Research PaperSmart management is key for successful diversification of field margins in highly productive farmland
Introduction
For more than 60 years, agricultural land in Europe has experienced a severe biodiversity decline (Baessler and Klotz, 2006, Stoate et al., 2009, Meyer et al., 2015, Sutcliffe et al., 2015). Agricultural intensification, field enlargement and mechanization (Tscharntke et al., 2005, White and Roy, 2015), but also changes in management practices (Alignier and Baudry, 2015), have led to a loss of semi-natural habitats, a general decrease of plant and animal species richness, and depletion of soil seed banks (Robinson and Sutherland, 2002, Simmering et al., 2013, Meyer et al., 2015). Field margins can be both habitat and refuge for many plant and animal species (Marshall and Moonen, 2002, Tscharntke et al., 2005, Hackett and Lawrence, 2014). But, especially in intensively used agrarian landscapes, repeated mulching or spraying reduces plant species diversity (Marshall and Moonen, 2002, Hahn et al., 2014), often resulting in species-poor, grass-dominated sites (Meyer et al., 2013), which are unable to fulfill important ecosystem services (e.g. habitat and food sources for pollinators). Contrary to perennial wildflower strips, which are created on arable land and are usually supported for five years by agri-environmental schemes, permanent field margins are situated on common land between farm tracks and arable fields. Although they are in an unfavourable conservation status in most European countries, only a few countries developed agri-environmental schemes to promote field margin biodiversity (e.g. Great Britain, Switzerland). In Germany, as well as in 15 other EU-28 countries, field margins are eligible as Ecological Focus Areas (EFA; Hart, 2015). Even though the Common Agricultural Policy (CAP, 2014–2020) requires that 5% of arable land be designated as EFA, most farmers favor fallows or cultivation of nitrogen-fixing crops instead of maintaining landscape elements like field margins (Pe’er et al., 2016).
In intensively managed agricultural landscapes, relying on spontaneous colonization of target plants is usually not successful (Kleijn et al., 1998, Török et al., 2011, Prach et al., 2015) due to depleted seed banks and the limited long-distance dispersal ability of many plant species (Bakker and Berendse, 1999, Nathan et al., 2008). Hence, active introduction of target species in restoration sites is recommended (Kiehl et al., 2010; Rydgren et al., 2010). Although grassland diversification experiments showed that the establishment success of target species is dependent on initial sward disturbance (Hofmann and Isselstein, 2004, Edwards et al., 2007, Pywell et al., 2007, Schmiede et al., 2012), there are still uncertainties about sward disturbance intensity.
After successful species re-introduction, implementing a suitable management regime is crucial for the maintenance of species diversity (Pywell et al., 2007, Öster et al., 2009, Auestad et al., 2016, John et al., 2016). In many European countries, field margins are usually mown once a year between September and February (Kleijn et al., 1998: France, Netherlands, UK; Bokenstrand et al., 2004: Sweden) or not at all (Hovd and Skogen, 2005: Norway). This might work well on marginal sites, but under nutrient-rich conditions it benefits competitive perennial grasses (Hansson and Fogelfors, 1998, Amiaud et al., 2008). In addition, late mowing destroys hibernating structures for invertebrates (Blake et al., 2013) and removes winter feeding sources for farmland birds (Vickery et al., 2009). By contrast, many forbs are able to regenerate quickly after early mowing, thus prolonging the flowering period until early autumn.
While many studies document the early establishment phase after species introduction into field margins (de Cauwer et al., 2005, Carvell et al., 2007, Blake et al., 2013), long-term studies are scarce (Bokenstrand et al., 2004, Smith et al., 2010). Successful examples of sustainable conversions of species-poor grass strips into flower-rich, highly-diverse field margins are completely missing. Even though grass strips are beneficial for some arthropod groups (Meek et al., 2002, Badenhausser and Cordeau, 2012), they meet neither the demands of nectar and pollen-feeding invertebrates nor those of farmland birds (Vickery et al., 2009). Converting species-poor grass margins into structurally and floristically diverse vegetation during summer and seed-rich habitats in winter are optimal prerequisites to provide feeding, nesting, and hibernating habitats for many animal species.
In general, problems with sowing high-diversity seed mixtures of regional wild plants include their rather high cost and difficulty in obtaining regionally produced seeds (Kiehl et al., 2010, Tischew et al., 2011). A cost efficient option is to introduce target species only in parts of restoration sites (Rayburn and Laca, 2013, Valkó et al., 2016). Combined with appropriate management, this might promote colonization of adjacent areas, but little is known about the immigration success of target species into adjacent species-poor grass margins (but see Smith et al., 2010).
In a large-scale field experiment in Germany, we examined the effect of sward disturbance intensity, mowing time and sowing on the development of field margin vegetation over seven years. We focused on the following questions: (Q1) Is sowing of a species-rich seed mixture a successful method to diversify species-poor grass margins? (Q2) Can adjacent target forbs establish in regularly mown, but unsown grass margins, and does mowing time influence their establishment success? (Q3) Does sward disturbance intensity influence the establishment of sown target species? (Q4) Which mowing time (June or September) is most successful in maintaining species-richness of restored field margins?
Section snippets
Study area
Our study area is situated on the border of the Strenzfeld campus of the Anhalt University of Applied Sciences in Saxony-Anhalt (N 51°49′01.00″, E 11°42′14.09″, 90–93 m above sea level). The climate is dry with 511 mm annual precipitation and a mean annual temperature of 9.7 °C (long-term mean 1981–2010, Deutscher Wetterdienst). Adjacent arable fields are used for conventional wheat, rape or maize production. The soil is a very productive Chernozem developed on loess substrate. Before the
General vegetation development
The original species composition of the plots was rather similar in 2010 before application of the treatments (Tables 1 and 2). However, already in 2011, the species composition in all sown plots (T1–T4) shifted considerably away from their initial positions along the first GNMDS-axis, whereas the plot trajectory of the T5 and T6 treatments (only mown, no sward disturbance, and no seeding) was much lower (Fig. 1). From 2011 until 2014, plots with a similar sward disturbance intensity remained
Diversification of species-poor grass margins by high-diversity sowing of wild plants
Compiling a high-diversity mixture of 49 wild plants allowed us to pursue different objectives: diminishing the risk of failure caused by environmental perturbations, and providing feeding, nesting, and hibernating habitats for different animal species. Species-rich vegetation hosting a variety of species traits can respond better to disturbances (Tilman et al., 2006, Proulx et al., 2010) and high diversity seed mixtures are known to increase the resistance of plant communities to climate
Conclusions
Our results, based on continuous monitoring over seven years, proved that sowing a high-diversity mixture of wild plants in combination with initial sward disturbance by harrowing and specific management is a very successful tool for diversifying species-poor grass margins. By selecting and sowing a high-diversity seed mixture of regional wild ecotypes, the emerging vegetation will strongly support pollinators such as butterflies and wild bees, thus enhancing the biological diversity of
Acknowledgements
This study was funded by the German Ministry of Research (grant no. 323011000033) and the German Federal Environmental Foundation (grant no. 31006-33/2). Matthias Stolle gave helpful advice concerning the selection of species for the seed mixture. We also thank the agricultural faculty, especially Dieter Orzessek and Stefan Gille, for their support. We are grateful to several colleagues and students, in particular Nele Adert, Heiner Hensen, Mark Pfau, Stefan Schreiter, and Lea Schubert, for
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