This study in Quebec compared the effect of Scirpus americanus rhizome removal by greater snow geese (Chen caerulescens atlantica) inside and outside two tidal-marsh sanctuaries. In autumn, geese foraged mostly within the sanctuaries, whereas in spring, they made greater use of the adjacent marshes. Annual removal rate of S. americanus rhizomes by geese represented 9.4, 12.2, 23.1, and 5.7% g m-2 of the autumn (maximum available) below-ground biomass within and outside the sanctuary, at Cap St. Ignace and Montmagny, respectively, and 18.5, 22.8, 58.8, and 30.1% g m-2 of the net below- ground primary productivity. Although the use by geese was not strongly related to the differences in production between grazed and ungrazed plots, the greatest impact of geese was observed in the Montmagny sanctuary where the most intensive use was recorded. Net-above ground primary production of S. americanus was significantly higher in exclosures within both sanctuaries but not in the marshes adjacent to them. Fewer inflorescences of S. americanus were produced following feeding by geese but there was no effect on the number of seeds per inflorescence. The present study has shown that intensive use by geese can reduce the production of S. americanus and enhance the growth of Zizania aquatica. The Scirpus marshes, however, appear very resilient and no irreversible trends in the composition or production could be detected.
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