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Effects of the Timing of Multiple Grazings by Geese on Net Above-Ground Primary Production of Swards of Puccinellia Phryganodes

D. S. Hik, H. A. Sadul and R. L. Jefferies
Journal of Ecology
Vol. 79, No. 3 (Sep., 1991), pp. 715-730
DOI: 10.2307/2260663
Stable URL: http://www.jstor.org/stable/2260663
Page Count: 16
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Effects of the Timing of Multiple Grazings by Geese on Net Above-Ground Primary Production of Swards of Puccinellia Phryganodes
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Abstract

(1) The effects of the frequency and timing of multiple grazings (including faecal input) by captive goslings of the lesser snow goose on net above-ground primary production (NAPP) and shoot nitrogen content of swards of the forage grass, Puccinellia phryganodes were examined. The effects of clipping and addition of nitrogen on the growth in pots of individual tillers of this grass were also investigated in a factorial experiment designed to separate the effects of each treatment. The experimental design of both investigations was closely based on the foraging behaviour of wild geese on Puccinellia swards. (2) The clipping of leaves per se which mimicked the leaf demography of shoots grazed by wild birds had a detrimental effect on shoot growth and NAPP. Addition of nutrients ameliorated the adverse effects of clipping on shoot growth. Plants which received nutrients, but whose leaves were not clipped, produced the highest amount of above-ground biomass. (3) During the summer of 1987 plots of Puccinellia were either grazed once every 12 days (one to six occasions), or once every 24 days (two or three occasions), or on three occasions at intervals of 12 days late in the season after the wild geese had left the marsh. Plots grazed on three occasions at intervals of 12 days from late June to early August during the period of rapid growth of above-ground biomass most closely mimicked the foraging behaviour of the wild geese. (4) Seasonal cumulative NAPP in 1987 was significantly higher in plots grazed on three occasions than in ungrazed plots, plots grazed once, or plots grazed repeatedly throughout the season. In 1988 plots were not grazed and a similar trend in NAPP was obtained, except that the NAPP of plots grazed late in the season in 1987 was low. The departure of wild geese in early August may be necessary if swards are to recover from the effects of defoliation. (5) During the summer of 1987, forage quality (N content) and NAPP together were highest in plots grazed on three occasions at intervals of 12 days--the treatment closest to the foraging pattern of wild birds. In July of the following summer shoot nitrogen content did not differ among the exclosed plots, irrespective of the treatment in 1987, and all values were lower than that for shoots from adjacent swards which were grazed and fertilized by wild geese. (6) The results show that NAPP and the nutritional quality of forage are dependent on faecal input, the number and timing of the grazing events and the phenology of plant growth. The role of the herbivore in maintaining grazing lawns of Puccinellia is discussed.

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