De Levende Natuur nummer 5 van 2019

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DLN 2019-5

Root voles and management of the Nieuwkoopse Plassen nature reserve 

M. van Schie & J. Zielman 

In the Nieuwkoopse Plassen nature reserve a population of the endemic root vole, Microtus oeconomus arenicola occurs. The effects of different management regimes were studied in a life trapping survey from 2010 until 2016. During this survey 920 individual root voles were trapped in different vegetation types under different management regimes. The effects of different management regimes on the presence of the root vole were examined. Highest relative density numbers were found for reed lands mown in winter and extensively grazed common rush grasslands. Fen-heath vegetations, mown in late summer or autumn have very low numbers of root voles. After mowing, these vegetations display very little regrowth until May. A dense vegetation cover of grasses or herbs is beneficial for the root vole. 

Choppering and intensive rotational grazing as alternatives to sod-cutting on wet heaths 

M. Wallis de Vries, R. Bobbink, E. Brouwer, R. Loeb & J. Vogels 

Restoration of habitat quality in wet heaths is an important task at both national and European levels. This requires not only hydrological restoration, but also a careful balance between different options for vegetation management and restoration of soil buffer capacity. Sod-cutting is a current measure to break down the grass encroachment by Molinia caerulea. Because of the radical influence of sod-cutting in soil, vegetation and fauna, we carried out a factorial experimental to investigate choppering and intensive rotational grazing as possible alternatives. In addition, liming was included as a complementary measure, not only against the actual acidification, but also to counter the temporary peak of ammonium that occurs right after sod-cutting, which can prevent the re-establishment of characteristic species. 

In Molinia-dominated wet heath, we still found a considerable species richness of bryophytes, macrofungi and invertebrates. This emphasises the importance to avoid drastic measures in restoration. After six years, choppering led to a similar recovery of the flora and macrofungi as sod-cutting, but a better recovery of the invertebrate fauna and common lizards. Intensive rotational grazing opened up the vegetation, but did not result in significant changes in species composition. Liming with granular calcium- magnesium carbonate at a one-time dosage of 2 ton/ha did restore soil buffering capacity without leading to eutrophication, even in the presence of organic soil, except in combination with sheep dung. We conclude that choppering offers a promising alternative to sod-cutting in wet heaths. In this study, intensive sheep grazing did not yet appear effective in restoring species richness, however. Finally, liming may successfully abate soil acidification even without prior sod-cutting. 

Is the amazon ant in the Netherlands and Belgium nearly extinct? 

A.J. van Loon, J. Noordijk, J. Cox, F. Vankerkhoven, K. Gielen & L. Crevecoeur. 

The amazon ant Polyergus rufescens is dependent on slave ant workers, that are raided as pupae, of several species (Formica ants of the subgenus Serviformica) that perform the colony tasks like foraging and brood caring. In the Netherlands and Belgium, this thermophilous species has been found on dry sandy soils in heath, nutrient-poor grasslands and wood edges. In the Netherlands there are quite a few historical locations, but in the last two decades the species was found in only three places. The absence of recent findings in the Netherlands, prompted a survey in De Plateaux, where amazon ants had been found in 2005. In June 2019, two nests of the amazon ant were discovered here, around 900 m away from the historic location. The site was a former landfill of debris and zinc cinders, that were partially removed. Although one site persisted up till now in both countries, the amazon ant is critically endangered. It probably became victim of a combination of a considerable reduction of the surface area of heaths, grass encroachment and development of rough vegetation in heaths as a result of nitrogen deposition, deterioration of the poor nutrient situation on sandy soils and the large scale fauna-unfriendly sod-cutting which has been in use into the 1990s. Habitat management for the amazon ant includes maintaining a large and connected open landscape (preferably through grazing) with sufficient density of slave ant species. Reluctancy towards sod-cutting is advised, since this reduces the density of slave ant species and may unintentionally remove a possibly present amazon ant nest. It is recommended to put the amazon ant on the International Red List of IUCN. 

Victor Westhoff (1916-2001) 

F. Saris 

In this biography we have shown that Westhoff’s development up to 1975 appears to mirror the development of nature conservation and nature management. Our main question is whether this parallelism illustrates Westhoff’s influence on Dutch nature conservation, or conversely, that Westhoff was in fact largely guided by developments in the field. 

In his own research, Westhoff focused primarily on developing and describing the classification of Dutch vegetation. The establishment of relationships based on ecological research with stationary factors and landscape processes has mainly been developed by others. If he had paid more attention to this, he might have been able to write more positively about the future of the wet meadow landscape. There is still a bright spot of hope for a better future: ecological restoration or recovery management as it is known started to bear fruit in the nineties. Westhoff saw this himself, although he almost had to be forced by several of his pupils: long-gone species and plant communities returned miraculously. He had not thought this possible and it gave him new courage. He was, however, barely aware of the contradiction: the conservationist who could be lyrical about the garden of The Netherlands, but on the other hand remained convulsive when it came to the potential of nature, standing on restored (blue) wet meadows, one of his irreplaceable Twente treasures. Here it appeared that if the circumstances and location factors are correct and appropriate, this rare type of vegetation can also be restored. 

The publication of his book Wild Plants in the early seventies was a highlight for Westhoff. His plant-sociological oeuvre took shape shortly before in the internationally-acclaimed study Plant Communities in the Netherlands (1969). These books would continue for a long time as standard works, in which his knowledge of nature and landscape, acquired during the preceding decades, was expressed in - with regard to Wild Plants - easily accessible texts. 

The years 1905, 1945 and 1975 mark the Thijsse generation, the Westhoff generation and finally the protest generation. It is always the younger generation that wants something and then actually enriches one another: it is just like elsewhere in society. Innovation within nature conservation always goes hand in hand with the classic struggle between young and old, between passionate and sedate, whether or not pensionable. Nowadays, nature conservationists worry about the future support, the youth. It is also about giving youth back their dreams, their own playing and research areas, not just as a marketing tool for member growth, but as a connection to the nature of their future, in the hope that there will be a new youth generation that nature conservationists say what nature protection should look like in the 21st century. 

Solar collectors facilitate high plant diversity on green roofs 

H.J. van der Kolk & P. van den Berg 

We compared the vegetation on green sedum roofs with and without solar thermal collectors, 7 and 8 years after construction. The vegetation on the roof with solar collectors harboured a much higher plant species richness and diversity, particularly for weeds and woody plants, than the roof without solar thermal collectors. Several species typical for moist soils were found on the roof with solar collectors. This plant diversity is likely facilitated by the shading of the solar collectors keeping the substrate moist. The solar collectors are placed in a 25° angle at 1.10 to 2.25m height, allowing sufficient light to reach the roof to promote plant growth. Several typical plant species from the surrounding areas at ground level were observed, suggesting migration of these plant species to the roofs. These results demonstrate that solar collectors can be combined with green roofs resulting in a beneficial synergy between generating solar energy and stimulating high plant diversity. We encourage others to monitor and report the vegetation development on green roofs, especially at locations where multiple roofs with different characteristics can be compared. 

The Dutch petrifying springs, the most polluted springs of Europe; 

H. de Mars, G. van Dijk, B. van der Weijden, A. Grootjans & F. Smolders 

In 2016 an extensive research was undertaken to collect data on water chemistry and bryophyte composition of about 150 petrifying springs in Dutch South Limburg and other parts of North western and Eastern Europe. The database also included data from an international sampling survey of 51 springs. The research aimed at a better insight into the abiotic conditions of the Dutch springs in general but also to determine threshold values for nitrate in petrifying spring water. The Dutch petrifying springs had by far the highest contamination with nitrate and ortho-phosphate. The mean nitrate concentration of the Dutch petrifying springs is 85 mg NO3 -/l. They also have an impoverished bryophyte flora compared to other European petrifying springs sampled. A redundancy analysis of the dataset showed that Cratoneuron filicinum and curled hook-moss Palustriella commutata are opposites when nitrate and ortho-phosphate are concerned. The presence and coverage of Cratoneuron filicinum appeared positively correlated [p=0,003] with nitrate pollution. However, Palustriella commutata is significantly associated with nutrient poorer conditions [p=0,003]. 

Based on the collected data, threshold values were determined for nitrate and ortho-phosphate in habitat type 7720 Petrifying springs. The only reliable threshold value for nitrate was determined at 18 mg NO3 -/l. For ortho phosphate this value is 0,04 mg PO4 3- /l. 

Contemplating the loss of northern wheatears in Gelderland, The Netherlands 

H. van Oosten 

The number of breeding Northern wheatears has declined strongly in The Netherlands, by 90% since the 1980s. This bird used to be a common sight in coastal dunes, on inland heaths and also on agricultural fields. Nowadays only few populations remain near the coast and inland. Its decline is probably mostly due to acidification and eutrophication as a result of excessive nitrogen deposition, causing the preferred short-grown foraging sites to be overgrown by tall grasses. Wheatears find their food on short vegetation, but the amount of their preferred habitat diminished in The Netherlands. The current study aims to determine whether the wheatear still occurs and can occur as a breeding bird on the heaths of nature area ‘the Veluwe’, in the province of Gelderland. Therefore, I determined whether suitable sites still remain in the nature area, regarding vegetation structure and potential nesting sites. Also I investigated the arthropod communities and compared the outcome to site Vogelduin, a dune area near Castricum where wheatears still occur. In spite of diligent searching, no breeding wheatears were encountered in 2018. Most likely, the species went extinct on the Veluwe between 2012 and 2018. All sites investigated showed a suitable vegetation structure and a suitable number of breeding sites, which increases the chance that other factors are causal to the species’ demise in these sites. A decline of insect biomass can be important in relation to the population decline. Yet, this is difficult to prove since general arthropod sampling, as I performed as well, may not relate to the few specific prey species wheatears feed to their young. Most important lesson is to start studies aimed to unravel bottlenecks for declining species in a much, much earlier stage, when large-scale factors may still be discovered. In a later stage, populations may continue to dwindle because of chance-effects, which, indeed, may cause the final decline but which may not be the cause of the initial large-scale decline.