Wet Dune Slacks Research Articles

Preference of wet dune species for waterlogged conditions can be explained by adaptations and specific recruitment requirements

The preference of wetland angiosperms for waterlogged soils has been explained by several hypotheses: (1) wetland species are adapted to waterlogging and sensitive to drought; (2) wetland species are tolerant to drought, but inferior competitors at drier conditions; (3) wetland species have narrow moisture optima for recruitment. We tested these hypotheses by the application of permanent and fluctuating water levels to experimental wet dune plant communities with four species that frequently occur in wet dune slacks ( Carex flacca, Molinia caerulea, Samolus valerandi and Schoenus nigricans) and one competitive species of drier conditions ( Calamagrostis epigejos). After 3 years, fluctuating water levels had led to lower total biomass production than permanent water levels, indicating that switching from aerated to anoxic soil conditions involved physiological costs. The collective biomass of wetland species was highest at permanently waterlogged conditions with interspecific variation in the biomass optima. At the nutrient-poor growth conditions of this long-term experiment, biomass of Calamagrostis was independent of water level treatment, thus the hypothesized competitive superiority of this species at drier conditions could not explain the biomass responses of the wetland species in this study. Instead, this is evidence for an ecological preference of adult wet dune plants for waterlogged to moist conditions. Recruitment of most wet dune slack species occurred in a narrower range of water levels than adult growth, indicating that recruitment requirements also pose a limitation to the distribution of these wet dune species. Incorporation of recruitment into nature management support models may improve their predictions.

Plant responses to rising water tables and nutrient management in calcareous dune slacks

Plant species of oligotrophic wet dune slacks have dramatically decreased as a result of desiccation and eutrophication. The aim of this study was to test in a field experiment the effects of restoration management in oligotrophic, wet dune slacks (groundwater level rise in combination with topsoil removal or mowing) on abiotic variables and on survival and biomass of four plant species. The effect of groundwater level rise on abiotic variables strongly differed between mown sampling locations and those with topsoil removal. At locations with a mowing treatment, a large rise in water tables led to increased N availability and higher reduced iron concentrations than at other locations. Such effects were absent at locations with recent topsoil removal. No effect of groundwater level rise on P-availability was found. Topsoil removal on average lowered N availability by 13%, P availability by 65% and Fe2+ by 56%. All phytometer species survived better in mown dune slacks than in dune slacks that had received topsoil removal. Survival of all species was negatively related to groundwater level rise. On the short term local extinction risks of small populations may be enhanced by rewetting and topsoil removal. On the long-term, however, such measures are crucial to maintain vegetation of oligotrophic wet dune slacks in a degraded dune landscape.

Effects of Ca- and Fe-rich Seepage on P Availability and Plant Performance in Calcareous Dune Soils

Ca- and Fe-rich seepage in wet dune slacks often sustains oligotrophic, species rich vegetation as a result of reduced P availability. While this effect has been attributed to pH buffering, we tested whether Ca- and Fe-rich seepage also immobilised P in calcareous soils with a strong pH buffer. Two oligotrophic species (Carex flacca and Schoenus nigricans) and two eutrophic species (Calamagrostis epigejos and Molinia caerulea) were planted in experimental sods. After 4 months supply with water of seepage or infiltration quality, with or without the addition of P, soil P fractions and a number of plant physiological responses were measured. A field validation was performed in a flow-through lake in calcareous dunes where the seepage flow had been restored recently. The readily available water soluble P fraction (Pw) was reduced by more than 80% by seepage, both in the greenhouse experiment and in the field, but the P Olsen-inorganic fraction was unaffected. All four test species had elevated N: P ratios in aboveground tissues when treated with seepage, indicating that seepage water had indeed reduced P availability to plants. Formation of dauciform roots by Carex flacca was diminished by P addition to less than 25% of treatments without P addition, indicating sensitivity to P availability, while seepage on average halved production of these root structures. Dauciform root formation by Schoenus nigricans was unaffected by the P addition and the hydrological treatment. Biomass of the test species in the experiment as well as vegetation biomass and relative abundance of oligotrophic species in the field were unrelated to seepage patterns, suggesting that compensatory mechanisms enabled the plants to sustain biomass production within the time frame of this experiment. In conclusion, Ca- and Fe-rich seepage can lower P availability in soils with a strong pH buffer. In the long term, this may create favourable conditions for species that have low P requirements or efficient P uptake. In the short term, however, existing vegetation seems to be resilient to changes in P supply.

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.

Does the seed bank contribute to the restoration of species‐rich vegetation in wet dune slacks?

AbstractQuestions: What is the contribution of the seed bank to restoration of species‐rich vegetation in oligotrophic wet dune slacks? Does the restoration management affect the seed bank?Location: Calcareous coastal dune slacks at the west coast of The Netherlands.Methods: Species composition of the seed bank and the above‐ground vegetation was sampled in dune slacks that had a variable extent of groundwater level rise in combination with either topsoil removal or mowing.Results: The seed bank had a high potential for restoration of species‐rich vegetation: 60 species were found in the seed bank of which 14 were characteristic of oligotrophic, wet dune vegetation. While topsoil removal almost completely removed the seed bank, groundwater level rise did not permanently submerge the seed bank of species of oligotrophic, wet conditions. Changes in abundance in the established vegetation were unrelated to species abundance in the seed bank. Of all new species establishments in the vegetation relevés, 76% occurred where the species was not found in the seed bank. The chance that presence of a species in the seed bank led to establishment the following year was only 11%.Conclusion: The seed bank was not the dominant source for newly establishing species following the large disturbance that was induced by restoration management. Changes in species abundance after the restoration impact were not related to species abundance in the seed bank, but to ongoing succession and current year dispersal. To attain a high number of new establishments, restoration projects should preferably be planned in the proximity of refuge populations, rather than relying on the seed bank alone.

Does the seed bank contribute to the restoration of species-rich vegetation in wet dune slacks?

Abstract Questions: What is the contribution of the seed bank to restoration of species-rich vegetation in oligotrophic wet dune slacks? Does the restoration management affect the seed bank? Location: Calcareous coastal dune slacks at the west coast of The Netherlands. Methods: Species composition of the seed bank and the above-ground vegetation was sampled in dune slacks that had a variable extent of groundwater level rise in combination with either topsoil removal or mowing. Results: The seed bank had a high potential for restoration of species-rich vegetation: 60 species were found in the seed bank of which 14 were characteristic of oligotrophic, wet dune vegetation. While topsoil removal almost completely removed the seed bank, groundwater level rise did not permanently submerge the seed bank of species of oligotrophic, wet conditions. Changes in abundance in the established vegetation were unrelated to species abundance in the seed bank. Of all new species establishments in the vegetation releves, 76...

Identifying key issues in environmental wetland research using scaling and uncertainty analysis

Quantification of the response of wetlands to environmental change and of climate change to wetland processes, is a pressing, but complex, issue. Findings of widely diverse investigations have to be generalised to identify the key issues and the gaps in knowledge. Two tools for generalisation, scaling and uncertainty analysis, were applied here to two case studies and act as stepping stones for analysis of key issues. The first case study is on methane emissions from wetland rice fields and the second is on the restoration of wet dune slacks. The type of problems encountered depends on the scale at which a process is studied. At the biogeochemical processes scale, knowledge on some key parameters, but especially on interactions between key parameters is limiting. At the vegetation scale, feedbacks between processes become especially important, whereas at even larger scales increasingly stringent approximations of interactions have to be made. Accumulating uncertainties at the landscape scale need careful evaluation. Apart from uncertainties in the approximations and their interactions, those introduced by spatial and temporal variability as well as by different data sources highly influence the accuracy of response estimates, while these last sources of uncertainty are neglected in many studies. Only by explicitly accounting for scaling effects and their resulting uncertainties, the interactions between wetlands and environment can be understood.

Changes in soil and vegetation during dune slack succession

Many rare plant species occur in Dutch wet dune slacks, particularly in the Junco baltici‐Schoenetum nigricantis. For nature management it is important to understand the processes controlling the presence of these basiphilous early successional communities, which is why we investigated vegetation and soil development during succession in coastal dune slacks. We compared 12 chronosequential stages of 0, 2, 4, 9, 10, 13, 25, 30, 43, 60, 70 and 85 yr in five different dune slack systems. In four of these locations turf had earlier been removed in order to restore the basiphilous pioneer stage.The main variation in the vegetation is related to the acidification/soil enrichment gradient and the salinity/maritime gradient. During succession, organic matter accumulates and acidification takes place. Maritime influence can buffer the soil and postpone the succession of basiphilous pioneer communities for many years. A significant correlation with age was found for 18 variables. Multiple regressions predicted changes in the vegetation (dependent variables: biomass, cover of Salix repens, Calamagrostis epigejos and Schoenus nigricans) as a function of acidification, organic matter accumulation, increase in available P and presence of Na in the soil.We conclude that natural ageing of the vegetation and the associated process of accumulation of biomass drive succession in this hydrosere. The underlying soil processes are acidification and organic matter accumulation. During succession dominance shifts from S. nigricans to S. repens or C. epigejos. Maintenance of the pioneer character of the habitat is only possible by local intervention or by natural or man‐induced dune forming. The effect of sod‐stripping depends on the environmental conditions and, in case of acidification, success is limited. Succession can be postponed by mowing.

Effects of microbial mats on germination and seedling survival of typical dune slack species in the Netherlands

The role of microbial mats in wet dune slack succession is often discussed. We tested if presence of microbial mats may retard dune slack succession by lowering the germination and seeding survival of successor species. This hypothesis was tested on a set of typical dune slack species of the Frisian Islands in two climate chamber experiments. The species were separated into early-, intermediate- and late successional species. There were large differences in germination rates between species (2% – >200% compared to the reference), but within a species the high germination rates were mostly found on sand without a microbial mat. Only the germination of Agrostis stolonifera appeared to be stimulated by the presence of a well-developed microbial mat, they were even higher than in the reference. Seedling survival also did not show different responses between successional stages. Seedlings placed on top of a microbial mat showed for most species lower survival rates compared to seedlings that were planted or placed on top of the sand. Growth was the only measured variable that differed between successional groups. Species of the early- and intermediate successional stages grew significantly better if a microbial mat was present whereas late successional species were not stimulated. Early and intermediate successional species seem to be favored by the presence of a microbial mat. An explanation for this may be that they can profit from the enhanced nitrogen availability caused by N2-fixation by cyanobacteria in the microbial mat.

An island biogeographical view of the successional pathway in wet dune slacks

Abstract. Wet dune slacks occur as small, naturally fragmented systems in the dune landscape, isolated from other slacks. We studied the effects of slack isolation and area on the rate and direction of primary succession in a chronosequence of dune slacks. The results indicate that important changes occur in community characteristics over a period of 50 yr. Total cover and number of species increase as a result of the endogenous succession process, during which organic matter and nutrients accumulate. Consequently, competitive interactions shift from competition for nutrients to competition for light. Local factors thus determine, at least partially, the community composition in the slack. However, differences in community composition with increasing age are smaller when the slack is more isolated or smaller, suggesting a slower succession rate and biomass accumulation. Together with a lower contribution of slow dispersing species in more isolated slacks, this indicates that species accumulation is dispersal limited and thus influenced by regional factors. The stochastic variation resulting from this dispersal limited species accumulation causes a divergent successional pathway.

An island biogeographical view of the successional pathway in wet dune slacks

Wet dune slacks occur as small, naturally frag- mented systems in the dune landscape, isolated from other slacks. We studied the effects of slack isolation and area on the rate and direction of primary succession in a chronosequence of dune slacks. The results indicate that important changes occur in community characteristics over a period of 50 yr. Total cover and number of species increase as a result of the endogenous succession process, during which organic matter and nutrients accumulate. Consequently, competitive interac- tions shift from competition for nutrients to competition for light. Local factors thus determine, at least partially, the com- munity composition in the slack. However, differences in community composition with increasing age are smaller when the slack is more isolated or smaller, suggesting a slower succession rate and biomass accumulation. Together with a lower contribution of slow dispersing species in more isolated slacks, this indicates that species accumulation is dispersal limited and thus influenced by regional factors. The stochastic variation resulting from this dispersal limited species accumu- lation causes a divergent successional pathway.

Restoration of Wet Dune Slacks on the Dutch Wadden Sea Islands: Recolonization After Large‐Scale Sod Cutting

AbstractThe effects of sod cutting were studied in a dune area on the Dutch Wadden Sea Island of Texel. Sod cutting was carried out in a range of different dune slacks in order to restore dune slack vegetation with many endangered Red List species. Sod cutting removed approximately 96% of the soil seed bank. Species abundant in the seed bank, notably Juncacea, also had a high frequency in the vegetation that established during the first year after the restoration measures. Many other species not registered in the seed bank or in the former vegetation also appeared. Species richness in the monitored plots exceeded that of uncut reference plots after a few years. Colonization rates were higher than extinction rates in most plots, indicating that a stable state has not been reached after 5 years. Differences in species richness between slacks appeared to be related to the occurrence of source areas nearby and availability of dispersal agents, such as flooding and animals.

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.

The effect of water supply on seed‐bank analysis using the seedling‐emergence method

1. When using the seedling‐emergence method to analyse a soil seed bank the greenhouse conditions should match the germination requirements of the species involved. Although the seedling‐emergence method is common practice in ecological studies, the germination characteristics of many species are not known, or are only partly known. 2. Before carrying out a large‐scale seed‐bank study in a wet dune slack, we tested the water requirements of the species in the seed bank, comparing a waterlogged soil with a moist soil. Four species germinated in significantly larger numbers in a waterlogged soil, seven species in a moist soil and 11 species showed no significant difference. When a species was present in low densities, it was often missed using the less appropriate treatment. 3. These results emphasize the need for a preliminary study before carrying out a seed‐bank analysis and show the danger of using a ‘standard’ method to analyse soil samples from different habitats.

Pollination ecology of Gentianella uliginosa, a rare annual of the Dutch coastal dunes

The pollination ecology ofGentianella uliginosa, a rare annual in The Netherlands, was studied in three relatively small coastal dune populations. In all populations, the species was fully self‐compatible: manual self pollination resulted in a seed set of 92–94%. Autofertility was also high (seed set of caged, unhandled flowers 80–85%). The homogamous flowers were visited byBombus pascuorum, which collected nectar and transported pollen sternotribically. Differences among populations in bumblebee visitation rate did not affect natural seed set. In one plot, seed set following natural pollination was lower than after manual selñng, and as high as after spontaneous selñng. As flowers here were also visited frequently, this might be due to pollen limitation through heterospecific pollen competition. Support for this hypothesis was derived from another population, where seed set after natural pollination was lower in a plot dominated by co‐floweringMentha aquaticathan in a plot with fewMentha.However, differences in water availability might also have caused this. In one population, seed sets of the cross pollination and spontaneous selfing treatments were lower than those of the manual selñng and natural pollination treatments. Whether the low outcrossing success can be attributed to small differences in the treatment of the experimental plants or to outcrossing depression remains unclear. We conclude that this annual species is most likely a predominant selfer. Its high autofertility is probably an adaptation to its wet dune slack habitat, where reproductive assurance may be important to cope with unpredictable fluctuations in water table, summer droughts and pollinator service.

The calcium demand of the calcicole sedge Schoenus nigricans

Summary Growth and reproduction of the perennial sedge Schoenus nigricans from calcareous wet dune-slacks was investigated in response to increasing proportion of calcium carbonate in dune sand to test the hypothesis that a high calcium supply is necessary to ensure a full life cycle of this calcicole species and counteract the population decrease in wet dune slacks. Plant growth and nutlet quality, but nor reproduction per se were significantly lower in plants grown for 30 months at 0.03% Ca compared to all other concentrations from 0.25 to 2% Ca. Plants at 0.03% Ca had the lowest nutlet mass. With increasing Ca supply, the Ca concentration in the roots increased more than that in leaves and shoots, where Ca was associated with the fractions being soluble in water and acetic acid. Ca-oxalate was not present. The insoluble Ca fraction was not affected by the external Ca supply, but was only related to the age of the tissue. A quotient of water-soluble calcium/water-soluble total cations of 0.15 seems to be sufficient for a good performance of S. nigricans in soils with 0.25% Ca or more, thus indicating a relatively low calcium demand of this calcicole species which has a high degree of Ca-tolerance.

Nutrient deficiency in dune slack pioneer vegetation: a review

A review of results of fertilization experiments in wet dune slacks is presented. In most cases the above-ground biomass appeared to be limited by nitrogen availability. Pri- mary phosphorus limitation was assessed only once in a dune slack where sod cutting had been applied very recently. In most other case studies phosphorus limits biomass production after nitrogen deficiency was lifted. Potassium availability is of minor importance for biomass production in this type of ecosystem. Singular nitrogen additions led to increased domi- nance of Carex and Juncus species as well as perennial grasses, such as Agrostis stolonifera and Calamagrostis epigejos. A combined addition of nitrogen and phosphorus led to total dominance of grasses, while the characteristic basiphilous pioneer species (including mosses) decreased or even disap- peared. Certain mechanisms are considered which may main- tain nutrient availability in slacks with basiphilous pioneer vegetation at a low level, despite of the accumulation of nutrients in the developing organic soil layer. Some implica- tions for management and further research are discussed.

Pedogenesis in coastal wet dune slacks after sod-cutting in relation to revegetation

The development of vegetation and soil was investigated in wet coastal dune slacks in North Holland, The Netherlands (52°36'N, 4°37'E). Sod cutting in the past has created a time series from 1 to 30 years, with an even older undisturbed site as reference. After sod-cutting Schoenus nigricans, a typical pioneer species of wet dune slacks, established and contributed together with Calamagrostis epigejos to more than 85% of the aboveground biomass. At the control site the biomass was 9.5 t dry matter ha-1.

Silicon in Developing Nuts of the Sedge Schoenus nigricans

Summary The accumulation of silicon in the nutlets of Schoenus nigricans , a sedge from wet dune slacks, was studied during nutlet development from ovule fertilization up to nutlet ripeness in plants from a submediterranean (Eastern Italy) and a temperate (North Holland) population at their natural sites. There was a rapid increase of the Si concentration between 4 and 10 weeks after fertilization of the ovules at the first position, but a delayed increase at higher spikelet position. Whereas the Si content of the surrounding tissues, i.e. shoot, bracts, and spikelet axis, remained below 3%, Si concentration in the pericarp of the nutlets at the first and second spikelet positions increased during nutlet development up to 34% Si. Nutlets at higher spikelet positions, having a shorter development time, contained significantly less silicon. Differences in the climate did not affect the Si concentration. Plants grown in desiccated dune slacks had a low silicification of the nutlets. Within the pericarp there was a strong Si gradient from inward to outward, accompanied by a similar trend of Ca, K and Cl, indicating a passive transport with the transpiration stream; the rapid increase of the Si concentration during pericarp development, however, made it necessary that a preferential and thus active Si transport to the pericarp was involved. The chemical Si-form in the pericarp was mostly biogenic opal, but correlation with the hydrogen concentration did not exclude the formation of Al silicates. The ecological consequences of pericarp silicification are shortly discussed.

Fruit development and performance ofSchoenus nigricansin coastal dune slacks of Europe: an extension of H. G. Baker's seed mass-altitude to a seed mass-latitude relationship

SUMMARY The development fruits (nutlets) and seeds of Schoenus nigricans, a perennial sedge of wet coastal dune slacks, was compared between populations of the Mediterranean Sea and the North Sea, to examine the hypothesis of adaptation in a geographical gradient (Baker 1972). The development period of nutlets and seeds lasted several months. The serial insertion of nutlets on the spikelet caused a negative mass gradient from adaxial to abaxial nutlets. Final nutlet mass (first adaxial spikelet position) of Mediterranean plants was 30% higher than that of North Sea plants, extending Baker's hypothesis of the inverse relationship between seed (nutlet) mass and geographical altitude to geographical latitude. The difference in final nutlet mass between plants from populations of the Mediterranean and North Sea persisted, when plants were grown under the climatic conditions of the North Sea plants. However, the nutlet mass, but not the seed mass of the Mediterranean plants, grown under Dutch climatic conditions, was less than at their original size. Start of flowering of the Mediterranean plants under Dutch climatic conditions was delayed by 1–2 months, compared with their flowering at the Mediterranean sites. Nutlets of Mediterranean plants ripened later than those of Dutch plants under the Dutch climatic conditions. The differentiation in nutlet and seed masses and plant height in the geographic gradient in Europe and its persistence under experimental conditions indicates a strong genetic component of population differentiation. The data do not support the hypothesis of ‘general-purpose genotypes’ (Schmid 1992) in this wet dune slack species.