure of restoration measures, particularly sod‐cutting and hydrological measures, in small wetlands on mineral soils in The Netherlands. Location: Twente, in the eastern part of The Netherlands. Methods: Success or failure of restoration measures has been assessed by comparing experimental plots with that in reference plots for (1) species richness, (2) re‐establishment of endangered species and (3) species composition, including life forms and Red List species. In total 119 samples were taken in 42 permanent plots in fen meadows, small‐sedge marshes, wet heathlands, and soft‐water pools. Topsoil samples were analysed for pH, Ca, Mg, Na, K and Cation Exchange Capacity. Gradient analysis was carried out by means of Canonical Correspondence Analysis. Results: Sod‐cutting, in combination with re‐wetting measures (infilling of ditches and drains), led to restoration of communities of soft‐water pools and small sedge marshes within five years. This rapid recovery is probably related to the presence of persistent seed banks of the component species of these communities. Complete restoration of fen meadows took longer (10–15 yr). Apparently, many species of fen meadows have short‐lived seed banks. Sod‐cutting of a degraded wet heathland and a soft‐water pool was only successful temporarily, probably as a consequence of low water tables. Conclusions: To prevent depletion of (persistent) soil seed banks, sod‐cutting in nutrient‐poor wetlands is not recommended in areas where the groundwater regime and the base status of the soil can not be restored to levels required by plant communities of wet heathlands.     

Vegetation development in dune slacks: the role of persistent seed banks

Abstract. The soil seed bank composition was determined at four sites in the dune slack ‘Koegelwieck’ on the Dutch Wadden Sea island of Terschelling. At three different sites in the slack, where sod-cutting experiments down to the mineral sand had been carried out, the established vegetation and seed bank were assessed after 5, 9 and 39 yr of undisturbed development, respectively. In addition, a fourth site in the slack was investigated, where vegetation development had proceeded for 80 yr since plant colonization of bare soil and where nowadays a vegetation dominated by Calamagrostis epigejos and Salix repens occurs. Together these four sites can be regarded as a chronosequence of dune slack formation. Clear time sequences were detected in the seed bank data. Many late successional species showed a significant increase in the number of seeds during the succession. Some of the early successional basiphilous pioneer species such as Anagallis minima, Centaurium littorale, Littorella uniflora, Radiola linoides and Samolus valerandi, showed either a decrease during the time of succession or an optimum in the vegetation while remaining present in the seed bank in low but detectable numbers. They could, therefore, play a role in re-establishment of the vegetation after sod-cutting. One of the target species, Schoenus nigricans, established within a few years after removal of the sod. However, no seeds of this species have been detected in the soil below either of the successional stages. Based on the species disappearance from the established vegetation and based on the independent data of Thompson et al. (1997) an estimation of seed longevity could be made for several Red List species of wet dune slacks.     

Hydrology of Dutch Cirsio‐Molinietum meadows: Prospects for restoration

Abstract. Fen meadows (Cirsio dissecti‐Molinietum) are seriously threatened by desiccation, acidification and eutro‐phication. In The Netherlands several projects were launched to restore damaged fen meadows. This review describes how successes and failures of these restoration projects depend on hydrological systems. Six hydrological systems have been distinguished, which all provide the site conditions required by this community. Nowadays, the best developed fen meadows are found in the higher Pleistocene landscape of The Netherlands, where they depend on base‐rich groundwater discharging from local or large groundwater systems. Fen meadows of the lower Holocene landscape usually occur in man‐made surface water systems. Almost all stands have been severely deteriorated. Restoration of fen meadows in the Pleistocene landscape is promising when the hydrology is only slightly disturbed or when hydrological measures are taken in combination with sod cutting. Restoration prospects of fen meadows in the Holocene landscape are low. Until now a complete regeneration of Cirsio‐Molinietum meadows has not been realized. Restoration measures failed to restore high pH values in the top soil. It is hypothesized that viable seeds of many target species lack in the soil seed bank. In addition, the dispersal capacities of these species seem to be limited.     

Ecological restoration of rich fens in Europe and North America: from trial and error to an evidence‐based approach

Fens represent a large array of ecosystem services, including the highest biodiversity found among wetlands, hydrological services, water purification and carbon sequestration. Land‐use change and drainage has severely damaged or annihilated these services in many parts of North America and Europe; restoration plans are urgently needed at the landscape level. We review the major constraints on the restoration of rich fens and fen water bodies in agricultural areas in Europe and disturbed landscapes in North America: (i) habitat quality problems: drought, eutrophication, acidification, and toxicity, and (ii) recolonization problems: species pools, ecosystem fragmentation and connectivity, genetic variability, and invasive species; and here provide possible solutions. We discuss both positive and negative consequences of restoration measures, and their causes. The restoration of wetland ecosystem functioning and services has, for a long time, been based on a trial‐and‐error approach. By presenting research and practice on the restoration of rich fen ecosystems within agricultural areas, we demonstrate the importance of biogeochemical and ecological knowledge at different spatial scales for the management and restoration of biodiversity, water quality, carbon sequestration and other ecosystem services, especially in a changing climate. We define target processes that enable scientists, nature managers, water managers and policy makers to choose between different measures and to predict restoration prospects for different types of deteriorated fens and their starting conditions.     

The effects of drainage on groundwater quality and plant species distribution in stream valley meadows

Conditions in fen meadows in Dutch stream valleys are influenced by both deep (Ca²⁺-rich) and shallow (Ca²⁺-poor) groundwater flows. The distribution patterns of phreatophytic (groundwater-influenced) plant species showed distinct relationships with the distribution of different groundwater types. Large fluctuations in the ionic composition of groundwater were observed in the upper peat layers of drained areas. Ca²⁺-rich groundwater was replaced by precipitation water to a considerable depth (1–1.5 m). These fluctuations in groundwater composition were less pronounced in undrained areas.It was observed that characteristic species of fen meadow communities and rare phreatophytic species were restricted to areas with high water tables, which were nourished by either Ca²⁺-poor or by Ca²⁺-rich groundwater. Few species showed a preference for drained areas, where replacement of groundwater types gave rise to the occurrence of an intermediate groundwater type, which was thought to be the result of an increased acidification of the top soil (increased influence of infiltration water).It was argued that the endangered species were best preserved in areas with an undisturbed discharge of natural groundwater flows.Abbreviations EC = Electrical conductivity     

Nutrient limitations in an extant and drained poor fen: implications for restoration

In a species-rich poor fen (Caricetum nigrae) and a species-poor drained fen, the difference in nutrient limitation of the vegetation was assessed in a full-factorial fertilization experiment with N, P and K. The results were compared to the nutrient ratios of plant material and to chemical analysis of the topsoil. A rewetting experiment with intact sods was carried out in the glasshouse and the results are discussed in view of restoration prospects of drained and degraded peatlands. In the undrained poor fen the above-ground biomass yield was N-limited while the vegetation of the drained fen was K-limited. Experimental rewetting of intact turf samples, taken in the drained site, did not change the biomass yield or the type of nutrient limitation. It was concluded that mire systems which have been subjected to prolonged drainage are inclined to pronounced K-deficiency, probably due to washing out of potassium and harvesting the standing crop. This may hamper restoration projects in degraded peat areas where nature conservation tries to restore species-rich vegetation types with a high nature value.     

Enhanced nitrogen loss may explain alternative stable states in dune slack succession

Ecological theory emphasizes competitive interactions between plant species when explaining primary succession in plants. Ecosystem processes, such as nutrient accumulation, are often regarded as independent, steering successional changes without being affected by the interacting plant species. We present experimental evidence that plant species in wet dune slack systems are able to affect ecosystem processes in their favor by reducing the rate of nitrogen accumulation by coupled nitrification–denitrification, promoting their competitive position. We compared denitrification rates of two early successional species having radial oxygen loss (ROL) with two non-ROL late successional species in a mesocosm experiment. The denitrification rates were significantly higher in mesocosms planted with early successional species Littorella uniflora (P<0.001) and Schoenus nigricans (P<0.05), relative to the rates found in the presence of non-ROL species, Carex nigra and Calamagrostis epigejos . We analyzed the consequences of enhanced nitrogen loss on the competition between an early and a late successional species by means of a simple theoretical model. Our analysis revealed that early successional species capable of ROL might retard successional changes and lock the ecosystem in an unproductive state for an extended period of time. We emphasize that understanding of ecosystem processes is crucial in successful conservation of high biodiversity vegetation in wet dune slacks.