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Increases in the Net Above-Ground Primary Production of a Salt-Marsh Forage Grass: A Test of the Predictions of the Herbivore-Optimization Model

D. S. Hik and R. L. Jefferies
Journal of Ecology
Vol. 78, No. 1 (Mar., 1990), pp. 180-195
DOI: 10.2307/2261044
Stable URL: http://www.jstor.org/stable/2261044
Page Count: 16
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Increases in the Net Above-Ground Primary Production of a Salt-Marsh Forage Grass: A Test of the Predictions of the Herbivore-Optimization Model
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Abstract

(1) The effects of grazing by captive goslings of the lesser snow goose on the vegetation of the La Perouse Bay salt marsh were investigated. On seven separate occasions during the summers of 1986 and 1987 goslings fed on different swards of Puccinellia phryganodes for up to 150 min. Net above-ground primary production (NAPP) and forage quality (amounts of nitrogen and carbon in tissues) of vegetation were measured in grazed and ungrazed plots. (2) Grazing early in the season resulted in increased NAPP of swards of Puccinellia, as predicted by the herbivore-optimization model. The 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. (3) 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. (4) A moderate period of grazing resulted in the maintenance of the nitrogen content of shoots (forage quality) throughout the season compared to the corresponding amount in shoots of ungrazed plants. Input of nitrogen from faeces alone was insufficient to account for the net accumulation of nitrogen in above-ground vegetation of swards which regrew following grazing. (5) The ability of swards to recover from the effects of grazing decreased over the course of the summer. With each successive experiment the maximum amount of regrowth decreased, and occurred in plots which received shorter grazing bouts, reflecting the increased foraging efficiency of the goslings. (6) The growth response exhibited by swards of Puccinellia when grazed depends upon the rapid recycling of nutrients via faeces, the intensity of grazing, and the phenology of plant growth. The ability of the herbivore-optimization model to predict the response of vegetation to the effects of grazing is relatively limited unless these and other factors are considered.

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