Effect of grazing by greater snow geese on the production of graminoids at an arctic site (Bylot Island, NWT, Canada)

1 Arctic ecosystems are sensitive to grazing because of their low overall net primary production. We therefore studied the effect of greater snow geese on the production of arctic graminoids. 2 At Bylot Island, NWT (73 degrees N), breeding snow geese graze Eriophorum scheuchzeri and Dupontia fisheri in lowland polygon fens. Vegetation in seasonal exclosures was sampled at 2-week intervals from mid-June to mid-August 1990, 1991 and 1993 (ungrazed areas) and compared with plant growth in short-term exclosures set up over grazed areas. Standing crop (above-ground biomass), net above-ground primary production (NAPP) and nitrogen content were determined for both plant species. 3 Goose faeces were used as an index of grazing intensity. Cumulative faeces counts increased from 2.2 faeces m-2 in 1990-9.9 faeces m-2 in 1993. 4 Peak above-ground dry biomass of ungrazed areas averaged 33 g m-2. In all years, goose grazing significantly reduced the above-ground biomass of Eriophorum and Dupontia. Over the 3 years, we estimated that geese consumed from 65 to 113% of the cumulative NAPP of Eriophorum, and from 30 to 78% of the cumulative NAPP of Dupontia. Consumption of Dupontia, but not Eriophorum, was related to grazing intensity. 5 Cumulative NAPP of grazed areas was slightly lower than that of ungrazed areas at the end of the summer. On an individual plant basis, cumulative NAPP of Dupontia, but not of Eriophorum, was reduced by grazing. Nitrogen content of plants after grazing was higher than in ungrazed plants. 6 Although grazed plants were able to grow new foliage, goose grazing did not enhance NAPP at Bylot Island as has been reported elsewhere. This could be either because grazing occurs too late in the season when the regrowth capacity of plants is low or because nutrients released from goose faeces are absorbed by mosses and are not immediately available to graminoids.