The influence of different grazing regimes on Phragmites ‐ and shrub vegetation in the well‐drained zone of a eutrophic wetland

. The effects of grazing by cattle and horses on vegetation development were studied in the well‐drained border zone of the Oostvaardersplassen nature reserve, a managed eutrophic wetland in the young Zuidelijk Flevoland polder in The Netherlands. At the start of the study period, 12 yr after the area was enclosed by dykes, the vegetation was dominated by Phragmites australis and tall herbs, particularly Cirsium arvense. Over the next 8 yr, different plant communities developed under different grazing regimes. In all communities, C. arvense was gradually replaced by Urtica dioica , and stands dominated by these two species expanded at the expense of P. australis. The shrub Sambucus nigra invaded the stands of C. arvense and U. dioica. Grazing affected the rate of these developments and the degree to which the grass Poa trivialis became dominant. When cattle were enclosed at a relatively high stocking rate in an area of Phragmites australis and tall herbaceous vegetation during summer, P. trivialis became dominant within 4 yr. The introduction of herbivores led to a faster spread of S. nigra , which contains cyanogenic glycosides which only ruminants can detoxify. Horses, as hind‐gut fermenters, did not feed on S. nigra . Grazing pressure and herbivore species, therefore, are two important variables that can be used to manage the development of Phragmites‐ and shrub vegetation: the greater the grazing pressure by cattle or horses the greater the area dominated by grasses, and a relatively high grazing pressure by cattle will retard S. nigra expansion.