May 2017

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English summaries

    • Fall and rise of the Fochteloërveen: results of 30 years of conservation efforts
    • W. Altenburg, W. Bijkerk, R. Douwes & N. Straathof
    • The Fochteloërveen is the remnant of a once large bog area in the northern part of The Netherlands. About 1400 ha of this Nature 2000 reserve consists of a mostly heavily degraded bog vegetation. The owner of the Fochteloërveen has taken measures by building a number of dams in the bog in 1984/85 and 1999-2001, in order to raise the average water level and to restore at least a part of the bog vegetation. As a result of these conservation measures the groundwater level in the bog as a whole has risen considerably. In the central part, with a remaining peat layer of > 1 m, groundwater has stabilized with fluctuations < 15 cm below surface level. Outside these central parts, where the remaining peat layer is < 1 m, groundwater fluctuations are larger, up to 50 cm below surface level.
      From the first rewetting measures in 1984/85 a number of characteristic bog species – especially Eriophorum vaginatum, Oxycoccus palustris and Andromeda polifolia –
      and several species of peat mosses have shown a strong increase. Until now the Fochteloërveen hosts 20 species of peat mosses, among which a number of less common and rare ones (e.g. Sphagnum affine, S. fuscum). S. cuspidatum has become by far the most abundant species, while also the hummock forming species S. papillosum and S. magellanicum have increased considerably. Molinia caerulea, the species that has dominated the Fochteloërveen during a large part of the 20th century, has shown a strong decrease after the start of the rewetting measures. Nevertheless this species is still abundant in the bog, and recently seems to show a slight recovery outside the central parts of the reserve. Surpisingly, minerotrophic peat mosses (especially S. fimbriatum, but also S. palustre and S. subnitens) have become abundant from 2002 on, especially in the central parts of the Fochteloërveen. The reason of this unexpected increase is unknown and needs further monitoring.
      The development of the bog vegetation as a result of the conservation measures is promising. Nevertheless there are worries about Molinia caerulea, the recent strong increase of minerotrophic peat mosses and the possible constraints of the high atmospheric nitrogen deposition. The owner of the bog should do the utmost to ensure optimal water levels, including crucial maintenance of the dams.

    • Beerzerveld: implications of landscape history for restoration of the bogs
    • J. Sevink, A.A.M. Kieskamp, N.W. Willemse, A.T.W. Eysink & A.J.M. Jansen
    • The Beerzerveld (part of Natura 2000-area ‘Vecht en Beneden-Regge’), located in a cover sand landscape to the South of the Vecht valley, consists of a mosaic of small raised bogs, classified as habitat type 7120 and drier habitats. Bogs originally developed through paludification, starting with the formation of a stagnant podzol and associated greasy humus layer (‘gliede’) in low-lying parts of the area. This paludification proceeded over time, resulting in raised bog habitats with a perched groundwater table. Radiocarbon datings showed that from the first centuries AD on, wind erosion of dry, fragile soils in cover sands led to widespread deposition of drift sand, covering the bogs. The original landscape was transformed into relatively high areas with dry to moist habitats and with peat deeply buried under drift sand, and often low-lying areas with truncated ‘dry podzols’. In a later period, lasting until the mid-20th century, local farmers dug pits to extract the peat from underneath the sand. They transformed the area into a man-made mosaic of pits, dumps and non-disturbed remains of the former sand-covered bogs, with additional dry habitats.
      After cessation of peat extraction, these pits, which still had a functioning ‘peat basis’, filled with water and peat growth started again. In earlier research, current bog formation was thought to be hampered by former lowering of the groundwater level, causing the peat basis to degrade and inducing increased vertical drainage from the bogs and their concurrent desiccation. However, we did not find evidence for such degradation and desiccation in monitoring data for the perched water table, since these showed that it has been stable over the last three decennia. Nor was evidence found in monitoring data and the reconstructed SHWT (Seasonal High Water Table) in soil profiles from dry sites for a significant lowering of the current groundwater level relative to that SHWT. Our research showed that for optimal bog formation at landscape scale, raising the ground water level is not effective. We strongly advocate to reduce the draining effect of ditches and lakes on the perched water table, as well as the increased evapotranspiration resulting from afforestation, being far more effective ‘internal’ measures than raising the groundwater level through the creation of ‘external’ hydrological buffer zones.

    • Food forest as diverse as a natural forest
    • J. Breidenbach, E. Dijkgraaf, B. Rooduijn, S.E. Nijpels-Cieremans & A.M. Strijkstra
    • Public concern for the ongoing loss of biodiversity in intensive agriculture is growing, and with that the interest in sustainable agricultural systems, such as food forests. A food forest produces food sustainably and is assumed to support local biodiversity. To find out if food forests can support local biodiversity, we investigated species richness and species composition of breeding birds, macro-moths and carabid beetles between April-July 2016 in the Dutch food forest Ketelbroek, and compared the data to an area with similar structure in the nearby Natura2000 forest De Bruuk. Main results show that the number of species is remarkably similar (breeding birds: 22 vs. 23 species; macro-moths: 169 vs. 168 species; carabid beetles: 35 vs. 27 species) in Ketelbroek vs. De Bruuk. Similarity of occurring species, however, was relatively low (breeding birds: 36.4%; macro-moths: 39.6%; carabid beetles: 8.8%), but that could well be ascribed to the current habitat characteristics and succession states of the investigated areas. The results suggest that for breeding birds, macro-moths and ground beetles Ketelbroek can support local species diversity. Thus, food forests appear valid as a nature inclusive agricultural system.

    • Birds of reedmarshes in the dynamic winterbed of the river
    • J.A. Sluiter
    • Since 1990s large-scale floodplain rehabilitation has been carried out in the Rhine-Meuse Delta. It was expected that the improvement of the ecological functioning of de riverine ecosystems would lead to large reed marshes in the winterbed and to an augmentation of the number of breeding birds f.i. great reed warbler and the eurasian bittern.
      Due to the hydrodynamics and pioneer circumstances after the floodplain rehabilitation no significant reed marshes developed. Special measures have to be taken for the development of sustainable reed marshes in the winterbed of the river. If water levels are controlled, periodic dryfall is possible and the reed can be mowed in winter, birds of reed marshes react instantaneously.

    • Conserving wild bees in agricultural landscapes: is sowing wildflowers the solution?
    • D. Kleijn, T.P.M. Fijen, I. Raemakers & J. Scheper
    • The decline of wild bees is generally considered to be caused by habitat destruction, exposure to pesticides, pests and diseases and lack of floral resources. Declining availability of host plants is the only factor that has been convincingly related to bee population trends, however, it is most likely the combination of these factors that hurts bees most. Measures to mitigate wild bee decline often aim to enhance flower availability, for example, by establishing wild flower strips along arable fields. A wealth of studies demonstrate that this enhances the densities of wild bees compared to control situations without wildflower additions. Recent studies are furthermore showing that such measures may also enhance bee densities in the surrounding landscape and on neighbouring crops which may in turn positively affect crop yield. Pollinator mitigation measures that enhance flower availability primarily benefit common species and threatened species that are listed in Red Data books are rarely observed in wildflower strips on farmland. This may be due largely because on farmland these species are constrained more by lack of suitable nesting sites than by food availability. Measures that aim to enhance both common and threatened species therefore need to consider nest site availability in their design. This can be done by targeting wildflower additions to areas near known nesting locations of threatened species. Alternatively, enhancing nest site availability can form an integral part of the mitigation measures, for example by creating patches of bare ground which are preferred by the largest number of wild solitary species.