Control of plant growth by nitrogen and phosphorus in mesotrophic fens

A fertilization experiment was carried out in 3 mesotrophic fens to investigate whether plant growth in these systems is controlled by the availability of N, P or K. The fens are located in an area with high N inputs from precipitation. They are annually mown in the summer to prevent succession to woodland. Above-ground plant biomass increased significantly upon N fertilization in the two mid-succession fens studied. In the late-succession fen that had been mown for at least 60 years, however, plant biomass increased significantly upon P fertilization. The mowing regime depletes the P pool in the soil, while it keeps N inputs and outputs in balance. A long-term shift occurs from limitation of plant production by N toward limitation by P. Hence, mowing is a suitable management tool to conserve the mesothrophic character of the fens.     

Botanical diversity in British mires

Although mires are sometimes regarded as a single, specific and even esoteric habitat-type, in their full range of development-calcareous fens to acidic bogs, lowland swamps to upland flushes, woodlands, sedgelands and mosslands-they occupy a wide range of environmental conditions and sustain a rich botanical resource. Here an outline is given of the types of mires that are widespread in Britain and their botanical resource. It is discussed with reference to some of the environmental variables and management conditions that have been shown to influence the species composition and diversity of their vegetation. Some implications that this has for the classification of mires are outlined.     

A new method for restoring ditches in peatlands: ditch filling with fiber bales

Ditching is a common practice to dewater wetlands, including peatlands, and ditch blocking is a common method for restoring wetlands because substrate is often unavailable for filling the ditches. However, filling has many advantages compared to blocking ditches. Our goal was to test whether ditches could be filled in a Colorado sloping fen (Chattanooga Fen) using bales created from shredded aspen (Populus tremuloides) tree‐fiber. We monitored water table levels before and after we filled two ditches (combined length of approximately 165 m × 3 m wide) as well as an undisturbed reference portion of Chattanooga fen. The reference site had stable water tables that rarely dropped more than 20 cm below the soil surface. The ditches had been dewatering large areas of the fen for at least 100 years. Filling the ditches with fiber bales resulted in a water table increase between 2 and 22 cm in an area up to 150 m below the ditch. Native sedges now cover the area where we filled the ditches, with no erosion or compression/settling of the ground observed and no water backing up behind the filled ditches. Filling the ditches with shredded fiber bales is a good option for restoration in wilderness areas, or areas lacking peat or mineral soil fill because it is a natural material that is easily transported and placed in the ditches.     

Effects of the loss of clonal integration on four sedges that differ in ramet aggregation

Although clonal growth is a dominant mode of plant growth in wetlands, the importance of clonal integration, resource sharing among ramets, to individual ramet generations (mother and daughter) and entire clones of coexisting species has not been well investigated. This study evaluated the significance of clonal integration in four sedge species of varying ramet aggregations, from clump-forming species (Clumpers –Carex sterilis, Eleocharis rostellata), with tightly aggregated ramets (rhizomes<1cm), to runner species (Runners –Schoenoplectus acutus, Cladium mariscoides), with loosely aggregated ramets. We manipulated clonal integration by either severing connections between target mother and daughter ramets or leaving connections intact, and then planted them in an intact neighborhood of a fen in Michigan, USA. We measured growth parameters of original and newly produced ramets over two growing seasons and conducted a final biomass harvest, to address four hypotheses. First, we expected integrated clones to accumulate more biomass than severed clones. However, final clone-level biomass and ramet production were the same for both treatments in all species although severing initially stimulated ramet production by Schoenoplectus and produced a more compact ramet aggregation in Cladium. Second, we hypothesized that mother ramets would experience a cost of integration, through reduced ramet or biomass production, while daughters would experience a benefit, through increased resource availability from mothers. Mother ramets of Cladium suffered a cost from integration, while Schoenoplectus mothers suffered a slight cost and Carex daughters saw a slight benefit. Finally, we hypothesized that integration would be more active in runner species than in clumper species. Indeed, we documented more active integration in runners than clumpers, but none of the study species were dependent upon integration for growth or survival once daughter ramets were established with their own roots and shoots. This study demonstrates that integration between established ramets may not be the most important advantage to clonal growth in this wetland field site. The loss of integration elicited varied responses among coexisting species in their natural habitat, somewhat but not completely related to their growth form, suggesting that a combination of plant life history traits contributes to the dependence upon clonal integration among established ramets of clonal species.     

Recent changes in the plant composition of wetlands in the Jura Mountains

Aim

To assess vegetation changes in montane fens and wet meadows and their causes over 38 years.

Location

Wetlands, Jura Mountains (Switzerland and France).

Methods

Plots were inventoried in 1974 and re‐located in 2012 (quasi‐permanent plots) on the basis of sketches to assess changes in plant communities. The 110 plots belonged to five phytosociological alliances, two in oligotrophic fens (Caricion davallianae, Caricion fuscae) and three in wet meadows (Calthion, Molinion, Filipendulion). Changes between surveys were assessed with NMDS, and changes in species richness, Simpson diversity, species cover and frequency and the causes of these changes were evaluated by comparing ecological indicator values.

Results

Changes in species composition varied between alliances, with a general trend towards more nutrient‐rich flora with less light at ground level. Species diversity declined, with a marked decreasing trend for the typical species of each alliance. These species were partly replaced by species belonging to nitrophilous and mesophilous grasslands. However, no trend towards drier conditions was detected in these wetlands. The largest changes, with an important colonization by nitrophilous species, occurred in the Swiss sites, where grazing was banned 25 years ago. As a result of floral shifts, many plots previously belonging to fens or wet mesotrophic meadows shifted to an alliance of the wet meadows, generally Filipendulion. Moreover, communities showed a slight trend towards more thermophilous flora.

Conclusions

The investigated wetlands in the Jura Mountains have suffered mainly from eutrophication due to land‐use abandonment and N deposition, with a loss of typical species. Areas with constant land use (grazing or mowing) showed less marked changes in species composition. The most important action to conserve these wetlands is to maintain or reintroduce the traditional practices of extensive mowing and livestock grazing in the wetlands, especially in areas where they were abandoned 25 years ago. This previous land‐use change was intended to improve fen conservation, but it was obviously the wrong measure for conservation purposes.     

The biogeochemistry of nitrogen in freshwater wetlands

The biogeochemistry of N in freshwater wetlands is complicated by vegetation characteristics that range from annual herbs to perennial woodlands; by hydrologic characteristics that range from closed, precipitation-driven to tidal, riverine wetlands; and by the diversity of the nitrogen cycle itself. It is clear that sediments are the single largest pool of nitrogen in wetland ecosystems (100's to 1000's g N m^-2) followed in rough order-of-magnitude decreases by plants and available inorganic nitrogen. Precipitation inputs (< 1–2 g N m^-2 yr^-1) are well known but other atmospheric inputs, e.g. dry deposition, are essentially unknown and could be as large or larger than wet deposition. Nitrogen fixation (acetylene reduction) is an important supplementary input in some wetlands (< < 1–3 g N m^-2 yr^-1) but is probably limited by the excess of fixed nitrogen usually present in wetland sediments.Plant uptake normally ranges from a few g N m^-2 yr^-1 to 35 g N m^-2 yr^-1 with extreme values of up to 100g N m^-2 yr^-1 Results of translocation experiments done to date may be misleading and may call for a reassessment of the magnitude of both plant uptake and leaching rates. Interactions between plant litter and decomposer microorganisms tend, over the short-term, to conserve nitrogen within the system in immobile forms. Later, decomposers release this nitrogen in forms and at rates that plants can efficiently reassimilate.The NO₃ formed by nitrification (< 0.1 to 10 g N m^-2 yr^-1 has several fates which may tend to either conserve nitrogen (uptake and dissimilatory reduction to ammonium) or lead to its loss (denitrification). Both nitrification and denitrification operate at rates far below their potential and under proper conditions (e.g. draining or fluctuating water levels) may accelerate. However, virtually all estimates of denitrification rates in freshwater wetlands are based on measurements of potential denitrification, not actual denitrification and, as a consequence, the importance of denitrification in these ecosystems may have been greatly over estimated.In general, larger amounts of nitrogen cycle within freshwater wetlands than flow in or out. Except for closed, ombrotrophic systems this might seem an unusual characteristic for ecosystems that are dominated by the flux of water, however, two factors limit the opportunity for N loss. At any given time the fraction of nitrogen in wetlands that could be lost by hydrologic export is probably a small fraction of the potentially mineralizable nitrogen and is certainly a negligible fraction of the total nitrogen in the system. Second, in some cases freshwater wetlands may be hydrologically isolated so that the bulk of upland water flow may pass under (in the case of floating mats) or by (in the case of riparian systems) the biotically active components of the wetland. This may explain the rather limited range of N loading rates real wetlands can accept in comparison to, for example, percolation columns or engineered marshes.     

Wetland dicots and monocots differ in colonization by arbuscular mycorrhizal fungi and dark septate endophytes

As an initial step towards evaluating whether mycorrhizas influence composition and diversity in calcareous fen plant communities, we surveyed root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate endophytic fungi (DSE) in 67 plant species in three different fens in central New York State (USA). We found colonization by AMF and DSE in most plant species at all three sites, with the type and extent of colonization differing between monocots and dicots. On average, AMF colonization was higher in dicots (58±3%, mean±SE) than in monocots (13±4%) but DSE colonization followed the opposite trend (24±3% in monocots and 9±1% in dicots). In sedges and cattails, two monocot families that are often abundant in fens and other wetlands, AMF colonization was usually very low (<10%) in five species and completely absent in seven others. However, DSE colonization in these species was frequently observed. Responses of wetland plants to AMF and DSE are poorly understood, but in the fen communities surveyed, dicots appear to be in a better position to respond to AMF than many of these more abundant monocots (e.g., sedges and cattails). In contrast, these monocots may be more likely to respond to DSE. Future work directed towards understanding the response of these wetland plants to AMF and DSE should provide insight into the roles these fungal symbionts play in influencing diversity in fen plant communities.     

Principal factors controlling the species richness of European fens differ between habitat specialists and matrix‐derived species

Aim

We present the first continental‐scale study of factors controlling the species richness of groundwater‐fed fens, comparing land snails, vascular plants and bryophytes. We separately analyse two ecologically distinct groups differing in conservation value and colonization/extinction dynamics, that is habitat specialists, and matrix‐derived species. Considering the island‐like nature of fen habitats, we hypothesize larger differences in the species richness–environment relationships between habitat specialists and matrix‐derived species than among the taxonomic entities.

Location

Seven European regions

Methods

Richness was counted at 373 well‐preserved fens with undisturbed hydrology using the same protocols. Relationships between the species richness and water pH, waterlogging, climate and geography were explored by GLMs.

Results

Land snail richness responded mainly to water pH, regardless of habitat specialization. Richness of vascular plant and bryophyte specialists was strongly driven by geographical location of the sites, while that of matrix‐derived species was driven by waterlogging and water pH. The richness of matrix‐derived species of all taxa significantly increased with the decreasing waterlogging. Residual richness of specialists of all taxa decreased towards southern Europe.

Main conclusions

In island‐like terrestrial habitats, differences between specialists and matrix‐derived species may outweigh differences among taxa, unless there is one strong physiological determinant of species richness such as pH in land snails. The richness of specialists seems to be strongly related to difficult‐to‐measure regional factors such as historical frequency and connectivity of fen habitats. The richness of matrix‐derived species depends mainly on local conditions, such as pH and waterlogging, determining the degree of habitat contrast against the surrounding matrix. Sufficient waterlogging maintains a high representation of habitat specialists in fen communities, and disturbance of water regime may cause the increase in the number of matrix‐derived species and potentially trigger successional shifts towards non‐fen communities.