Foraging by geese, isostatic uplift and asymmetry in the development of salt-marsh plant communities

The response of different salt-marsh plant communities to grazing by lesser snow geese and isostatic uplift was examined at La Perouse Bay, Manitoba on the Hudson Bay coast. Results are based on direct experimental manipulation of swards and previously published floristic and biomass data. At the seaward end of the salt-marsh the vegetation is composed of Puccinellia phryganodes and Carex subspathacea. This assemblage of species is also present in the upper marsh, but only as a consequence of intense grazing by geese. Grazed swards of these species persist over a period of at least 10 years. When swards are protected from grazing in the upper marsh, rapid changes in floristic composition of vegetation occur. There is a substantial increase in above-ground biomass and the vegetation is dominated by Calamagrostis deschampsiodes, Festuca rubra and dicotyledonous plants. When these swards are grazed the species composition does not revert to the Puccinellia-carex assemblage. In the presence of grazing this change from the Puccinellia-carex assemblage to the Calamagrostis-Festuca assemblage occurs gradually as a consequence of changes in edaphic conditions associated with isostatic uplift. The geese delay, the rate of vegetational development associated with the effects of isostatic uplift but they cannot arrest it. Both types of plant assemblage may become extinct locally as a result of grubbing by the geese for roots and rhizomes of graminoid species. Habitat conditions are sufficiently altered following grubbing that the process of succession recommences from bare sediment. The results of changes in floristic composition, above-ground biomass and replacement rates of vegetation are examined in relation to predictions of multiple-state models which emphasize the directional asymmetry in the development of plant communities.