Abstract
Previous studies have shown that the growth of fresh-water grasses and sedges eaten by breeding colonies of Snow Geese responds weakly to nitrogen additions, and also is poorly able to compensate within the same season for tissues lost to geese. These results contrast with the rapid responses to grazing and fertilization that have been observed in salt-marsh species. A possible explanation is that the mosses prominent in fresh-water wetlands rapidly sequester added nitrogen, preventing access by forage species to the fecal inputs provided by foraging geese. To investigate this hypothesis, I added ecologically realistic amounts of ammonium and nitrate labelled with 15N to the surface and rooting zone of experimental plots in fresh-water wetland vegetation at two Snow Goose colonies. Results indicate that the presence of mosses did not prevent forage species from rapidly taking up ammonium and nitrate added either at or below the moss surface. Nonetheless, most of the added 15N was absorbed by the moss layer; consequently, mosses tend to divert nitrogen away from forage species and into long-lasting peat. In the long term, this may reduce the ability of fresh-water forage plants to recover from damage by increasing populations of Snow Geese.