A positive feedback: Herbivory, plant growth, salinity, and the desertification of an Arctic salt-marsh

1 A 2-year study is described which suggests that a positive feedback process results in the destruction of salt-marsh swards and the exposure of bare sediments at La Perouse Bay, Manitoba, Canada. Lesser snow geese initiate the process by grubbing for roots and rhizomes of salt-marsh graminoids (Puccinellia phryganodes and Carex subspathacea) in spring. The increased rates of evaporation from sediments beneath disturbed or destroyed swards in summer result in high soil salinities that adversely affect the growth of the remaining grazed plants. 2 Above-ground biomass and soil salinity differed between sites in the salt marsh. Soil salinity was inversely related to above-ground biomass and shoot density of Puccinellia phryganodes. Increased biomass led to reduced soil salinity at sites where exclosures were erected. 3 Plant growth, measured as the rate of leaf births on Puccinellia shoots, was reduced by high soil salinities at sites where exclosures were erected. 4 Leaf demography of transplanted experimental plants of Puccinellia differed in 1992, but not 1991, between plants transplanted into sites with different amounts of above-ground biomass. Leaf births and deaths were highest for plants grown in sites where above-ground biomass was high and lowest for plants transplanted into bare sites. Grazing had no effect on leaf demography in 1991 and only marginally increased the rate of leaf deaths in 1992. 5 Growth of transplanted individuals of Car er subspathacea was similarly highest at sites where the standing crop of Puccinellia and Carer was high and was lowest in bare sites. 6 Algal crusts, which formed on bare or poorly vegetated sites, also reduced the growth of Puccinellia plants. 7 The effects of this deleterious positive feedback on plant growth are discussed in relation to changes occurring in the lesser snow goose colonies at La Perouse Bay and elsewhere.