De Levende Natuur nummer 5 van 2016 (English summary)

Mechanisms underlying the distribution of stream corridor plant species from river levees

J.P.M. Lenssen, I. Niemeijer, G. Boedeltje & F.L. Baarspul

Stream corridor plants from high, sandy levees in river forelands are of great importance for nature conservation since this group includes many rare and endangered species. Since the 1960’s these species have declined dramatically, but recent river restoration projects seem to have turned the tide. In these projects agricultural use is abandoned and the fields are left to natural processes such as disturbance by wind and water and extensive grazing.

Although many species have benefited from these measures, others have stayed behind. We investigated whether the successful species have a higher spatial mobility that would allow them to take advantage from the frequent disturbances. Data were obtained from an intensive field survey on  the sandy levees along the River Waal and Nederrijn in the east of The Netherlands (Gelderse Poort) in two periods, 2004-2007 and 2011-2013. A total of 64 sandy stream corridor species were included in the survey. Their presence and abundance was recorded with GPS to the nearest 15 meter. This enabled us to assess whether a species in the 2011-2013 period had persisted at its previously recorded site or had colonised new sites.

Colonisation appeared to be the dominant process in regulating population dynamics. Most species were found  in new sites in the second survey period, regardless whether they had an overall increase or decrease in the study period. Colonisation was significantly and positively related to abundance in the previous period. In addition, the ability of seeds to remain floating on water or adhere to animal fur also seemed to result in a higher degree of colonisation. Based on these findings we argue that initial population size may be a critical factor determining potential for expansion. Management of stream corridor plants should at least facilitate exchange between remaining populations by using natural dispersal vectors such as water and animals. In addition, reinforcement of small populations by sowing or planting seedlings may also be considered.

Drivers of plant diversity in urban wildscapes

M.C.A. van Aar, J. Limpens & J. van Ruijven

Urban wildscapes can host a remarkable amount of plant species. It is yet unknown which factors drive this observed diversity.

We assessed the effect of 17 environmental factors on species composition and plant diversity (species richness and evenness) in 28 sites divided over 3 major cities in The Netherlands. We used Detrended Correspondence Analysis (DCA) to assess the variance in species composition between sites and cities and linear mixed models to determine the effect of the environmental factors on plant diversity.
We found typical urban species in all three cities, as well as some annual pioneer species. Species composition clearly differed in the three cities, this variation seems based on environmental factors as well as on the species pool in the surrounding area. We found that the environmental factors soil organic matter, age, size and low vegetation and build-up area in a 500 m radius had the biggest influence on species composition.
We found that none of the factors had a significant effect on species richness, but we did find effects on evenness. The soil factors organic matter (negative effect) and pH (positive effect) explain the most variation in evenness. The factors disturbance, rubbish and rubble all explain a part of the variation in evenness as well and have a positive effect. Age has a negative effect on evenness when looking at sites with similar organic matter content.

We conclude that the diversity is highest in sites with vegetation of early succession stages – these are sites with low organic matter content and/or young sites – where disturbances, rubble and/or waste are present. Implications for management are amongst others that a high turnover (several years) of sites can contribute to the diversity of urban wildscapes in the city in general, as long as the new sites are sandy. Even though the species composition differed between the studied cities, significant effects were still found for a number of environmental factors. Therefore the conclusions and management implications of this study can be extrapolated to cities comparable to the studied ones. This includes most Dutch cities.

Influence of livestock grazing on biodiversity of salt marshes

P. Esselink, B.J. Ens, D.D.G. Lagendijk, F.S. Mandema, S. Nolte, J.M. Tinbergen, R. van Klink, M.F. Wallis de Vries & J.P. Bakker

Livestock grazing is often recommended to preclude the development of European salt marshes into a species-poor late-successional stage that is frequently dominated by Sea couch (Elytrigia atherica). It remains unclear, however, how grazing may be optimized for conservation management in order to maintain relatively high levels of biodiversity. To address this question, a grazing experiment was implemented at Noord-Friesland Buitendijks, a mainland salt marsh along the Dutch coast of the Wadden Sea. In an experimental approach, the effects of livestock species and stocking density were studied by means of summer grazing in paddocks of 11 ha, viz.: horse and cattle grazing with high (1 animal/ha) and low (0.5 animal/ha) stocking densities. There was a clear difference between the effects of horse and cattle grazing on the vegetation. Horses require more energy, are more mobile, consume more biomass, and moved greater distances from the watering points even at low stocking densities. Cattle preferred the higher salt marsh and grazed closer to the waterpoints. At low stocking densities, a spatial mosaic of short and tall vegetation developed, of which the scale was partly determined by livestock species. Flower-visiting insects and their most important food plant (Aster tripolium) were most abundant at low stocking densities.

No differences were found in the density of breeding birds among the different grazing regimes, which could be due to the relatively small scale of the experiment and the short duration of the study (three years). The breeding success of birds, however, was strongly negatively impacted by horse grazing, as horses, through their higher activity levels, trampled many more nests than cattle. During autumn, geese had a clear preference for areas with high stocking densities which were characterised by short and homogeneous vegetation.

Voles seemed to be most abundant at low stocking densities with cattle.

We showed that different grazing regimes benefit different taxa, with the most favourable effects occurring at low cattle densities. We conclude, however, that the overall diversity of salt marshes benefits the most by establishing and maintaining a spatial mosaic of different management regimes: intensively grazed, lightly grazed, and ungrazed areas. The extent to which this spatial variation should be varied also at a temporal scale remains to be investigated.

New lady’s tresses orchids in The Netherlands and Belgium

B. Gravendeel

The number of species of lady’s tresses orchids (Spiranthes) in The Netherlands and Belgium increased since 2011. With a combination of morphology and DNA barcoding, plants of Dutch and Flemish populations were identified as S. lucida, S. spiralis, S. romanzoffiana and two man-made hybrids between S. cernua ‘Chadd’s Ford’ and S. odorata sold at garden centers and webshops. The hybrids can be distinguished from each other by their DNA barcodes, the different shape of the tip of the lip and their flowering time. Plants with S. cernua ‘Chadd’s Ford’ as mother and S. odorata as pollen donor have flowers with a lip with a blunt tip that are open in August-September. Plants with S. odorata as mother and S. cernua ‘Chadd’s Ford’ as pollen donor have flowers with a lip with an acute tip that are open in September-October. Contrary to earlier reports, S. aestivalis has not reappeared in the Netherlands. Greenhouse experiments showed that the man-made Spiranthes hybrids are self-pollinating. Regeneration does not only take place by seeds but also by underground shoots. At five different localities, an explosive increase in population size of man-made hybrids was observed between 2011 and 2015 from less than 10 up to over 100 flowering plants. At one locality, ant nests containing larvae of a rare syrphid fly species (Microdon myrmicae) are being trampled by visitors entering the area during the orchid flowering season. To protect endangered native biodiversity it is recommended to remove the man-made Spiranthes hybrids from nature reserves to prevent dispersal.