RESPONSES OF SALT-MARSH VEGETATION TO GRAZING BY LESSER SNOW GEESE (Anser caerulescens caerulescens).

SUMMARY:

1. The mechanisms by which grazing by lesser snow geese bring about an increase in cumulative net above-ground primary production (NAPP) of salt marsh vegetation at La Pérouse Bay, Manitoba were investigated.

1. In the absence of grazing, cumulative NAPP was approximately 80 g m-2 in swards dominated by Puccinellia phryganodes in 1982 and 1983. Grazing by snow geese resulted in increases in cumulative NAPP of 46% in 1982 and 106% in 1983. These increases were significant (p < 0.05).

1. There was a significant correlation between values of leaf Brea index and live standing crop in grazed and ungrazed sites (p < 0.05).

1. Grazing stimulated tillering in P. phryganodes. This increase in axillary shoot production accounted for the increase in cumulative NAPP of grazed sites compared with ungrazed sites in 1982 and 1983.

1. The rate of appearance of leaves and the rate of death of leaves was similar on main shoots of P. phryganodes in both grazed and ungrazed sites. However, the total number of leaves produced on axillary shoots of grazed plants was substantially higher than that of ungrazed plants. The average age of a leaf when grazed was 14 days, whereas the mean age of ungrazed leaves was 35 days.

1. Significant increases in live standing crop were observed in exclosed plots fertilised with fresh snow goose drqppings compared with that in control plots which received no droppings (p < 0.05).

1. Rates of nitrogenase activity, measured by acetylene reduction, were consistently greater in grazed than in ungrazed sites in 1983.

1. Grazing maintained the total nitrogen content of P. phryganodes at a level of 2.4% on a dry weight basis, while that of ungrazed shoots of P. phryganodes declined by 30% two weeks after hatch.

1. In the absence of snow geese, over a period of four years plant species diversity increased in exclosures while live standing crop declined and litter accumulated. The soil environment of ungrazed sites was cooler than that of grazed sites.

1. It was concluded that the intensive grazing activities and colonial feeding behaviour of snow geese result in higher nitrogen availability for plant growth in grazed sites, and the maintenance of better quality forage as a result of the higher nitrogen content. Nitrogen is supplied from goose droppings and increased rates of nitrogen fixation in grazed sites. P. phryganodes, a grass, is able to withstand heavy grazing, and plants respond by producing tillers so that the amount of forage 13 increased significantly.