Effects of summer grazing by reindeer on composition of vegetation, productivity and nitrogen cycling

In this study, we investigated the effect of reindeer grazing on tundra hea th vegetation in northern Norway. Fences. erected 30 yr ago, allowed us to compare winter grazed, lightly summer grazed and heavily summer grazed vege tation at four different sites. At two sites, graminoids dominated the heav ily grazed zone completely, while ericoid dwarf shrubs had almost disappear ed. In the other two areas, the increase of graminoids was almost significa nt. At one of the sites where graminoids dominated the heavily grazed area. we also measured plant biomass, primary production and nitrogen cycling. I n this site: heavy grazing increased primary production and rate of nitroge n cycling, while moderate grazing decreased primary production. These resul ts were inconsistent with the view that the highest productivity is found a t intermediate grazing pressure. These results rather support the hypothesi s that intensive grazing can promote a transition of moss-rich heath tundra into productive, graminoid-dominated steppe-like tundra vegetation. More o ver the results suggests that intermittent intensive reindeer grazing can e nhance productivity of summer ranges.