Ir. J. Bokdam : Nature conservation and grazing management

The biodiversity of low-productive grasslands and heathlands in Europe is threatened by wood encroachment and atmospheric nutrient inputs. The question whether and how free-ranging grazers can maintain or restore these habitats is important since this grazing strategy is being applied on a wide scale in conservation areas. In this thesis, results of 20 years of vegetation monitoring and investigations of habitat and diet selection and nutrient transports by cattle were used to test the ‘Resource-mediated Successional Grazing Cycle’. The free-ranging cattle in the study area displayed opportunistic habitat use and acted as a driving force for cyclic shifts in the successional mosaic. They created grazing lawns in canopy gaps in the woodland by suppression of the tall understory plants and by introducing grass seeds in their dung. The soil nutrient pool of the grazing lawns was depleted because of nutrient redistribution to resting sites in the woodland. Soil nutrient depletion favoured the invasion of woody pioneers (Calluna vulgaris, Pinus sylvestris and Betula pendula) in the grass lawn. It was concluded that free-ranging grazers create temporary open, nutrient-poor habitats. The model is used to predict the effects of the grazed area and the herbivore assemblage on the size and longevity of these open patches