IMPACT OF GRAZING AND ATMOSPHERIC NITROGEN DEPOSITION ON THE VEGETATION OF DRY COASTAL DUNE GRASSLANDS

A five-year experimental study was carried out to examine the combined effects of grazing and atmospheric nitrogen deposition on the vegetat ion of three dry dune grasslands: one short species-rich, one short sp ecies-poor, and one predominated by tall graminoids. Additional fertil ization with nitrogen had no significant effect, neither in grazed nor in non-grazed plots. Exclusion of grazing by rabbits resulted in an i ncrease in the frequency of perennial graminoids and a decrease in the frequency of annual graminoids and herbs. Nevertheless, species diver sity remained the same in the species-rich grassland. During the exper iment, the above-ground biomass increased in all non-grazed plots and the amount of bale soil and mosses decreased. The vegetation changes o ccurred mainly within one year after the exclusion of grazing. An exce ption is the grass-dominated site where the amount of Calamagrostis ep igejos increased gradually from ca. 20 % in the first two years to abo ut 50 % in the fourth and fifth year. Grazing by rabbits seems essenti al to prevent graminoids to become predominant in the dry dunes. If gr aminoids are dominant, grazing by horses can be an appropriate method to restore the original grassland vegetation. After six months of graz ing by horses the grass-dominated site showed a decrease of the freque ncy of perennial graminoids, from 95 % to 80 %, and an increase of the frequency of perennial herbs, from 2.5 Cio to between 13 and 20 %.