De Levende Natuur nummer 4 van 2013 (English summary)

Re-creation of nature on former arable lands: a targeted search for chances!

M. van Mullekom, E.C.H.E.T. Lucassen, M.J. Weijters, H.B.M. Tomassen, R. Bobbink & A.J.P. Smolders

The transformation of ex-agricultural lands into low productive biodiverse dry and wet semi-natural grasslands and heathlands in The Netherlands only turned to be successful in case phosphorus (P)-limited conditions can be achieved in the soil top-layer. Due to many years of heavy fertilization, the (deeper) soil has often become strongly enriched with nutrients. Without reduction of excessive phosphorus, dominant monotonous growth of Holcus lanatus (dry soil) and Juncus effusus (moist to wet soil) mostly occurs on abandoned agricultural lands. By mowing or mining, in combination with biomass removal, a reduction in the total P concentration of the topsoil can mostly only be realized on the longer term. In contrast, top soil removal turned to be a very effective measure to achieve nutrient poor conditions in the soil top-layer on the short term. The Olsen-P concentration, in combination with the total-P concentration in the soil, is a strong indicator for the potential growth of J. effusus and the species-richness that will develop on abandoned agricultural lands. Therefore analysis of these parameters in depth gradients can give a quick and accurate indication of the depth at which P availability is sufficiently low and the time it takes to reach sufficiently low P concentrations through mowing or mining (with or without partial soil removal). In addition, analyses of the total calcium concentration in the soil give accurate knowledge on the type of nutrient poor nature that can develop under dry to wet conditions. Field experiments showed that improving the soil property by only lowering the phosphorus availability, does not always lead to success. Additional information on the hydrology and groundwater quality is required. Also additional measures are often needed, including restoration of the hydrology (removal of former drainage ditches), lime application to undo soil acidification, and re-introduction of plant species (in the absence of a vital seed bank and/or donor site in the neighbourhood).

Water quality in Dutch peat meadow areas

A.J.P. Smolders, J. van Diggelen, J. Loermans, G. van Dijk, M. van Mullekom & L.P.M. Lamers

Over the last 1000 years anthropogenic activities have completely changed the landscape of the western lowland parts of the Netherlands. Once dominated by peat marshes, the landscape is nowadays dominated by the typically Dutch peat meadows. Due to former inundation by the sea, these peatlands are rich in reduced sulphur, although most of the brackish characteristics have been lost due to the isolation of the former Zuiderzee from seawater entrance.

Oxidation of the peat layer, owing to drainage, results in strong sulphate mobilization because of the oxidation of reduced sulphur compounds.  Sulphate may provoke the mobilization of phosphate from peat sediments indirectly leading to eutrophication of the surface waters. The oxidized top layers of the peat soils are rich in iron, and also show high phosphorus concentrations  due to the immobilization by oxidized iron. During high water levels, however, iron becomes reduced and phosphorus becomes mobilized at the same time, leading to eutrophication of the surface waters.

In this article, several measures that may allow an improvement of the water quality, amongst which restoring the brackish nature of the systems, are discussed. In general, however, many of these measures do not tackle the basic problem, which is the ongoing subsidence of the land due to drainage and the resulting peat oxidation. Eventually the peat meadow landscape is bound to vanish if radical changes are evaded.  The restoration of peat forming marsh vegetation is a sustainable solution that will not only bring an end to land subsidence, but will also create valuable nature and strongly improve surface water quality. High concentrations of phosphorus in peats, however, will hamper the development of peat forming vegetation. In order to create phosphorus limited conditions,  stripping of mobilized phosphorus after inundation by growing Azolla filiculoides might provide an innovative alternative to top-soil removal.

The impacts of increased atmospheric N deposition: are restoration measures still necessary?

R. Bobbink, L.J.L. van den Berg, H.B.M. Tomassen & M.J. Weijters

Atmospheric nitrogen deposition, from both oxidised (NOy) and reduced (NHx) compounds, is nowadays one of the main threats for biodiversity in European (semi-) natural ecosystems of high conservational value. Long-term nitrogen input from the atmosphere may cause eutrophication, soil acidification and/or ammonium toxicity. The severity of these impacts depends on the biogeochemistry of the particular ecosystem, but is especially severe under oligo- to mesotrophic, weakly buffered soil conditions.

Long-term field trials have been set up in deteriorated ecosystems such as grassland and heathland sites since the early 1990s to counteract the severe impacts of N pollutants. The first aim was to restore former soil conditions, as we feel that rehabilitation of ecosystems should start with recreating appropriate abiotic conditions. Removal of the vegetation and top soil (‘sod cutting’), liming, hydrological measures or a combination of them were used depending on the actual biogeochemical constraints after the degradation. The effectiveness was mostly evaluated by following the soil chemistry and plant composition. In this paper an overview of the experimental restoration measures and the main factors of success or failure have been presented. In most cases a combination of measures proved to be successful in restoring appropriate soil conditions and a low productive sward. A full recovery of plant diversity was, however, seriously limited when the characteristic species had already disappeared, especially in dry conditions or when it was impossible to increase the soil buffer capacity after acidification. Additional measures to counteract the dispersal limitation of many endangered species may be needed. Finally, it is concluded that a decrease in atmospheric N deposition is still needed in several areas of the Netherlands to protect sensitive ecosystems.

Biogeochemistry as a tool to maintain variation in moorland pools

E. Brouwer & E.C.H.E.T. Lucassen

The biodiversity of atlantic moorland pools and shallow softwater lakes has become seriously threatened due to acidification, eutrophication and drainage. In the past thirty years, a set of restoration measures has been developed and applied in these lakes. Also, new insight was gained in essential processes such as carbon limitation and internal eutrophication. Thus we were able to restore species rich, early phases in the development of these lakes. The focus now shifts towards the recovery of late successional stages and the restoration of the internal variation within lakes. In this article, several measures are suggested: 1) removal of nitrogen and immobilisation of phosporus by short periods of drying or by stimulating a vegetation of isoetid macrophytes. 2) Reinforcing internal gradients in carbon availability by selective removal of organic sediments and/or high vegetation on the shores. 3) Reinforcing internal gradients in pH, nutrients and carbon by restoring groundwater quantity and quality, which may include restoration measures in the catchment area or adaptations in the water level.

Waters and wetlands in a changing climate: linking management to science

L.P.M. Lamers, M. Poelen, L.J.L. van den Berg, J.J.M. Geurts, J.G.M. Roelofs & A.J.P. Smolders

Although water quality has improved in many areas of the Netherlands, the ecological quality is still poor in general, mainly due to eutrophication as a result of intensive agricultural activities. Although the restoration of water clarity is the first focus in order to enable the growth of submerged vegetation, this may still lead to low biodiversity. One reason for this is the phosphate legacy of underwater sediments, that facilitates massive growth of fast growing, highly competitive species including Ceratophyllum demersum and Elodea nutallii, but also of invasive species such as Cabomba caroliniana.

Applied research including experimental approaches in the field and laboratory not only generates knowledge about key factors and processes in wetland functioning and deterioration, but also provides indispensable information to design diagnostic and prognostic tools, and for decision support in wetland management.
As a result of global climate change, anaerobic episodes will occur more frequently, leading to higher risks of internal phosphate loading and concomitant algal or cyanobacterial blooms. This will provide bigger challenges to water management and nature management, and require additional measures. Although it is obvious that wetlands play an important role in climate proof water management, care should be taken to avoid large-scale eutrophication as a result of inverse water quality or soil quality. At the same time, novel directions such as phosphorus recycling, carbon sequestration and water retention should be included in future wetland management.

In order to be able to define successful management and restoration programs at the landscape scale, close collaboration among nature managers, water managers and wetland scientists is vital, especially in a changing climate. With respect to wetland policy, all benefits of wetland ecosystem services should be included in cost-benefit calculations.

Species introduction by hay application after restoration: impatience or wisdom?

R. Loeb & M. Weijters

A panel discussion at the 10th anniversary symposium of B-WARE Research Centre raised the question whether species introduction by hay application is always necessary after restoration. The lack of seed sources is one of the main problems for restoration after the restoration of abiotic conditions. Ethical questions concerning different forms of species introduction have however led to discussion among nature management organisations and public. Agricultural practices before restoration have generally led to the absence of a seed bank and dispersal from outside the area is often hampered by the distance to other populations. Moreover, soil biota might as well play an important role in vegetation development, so soil inoculation might also be necessary. It is already known that two-third of the Dutch Red List plant species did not profit from restoration programs, dispersal problems being one of the main causes. Proper decision making needs to be fed by more facts about the chances of successful restoration with or without introduction of species. Also more scientific knowledge is needed to predict the dispersal success probabilities of separate species based on results achieved in recent restoration programs in order to improve restoration success in future.