Long-term monitoring of the restoration and development of limestone grasslands in north western Germany: Vegetation screening and soil seed bank analysis

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Abstract

We analysed the long-term development of limestone grasslands in West- and Central Europe (Mesobromion: Gentiano-Koelerietum) under conditions of restoration comparing vegetation changes with the composition of its corresponding soil seed bank over a time period from 1992 to 2005. Because of optimal climatic, edaphic and geomorphologic conditions restoration of vegetation developed gradually into the optimal phase of the typical limestone grassland vegetation type. However, the development of the soil seed bank compared to that of the actual vegetation cover is much slower, and even after 13 years all three study plots did not reach the optimal phase compared to soil seed bank composition from well-developed control vegetation nearby. This finding agrees with the assumption that after changes of management limestone grasslands need decades or even centuries to develop into self-sustaining species-rich habitats. We also monitored the orchid Ophrys insectifera as target species and characteristic species of this type of plant association over the last 27 years. Here, population density increased continuously over time indicating that vegetation restoration efforts of the last decades have had a positive effect.

We further documented that the data obtained for the soil seed bank largely depend on the applied method used for the characterization of the soil seed bank. The seedling emergence and the rinsing method provided different results, and in case of our study area the rinsing method was mostly superior. The rinsing method detected up to 20fold higher total seed numbers.

Introduction

Limestone grasslands in Central Europe are particular references of past traditional, and actually mostly abandoned land usage forms. Characteristics for these kinds of vegetation types, which were developed over centuries by deforestation, grazing and mowing, are poverty of nutrients, extensive usage and frequently rapid drainage of water through the native limestone rock. This also summarizes the most important reasons for its threat: changes in agriculture and increase of land use in a general sense, abandonment of extensive mowing or grazing because of low productivity, direct or indirect (e. g via air pollution) entry of nutrients, and here in particular plant disposable nitrogen. These grasslands are of great importance for biodiversity protection because of their high species richness and many ecological interactions between many different organisms (Bernhardt et al., 1995, Köhler et al., 2005). In various countries long-term vegetation monitoring experiments have been established considering various aspects of environment changes and taking into account the special quality of this vegetation type (Dierschke, 1985, Dzwonko and Loster, 1998, Hansson and Fogelfors, 2000, Kahmen et al., 2002, Koch and Bernhardt, 1996, Köhler et al., 2005, Novák and Prach, 2010, Pärtel et al., 1998, Ryser et al., 1995, Willems, 1985).

The peculiar situation and endangerment of limestone grassland is also obvious from a glance at the Red Data Books (Garve, 1993, Willems, 2001). In Lower Saxony, the German federal state where the herein presented area belongs to, approximately 30–40% of plants typical for these kinds of grasslands are endangered. However, endangerment and declining distribution area of limestone grasslands, which are among the habitats most rich in plant species in Central Europe, is not only obvious for north western Germany but for whole Central Europe (Bergmeyer and Novak, 1988, Mattern et al., 1980, Poschlod et al., 1991, Schmitt and Schmidt, 1992, Zoller et al., 1986).

Some groups of plants characteristic for limestone grasslands are more sensitive to environmental changes and disturbance than others. The orchids for example are often highly specialized ecologically, and we do not wonder about the fact that among 44 endangered orchid species in Lower Saxony five species are either extinct or went lost. Among the remaining 39 taxa there are 19 species with main distribution on limestone grassland. Several of these orchids are found in our investigation area at the north western or northern margin of their main distribution range (e.g. Ophrys apifera, and O. insectifera, submediterranean-subatlantic chorotypes) and are subjected to an increasing risk of extinction. Various other species in closest proximity to our study plots as well demonstrate that endangerment might be increased when species occur at the margin of their distribution range or represent simply outposts such as Melica nutans, Actaea spicata (both from orchid-rich open woodlands), or Helictotrichon pratense. In summary, it is well known that any sustainable continuation of existence of limestone grasslands and measures to improve this depend on continuation or resumption of traditional, extensive forms of land usage – which, however, often is hampered by financial constraints.

Although we gained much knowledge about the impact of different forms of land usage (e.g. cattle grazing versus mowing) on the actual and aboveground vegetation, much less is known about the dynamics and development of the corresponding soil seed bank (Koch and Bernhardt, 1996, Poschlod, 1993, Poschlod and Jackel, 1993, Poschlod and Jordan, 1992). But this knowledge is virtually important for any long-term development of species-rich limestone grassland, because of different and specific-specific ways of propagation (e.g. seed dispersal versus clonal growth), persistence over long and sometimes disadvantageous time periods (e.g. long-lived perennials, ability to build up a soil seed bank), colonization ability (e.g. long-distance dispersal via seed) or the maintenance of additional genetic variation in the soil seed bank contributing to the overall genetic diversity of the population, but not contributing continuously to the gene pool. Consequently, a monitoring that compares the development of the actual vegetation with the corresponding soil seed bank should accompany restoration efforts at least for some case studies.

Herein we focus on a study area in Lower Saxony, county Osnabrück, in the north western part of Germany (Nature Reserve “Silberberg” near Hagen a.T.W.). In this area the geological situation locally changes rapidly over short distances creating isolated habitats. In case of the Nature Reserve “Silberberg” Permian Zechstein-age anhydrite rocks cover at the southward exposed slopes the younger Triassic red sandstone indicating an old reversal of the rock formations, whereas at the northern slopes the red sandstone only is found. The closest limestone outcrops are several kilometres apart. The study area is optimal for a case study because of its good floristic documentation over the last 100 years and the establishment of several permanent investigation plots twenty years ago. In addition, the study area in the western Teutoburg Forest represents the north western distribution limit for such species-rich limestone grasslands and is an example for restoration efforts at the margin of its ecologically optimal range.

We conducted this study in order to provide a detailed plant biodiversity screening for a highly endangered vegetation type and to address the following issues:

  • (i)

    providing data for the soil seed bank, that is up to present not so well known for limestone grassland;

  • (ii)

    comparing the two most frequently used methods to quantify soil seed bank composition; and

  • (iii)

    developing ideas about dynamics and time periods needed to restore or develop limestone grasslands at the margin of their major distribution range.

Section snippets

Study area

The study was conducted at the Nature reserve “Silberberg”, Hagen a.T.W., 10 km southwest of Osnabrück (52°12′N, 7°16′E, 135–160 m a.s.l.). Climate is Atlantic with moderate and humid winters and a short and not very hot and dry summer season. Mean annual temperature is 9.4 °C and the area receives on average 856 mm of rainfall annually, more or less equally distributed all over the year (data for Osnabrück). The study area has a long history of human impact (mining for silver and zinc dates back to

Aboveground vegetation developed gradually into limestone grassland

Data of the vegetation monitoring are provided in Table 1, and in total we observed 98 taxa of higher vascular plants on our investigation plots (23% phanerophytes, 3% chamaephytes, 3% geophytes, 67% hemicryptophytes, 4% therophytes). It is demonstrated that the control vegetation on plot C (Fig. 1) did not change during our monitoring time frame (1992–2005) and had a constant species composition and species number of 53 taxa. In contrast, species composition as well as species number on plots

Aboveground vegetation: patterns and processes

Our study was originally initiated to monitor and evaluate long-term effects of limestone grassland restoration efforts. Our study area is a representative example for mesic calcareous grasslands that occur all over central Europe and are characterized by a long history of providing an open landscape over centuries (e.g. via extensive grazing, clearance, mining activities). They are also characterized by subsequent continuous fragmentation and decrease of total area size. Our data provide

Conclusions

The development of restored limestone grassland in species-rich vegetation is most influenced by the colonization capacity of the directly neighboring source populations (by vegetative means or by short lasting/temporarily seed rain). In most cases this seed rain is not sufficient to create a significant and typical soil seed bank of limestone grassland within few years. For more than one decade the influence of adjacent vegetation on the development of the restoration sites is of outstanding

Acknowledgements

We would like to thank all local people providing support for collecting the orchid data and maintaining the continuous vegetation management regime (AG Natur und Umwelt Hagen a.T.W.). The data for 1992/1993 were collected by two former diploma students: Sabine Jürgens and Sigrid Kassenbrock. We are very grateful for continuous support and permissions to the local government, technical authority for environmental concerns, county Osnabrück, namely H.H. Escher and F.J. Brockmeyer.

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