Role of Ice Scouring and Goose Grubbing in Marsh Plant Dynamics

1 The effects of scouring by ice and of grazing by the greater snow goose (Chen caerulescens atlantica) on the substrate and vegetation of a brackish tidal marsh in the St Lawrence estuary were studied from 1985 to 1987. During spring and fall migratory stopovers, the birds fed mainly on Scirpus americanus rhizomes. 2 The presence of ice-made depressions was related mainly to the physical characteristics of Scirpus marshes. Goose grubbing has no significant effect on the annual dynamics of these depressions. 3 In exclosures, i.e. iii the absence of geese, ice drift has little influence on species abundance and net above-ground primary production of the marsh except at high levels of disturbance. 4 In ice-made depressions, goose grazing reduced net primary production by approximately 45% and facilitated colonization by competitive species such as Zizania aquatica, Sagittaria spp. & Scirpus torreyi. History or frequency of disturbance seems to play an important role in this process. 5 By exploiting ice-made depressions, snow geese contribute to an increase in the heterogeneity but a decrease in the primary production of the Scirpus marsh.