July 2008

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English summaries

    • Possible mechanism for spontaneous establishment of Calluna vulgaris in a recently abandoned agricultural field
    • Wal, A. van der & W. de Boer
    • The conversion from arable land into Calluna vulgaris-dominated heathland is difficult. The growth conditions, e.g. nutrient availability, on ex-arable sandy soils differ markedly from those of heathland and will favour fast-growing plant species. Succession towards Calluna-dominated heathland is expected to take decades, unless intensive restoration management is applied.Here we report a possible mechanism to explain the occurrence of Calluna patches in an agricultural field abandoned in 1994 within a dominant vegetation of grasses and forbs. All roots sampled from the Calluna patches were colonized by ericoid mycorrhizal (ERM) - and other endomycorrhizal fungi. Nitrogen mineralization of soil organic N was much lower in soil under Calluna patches than in the non-Calluna patches, although other soil characteristics did not differ. The N:P ratio in Calluna shoots was much higher than that in shoots of grasses and forbs, indicating that the latter were more N limited. These results indicate that the association with ERM fungi is probably providing the host competitive superiority for nitrogen, even in a soil with low organic matter content.Our results suggest that the conversion from arable land into heathland may be accomplished by the immediate establishment of Calluna seedlings and ericoid mycorrhizal inoculum when agricultural activities cease. This needs, however, to be tested in controlled experiments.
    • Pine martens in fenlands
    • Messemaker, R. & H.J.W. Wijsman
    • Historically fenlands formed not a typical habitat for Pine martens (Martes martes) in The Netherlands. Over the last years alder forest increased in age and Pine martens are invading the lowlands and are increasing in numbers. They have nests in alder trees but also in the nests of Buzzard and Goshawk, predators which also invaded these new forests.
    • Studies on avian reproduction are essential for nature management and conservation
    • Turnhout, C.A.M., H. Schekkerman, B.J. Ens & K. Koffijberg
    • In this paper we present an overview of the benefits of information on breeding biology of birds for nature policy and management issues. We distinguish three main purposes: (1) evaluation of effects of habitat management, (2) identify possible causes of population changes and (3) early warning for population declines in long-lived species. We argue that management actions can be more directly evaluated by assessing reproductive success rather than by using counts of population numbers. Counts only present trends in numbers, but do not give any information on the underlying biological processes. Hence, it is difficult to design effective conservation measures. By simultaneously monitoring population numbers and demographic rates it is possible to identify the stage of the life-cycle affected by environmental change, indicate likely causes of population change and distinguish between anthropogenic changes and natural population fluctuations. We advocate a more evidence-based management and conservation strategy, combining (1) a long-term national integrated population monitoring that focuses on a large set of species and (2) in-depth research of management actions on a shorter term, carried out by qualified volunteers and professionals respectively.

    • Restoration of softwater lakes on abandoned agricultural lands in ‘de Valkenberg’ (The Netherlands)
    • Lucassen, E., A. Smolders, R. Gerats, E. Brouwer, P. van den Munckhof & J. Roelofs
    • In 1999, three softwater lakes situated on abandoned agricultural lands in the nature reserve ‘de Valkenberg’ were restored by filling up drainage ditches in combination with soil removal. On one location, the upper 25 cm of soil was removed instinctively. On two other locations, the soil profile (0-70 cm) was analysed on four parameters that play an important role in binding phosphate and the creation of nutrient-poor conditions: the concentration of total P and plant available P (Olsen-P) which should not exceed 3000 and 300 µmol/l soil and the Ca/P and (Fe-S)/P ratio in the soil that should be much higher than 2 to prevent mobilisation of phosphate to the surface water. Based on the measurements, the upper 25 cm of soil and 20 cm of underlying phosphate enriched mineral sand were removed on the two locations. The results show that the total-P and Olsen-P concentrations in the new soil top layer exceeded the critical values of 3000 and 300 µmol/l soil, in case only 25 cm of soil was removed. As a result, a eutrophic lake developed with dominant growth of algae, lemnids and fast growing wetland plants including Juncus effusus and Glyceria fluitans. In case 45 cm of soil was removed, all four parameters met the critical values. Here, two nutrient-poor softwater lakes developed with dominant growth of plants species characteristic of oligotrophic and weakly buffered conditions including Pilularia globulifera, Hypericum elodes, Potamogeton polygonifolius, Eleocharis multicaulis, E. palustris, Eleogiton fluitans and Littorella uniflora. The system is already stable for 8 years. This research indicates that soil removal can be a successful option to restore softwater lakes on abandoned agricultural lands in case the availability of phosphate is taken into account. However, attention should be payed to the availability of a persistent seed bank, since there is the risk that a viable seed bank is removed with the soil. Remaining nearby populations might act as donor sites.
    • Regeneration of matgrass swards on abandoned agricultural grasslands in southern Limburg
    • Smits, N.A.C., H.P.J. Huiskes, J.H. Willems & R. Bobbink
    • The plant biodiversity of nutrient-poor grasslands on hillslopes in southern Limburg is still declining. Within the framework of OBN, research was started in 2004 in which the development of these grasslands from intensively used agricultural grassland also is studied. Fifty experimental plots were put up in 2005 on two different fields to investigate the effects of different restoration practices to restore matgrass swards. Two aspects are considered: firstly, the removal of nutrients in the soil, secondly, the dispersal possibilities of characteristic species. This article describes the results of these small-scale experiments (3x3m) between 2005 and 2007. The combination of sod cutting and hay transfer (from a well-developed reference site) appears to be the most successful way to restore these valuable grasslands. In 2007 an extended experiment was started with plots of 0,75 ha. These plots will be used to study the large-scale effects, including the restoration of fauna in these grasslands.

    • Aspects of Wild boar management on the Veluwe
    • Groot Bruinderink, G.W.T.A. & D.R. Lammertsma
    • From 1987 to 1997 the diet and fitness of two populations of Wild boar, which received no supplementary feeding, were studied in the Dutch Veluwe area. In both populations the numbers were controlled by hunting. Composition of stomach contents depended on season, mast availability and area specific factors. Mast is the staple food of the Wild boar, but if no mast is present, broadleaved grasses may offer a substitute for their energy intake. Density dependence was found for the decrease in mast consumption and condition (body weight) from autumn to winter. Mast determined the condition and reproductive success of the Wild boar. We used this unique relationship to model carrying capacity in terms of a threshold density in winter above which the average body mass is density-dependently reduced. In practice, the number of Wild boar turns out to be the threefold of the model predictions.We suggest that this may be partly the effect of inaccurate data used in the model. But is also the outcome of a sex biased hunting regime, which is aimed at a relatively large cull of adult male boar. Also a milder winter climate increases survival of piglets. However, little is known on the ecology and population dynamics of a Wild boar population which is not controlled by man. Long-term ecological studies on such a population will provide the answers that may be used in choosing future Wild boar management strategies.