Nature conservation and grazing management. Free-ranging cattle as a driving force for cyclic vegetation seccession

Key-words : biodiversity, herbivory, wilderness, non-linear dynamics, mosaic cycling, grassland, wood encroachment, forest, Bos taurus , Calluna vulgaris , Deschampsia flexuosa.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.