May 2019

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English summaries

    • A system based approach to control Crassula helmsii
    • J.M.M van der Loop, H.H. van Kleef & E. Brouwer
    • The non-native Australian swamp stonecrop (Crassula helmsii) is expanding rapidly in the Netherlands. Dominance of stonecrop locally threatens species characteristic for soft waters. Eradication of stonecrop is almost impossible as it easily grows back from seeds and fragments. Therefore, we explore ways to cope with this new species by looking for situations where the species is not dominant and identifying the abiotic and biotic factors determining its abundance. A field study was performed in order to determine suboptimal conditions for stonecrop growth. Submerged, growth of stonecrop is limited by the availability of carbon dioxide. Stonecrop abundance was negatively correlated with cover of native species. The effects of nutrients and competing species (shoreweed Littorella uniflora and marsh St. John’s wort Hypericum elodes) on growth of Australian swamp stonecrop were tested experimentally. On nutrient poor soil, growth of stonecrop was minimal. Phosphorous additions had a minor positive effect on stonecrop growth. Under elevated nitrogen concentrations growth was greatly enhanced. Marsh St. John’s wort was, in a low density, growing equally well. Shoreweed, in particular it’s root system, exhibited stronger growth than stonecrop.  A third experiment was conducted in order to determine whether a closed canopy of native species (six-stamened waterwort Elatine hexandra and shoreweed) can prevent settlement of Australian swamp stonecrop. Goose dropping were added to simulate dispersal by geese and to determine if nutrient addition by goose faeces is able to mitigate the competition by native species. The closed canopies of native species strongly reduced settlement of stonecrop as compared to bare soil. In particular on bare soil, goose faeces increased growth of stonecrop. On the other hand, growth in a closed canopy of native species was very slow even with the addition of faeces. To summarize: it is unlikely that Australian swamp stonecrop can become invasive in well-vegetated nutrient poor ecosystems. However, when patches of bare soil are created and nitrogen is available, stonecrop will be able to spread rapidly and become the dominant species. Based on the results of these studies, preliminary management actions have been formulated to control invasions of Australian swamp stonecrop in nutrient poor ecosystems. Three measures have been identified as possibly effective for stonecrop control. (1) Reduce and minimize nutrients. Input of nitrogen through atmospheric deposition, surface water and waterfowl should be minimized. (2) Reduce dominance of stonecrop. This may be achieved by covering swards with heavy plastic foil, sod cutting, boiling water and prolonged inundation of carbon poor water. (3) Assist native species in recolonizing the area by introduction and an intensive monitoring and removal of stonecrop settlings. Application of these measures is still in an experimental phase. Results are expected in a few years.

    • Occurrence of river lamprey, river clubtail and freshwater mussels in the river Waal.
    • M. Dorenbosch, van Kessel, F.P.L. Collas, L.H. Jans, M.M. Schoor & R.S.E.W. Leuven
    • River lamprey (Lampetra fluviatilis), river clubtail (Gomphus flavipes) and native freshwater mussels are important indicators for the ecological status of Dutch large rivers. Unfortunately, not much is known about the occurrence of their larval stages in these rivers. The presence of both species in various habitats of the river Waal (i.e., groyne fields, shore channels, secondary channels) was recorded using seine netting and hydrodredging. In total, 27 larvae of river lamprey and 57 larvae of river clubtail from all year-classes were caught. Both species were present in the three studied habitats. Both larvae of river lamprey and river clubtail were most abundant in shore channels. Juveniles of swollen river mussel (Unio tumidus, n=77) and duck mussel (Anadonta anatina, n=30) were caught in the three studied habitats, whereas four specimens of painter’s mussel (Unio pictorum) were only caught in the shore channel. Swollen river mussel was most abundant in side channels, duck mussel in shore channels. Only a few specimens of these three mussel species were caught in groyne fields. The results indicate that littoral zones in groyne fields of a regulated river such as the river Waal, can harbour critical benthic species but relative numbers are significantly higher in side channels (native mussels) and shore channels along longitudinal training dams (larvae of river lamprey and river clubtail). The environmental conditions in these shore channels, such as continuous water flow, presence of sheltered zones and absence of ship navigation, have a positive effect on the presence of larvae of river lamprey and river clubtail (both Natura 2000 species). During low river discharges water flow in side channels is strongly reduced, resulting in less suitable habitat for both species. However, native mussels do occur in higher numbers in side channels.

    • Differences between an up- and downstream population of the river-clubtail in River Waal
    • D. Schut & J.P.M. Lenssen
    • After a 93 years absence in the Netherlands, larvae of Stylurus flavipes were found again in the River Waal in 1996. Since then the species seems to have expanded its range over the entire river, but it is unknown how well the population is actually doing. Using counts by voluntary observers we tried to assess the population fluctuation in two trajectories of the River Waal: the upstream and hydrologically dynamic ‘Gelderse Poort’ and the downstream stretch at Loevestein, with far more moderate hydrodynamics. Average numbers of larvae per 250 m stretch did not clearly differ between both areas and seemed sufficient for a viable population. However, at Gelderse Poort, larvae appeared to be more aggregated, with more than 75 % of all the larvae emerging at a single stretch. Analysis of timing of emergence of larvae indicated that larvae at Loevestein emerge at a considerably lower watertemperature sum than larvae at Gelderse Poort. This difference in required temperature sum indicates a slower development of last instar larvae at Gelderse Poort, most likely due to a lower food availability and/or higher predation pressure. Both the slower larval development and the higher aggregation indicate that the population is more vulnerable at the highly dynamic Gelderse Poort. Probably the river bottom here provides only a few mesohabitats with detritus and fine-grained soils, the mesohabitats considered to be most suitable for larval development.

    • Very large beech trees in the Sonian forest (Belgium)
    • K. Vandekerkhove, M. Vanhellemont, A. Leyman, P. Van de Kerckhove & M. Esprit
    • The frequent occurrence of very large trees (diameter at breast height ≥ 80 cm) is a typical element of both primary and secondary old-growth forests. We analyzed the characteristics of very large trees in one of the few remaining stands of lowland old-growth beech forest in Northwestern Europe, regenerated around 1775 and left unmanaged since 1986. Based on repeated full dendrometric surveys, we examined the density, diameter range, increment, mortality rate and spatial distribution of very large trees of beech (Fagus sylvatica). In order to evaluate the results, we compared the beech trees of Sonian forest to an extensive set of primary and secondary old-growth beech forests in Europe. The very large beech trees in our study site increased in density from 31.5 to 34.3 trees per ha between 1986 and 2011. In the old-growth beech forest reference sites, the density of very large trees was generally much lower: between 5 and 20 trees per ha (median 13.1). The trees reached a median diameter of 97 cm (mean 98.9, max 159 cm) and heights of over 45 m, which probably makes them the largest European beech trees in the world. Although the very large beech trees are over 200 years old, they still show a high average diameter increment of 4.7 mm per year and a low mortality rate (0.9% per year), indicating that they are still vital. The regular spatial distribution pattern of the very large trees in the studied stand clearly differed from a typical old-growth stand, in which very large trees are randomly distributed. The extraordinary densities and dimensions of the very large trees in our study site can be explained by the favorable climate and site conditions that promote high increments, in combination with the former management interventions of tending and thinning that resulted in continuous non-suppressed growth. Although derived from a very specific case with particular conditions, the observations on old beech trees in Sonian forest may be relevant to other beech forests, as they tend to challenge certain baseline assumptions for tree size and longevity potential of beech in Northwestern Europe.

    • The oak-hornbeam forest in the Netherlands: an endangered woodland ecosystem of stagnosols, fallen off the radar
    • R.J. Bijlsma, H.G.J.M. Koop & E.J. Weeda
    • Until the 1960s parts of the eastern and southern Netherlands became waterlogged in winter and early spring due to stagnation of rain water and poor drainage especially in areas with boulder clay, Pleistocene river clay or impermeable soil layers (stagnosols). During summer these kind of soils can dry out strongly. Following German vegetation ecologists, the species-rich woodlands characteristic for this regime of periodic wetting and drying of base-rich soils were included as oak-hornbeam forest by Van der Werf (1991) in his overview of Dutch forest types. However, in the recently updated national vegetation classification oak-hornbeam forest is interpreted as independent of groundwater regime, jeopardizing restoration efforts of the corresponding Natura 2000 habitat type. Improved drainage including the establishment of large-scale ridge and furrow landforms in forestry resulted in a deterioration of oak-hornbeam woodlands due to chronic drought and corresponding acidification. Nowadays, only a few good examples can still be found in the Netherlands. Since definitions of Dutch Natura 2000 habitat types are based on the national vegetation classification, oak-hornbeam forests of periodically wet, base-rich soil are at risk of becoming out of sight regarding proper restoration measures for habitat type 9160 (Sub-Atlantic and medio-European oak or oak-hornbeam forests of the Carpinion betuli). We describe a recent project aimed to restore the relief and water relationships in a ridge and furrow forest in the Natura 2000 site Ulvenhoutse bos. Proper restoration is still hampered by knowledge gaps regarding the functioning of the relief and ground water regime in reference sites and by little practical experience with restoration measures in ridge and furrow landforms.