INCREASES IN THE NET ABOVEGROUND PRIMARY PRODUCTION OF A SALT-MARSH FORAGE GRASS - A TEST OF THE PREDICTIONS OF THE HERBIVORE-OPTIMIZATION MODEL

The effects of grazing by captive goslings of lesser snow goose Chen caerulescens caerulescens on the vegetation of the La Perouse Bay salt marsh, Manitoba, were investigated. Goslings fed on different swards of Puccinellia phryganodes for up to 150 min. Net above-ground primary production (NAPP) and forage quality of vegetation were measured. Grazing early in the season resulted in increased NAPP of swards of Puccinellia, as predicted by the herbivore-optimization model. Greatest enhancement of production compared to that in ungrazed swards occurred in plots which received a low to moderate period of grazing (30-90 min). When the period of grazing was 120 or 150 min, NAPP was reduced. The increase in NAPP above that of ungrazed swards was dependent upon the presence of goose faeces. In the absence of faeces NAPP did not increase, indicating the deleterious effects of clipping per se by the geese on the regrowth of swards. A moderate period of grazing resulted in the maintenance of the N content of shoots (forage quality) throughout the season compared to the corresponding amount in shoots of ungrazed plants. Input of N from faeces alone was insufficient to account for the net accumulation of N in above-ground vegetation of swards which regrew following grazing. The ability of swards to recover from the effects of grazing decreased over the course of the summer. The growth response thus depends upon rapid recycling of nutrients via faeces, intensity of grazing, and the phenology of plant growth. -from Authors