Nature conservation and grazing management Free-ranging cattle as a driving force for cyclic vegetation succession Jan Bokdam Promotor Prof. dr. F. Berendse Hoogleraar Natuurbeheer en Plantenecologie, Wageningen Universiteit Promotiecommissie Prof. dr. R. Marrs University of Liverpool, United Kingdom Prof. dr. J.P. Bakker Rijksuniversiteit Groningen Prof. dr. ir. H. van Keulen Wageningen Universiteit Prof. dr. J. Sevink Universiteit van Amsterdam Nature conservation and grazing management Free-ranging cattle as a driving force for cyclic vegetation succession (met een Nederlandse samenvatting) Jan Bokdam Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit Prof. dr. ir. L. Speelman in het openbaar te verdedigen op vrijdag 11 april 2003 des namiddags te vier uur in de Aula. ISBN 90-5808-791-3 Printed by Ponsen & Looyen b.v., Wageningen. This study was carried out at the Nature Conservation and Plant Ecology Group, Department of Environmental Sciences, Wageningen University, Bornsesteeg 69, 6708 PD Wageningen, in co-operation with the Vereniging Natuurmonumenten, ’s Graveland. Most of the chapters in this thesis have been or will be published as articles in scientific journals. You are kindly requested to refer to these articles rather than to this thesis. This thesis should be cited as follows: J. Bokdam (2003) Nature conservation and grazing management. Free-ranging cattle as a driving force for cyclic succession. PhD thesis. Wageningen University, Wageningen, The Netherlands. voor Margaret, Johan, Maarten en Wouter Alles hangt met alles samen Alles verandert - Panta rhei (Heraclitus, ca. 500 v.C.) Abstract Bokdam, J. (2003). Nature conservation and grazing management. Free-ranging cattle as a driving force for cyclic vegetation succession. PhD thesis. Wageningen University, Wageningen, the Netherlands. 224 pp. This thesis examines the suitability of controlled and wilderness grazing as conservation management tool for open, nutrient-poor habitats. It tests the Resource-mediated Successional Grazing Cycle (RSGC) theory, which claims that free-ranging large grazers can act as driving force for successional woodland-grassland cycles. The hypotheses were that free-ranging cattle create grassland in woodland (i), deplete the nutrient pools of these grasslands (ii), induce invasion of woody pioneers because of this depletion (iii). During the subsequent woodland phase the soil nutrient pool is replenished (iv). The study was carried out in the Wolfhezerheide, The Netherlands. The vegetation was composed of 19 ha of woodland, 33 ha of grassland and 9 ha of Calluna heathland. The area was year-round grazed by free-ranging cattle (90 kg ha-1 yr-1) after 1983. Atmospheric N-deposition varied from 25-60 kg ha-1 yr-1 depending on the vegetation. Vegetation change was monitored. Habitat and diet selection were quantified by 24-h observation and bite counting. On the basis of these data, nutrient balances were assessed for the whole area, the successional communities and excreta and excreta-free patches in the grassland. The results showed that cattle induced grasslawns in gaps in the heathland and woodland. They suppressed woody and tall herbaceous species in the woodland undergrowth and introduced grass seeds by dung deposition. Nutrient removal only exceeded the atmospheric inputs on excreta-free patches in the grassland for phosphorus and potassium. Nutrients were removed from these sites by off-take and gaseous excreta-N losses. Cattle also reduced the interception deposition. Nutrient depletion favoured the invasion of pioneer dwarfshrubs (Calluna) and trees (Pinus sylvestris and Betula spp) in the grass lawns. This result was related to the carbon-based chemical resistance of the involved woody pioneers. Cattle and wild herbivores only prevented wood encroachment on relatively nutrient-rich grasslands. Wood invasion and abandoning by the cattle induced replenishment of the soil nutrient pool. The results confirmed the RSGC hypotheses and emphasised the interaction of herbivory and plant resources as determinants of cyclic plant succession. These findings implicate that wilderness grazing may generate temporary open, nutrient-poor habitats in dry uplands, but incompleteness of the abiotic gradient, successional mosaics and herbivore assemblage will restrict the openness and soil depletion. Under these circumstances, conservation managers might prefer controlled grazing. Forced summer grazing, nutrient removal from grass lawns by housing of the animals during the night and additional removal of unwanted species seemed to mimic natural herbivory. Key-words: biodiversity, herbivory, wilderness, non-linear dynamics, mosaic cycling, grassland, wood encroachment, forest, Bos taurus, Calluna vulgaris, Deschampsia flexuosa. Contents Chapter 1 General introduction 9 Chapter 2 Effects of grazing by free-ranging cattle on vegetation 27 dynamics in a continental north-west European heathland Chapter 3 Effects of browsing and grazing on cyclic succession in 63 nutrient-limited ecosystems Chapter 4 Free-ranging cattle as opportunistic exploiters of a 87 successional woodland-grassland-heathland mosaic Chapter 5 Free-ranging cattle as nutrient movers in a successional 117 woodland-grassland heathland mosaic Chapter 6 Forage quality as limiting factor for cattle grazing in 149 isolated Dutch nature reserves Chapter 7 Free-ranging cattle as a driving force for resource- 171 mediated successional grazing cycles: a synthesis Chapter 8 Grazing as a conservation management tool for open, 189 nutrient-poor habitats Summary 203 Samenvatting 211 Dankwoord 221 Curriculum vitae 223 Chapter 1 General introduction Jan Bokdam Chapter 1 Open, nutrient-poor habitats are crucial for biodiversity. Sand lizard (Lacerta agilis) male sunbathing on a dead grass tussock. Wolfhezerheide, August 1987 (photo Maurits Gleichman). 10 General introduction General introduction Large grazers as a management tool in nature conservation areas OPEN NUTRIENT-POOR HABITATS The ideal way to conserve populations of wild plants and animals is by protecting their pristine, evolutionary habitats (Wilson 1999). Unfortunately, this strategy can only be applied to a declining number of species and areas. This raises the question about suitable and feasible alternatives. The rich flora and fauna of open, nutrient-poor habitats in Europe depend today mainly on semi-natural pastures (Webb 1986; Bignal, McCracken & Curtis 1994; Bal et al. 2001; Elligsen, Beinlich & Pflachter 1998). These grasslands and heathlands are the result of traditional pastoralism (Webb 1998). During several millennia, farmers herded their livestock in periodically flooded nutrient–rich wetlands and in nutrient- poor drained higher uplands. In both abiotic zones, pastoralism prevented sucession to woodland (Gimingham 1972; Pott & Hüppe 1991; Webb 1998; Tubbs 2001). These semi-natural pastures in the woodland domain differ from low-productive pastures in the permanent open domain (coastal zones, salt marshes, riverbanks, bogs and alpine zones) because the harsh conditions and lack of woodland-grassland mosaics in the open domain affect the species composition. Since 1850, traditional livestock farming has gradually lost its economic and social feasibility. It has been replaced on many places by modern farming systems characterised by high energy and nutrient inputs and high outputs of products and pollution. In other areas, traditional pastures were abandoned and turned into woodland (Anonymus 1988). In the remnant open areas, often designated nature conservation areas, biodiversity is suffering from fragmentation and eutrophication (Anonymus 1988; Marrs 1993; Jefferies and Maron 1997; Bobbink, Hornung & Roelofs 1998; Bakker & Berendse 1999). Traditional pastoralism has only survived in economically marginal areas (Bignal, McCracken & Curtis 1994), in marginal climates (Schütz et al. 2000) and in areas with marginal land use rights (Tubbs 1986). Sustained 11 Chapter 1 traditional farming, as conservation strategy for open, nutrient-poor habitats, will increasingly demand financial subvention by the European Union, national governments and private nature conservation organisations. The sustainability and efficacy of such agri-environment schemes has been questioned (Baldock et al. 2000; Kleijn et al. 2001). Nature conservation in Europe must therefore develop and test new management strategies for the conservation of nutrient-poor, open landscapes. Mechanical management may be in certain cases a suitable substitute for traditional farming (Anonymus 1988; Bakker 1989; Marrs 1993) but without marketable products this method is expensive for large areas (Werpachowski 2002). Controlled grazing using free-ranging livestock or wild herbivores, combined with minimal mechanical management is a second option for the replacement of traditional farming (Bal et al. 2001; Bokdam & Gleichman 2000) It has the advantage of control and marketable animal products (Kuit & Van der Meulen 1997; Prins, Grootenhuis & Dolan 2002). A third option is wilderness grazing (Baerselman & Vera 1989). This thesis examines the suitability of the last two options. GRAZING MANAGEMENT STRATEGIES Since 1960, cattle, horses and sheep have been increasingly used for the management of semi-natural pastures in the Netherlands (Oosterveld 1975; Thalen 1984; Bakker & Londo 1998) and abroad (Wells 1965; Welch 1997; Gordon et al. 1990; Bülow Olsen 1980; De Blust & Schneiders 1989; Buttenschøn & Buttenschøn