Experimental studies of the effects of grazing by captive goslings of the lesser snow goose on the vegetation of a subarctic salt marsh.
SUMMARY:1.The effects of grazing by captive goslings of the Lesser Snow Goose on the vegetation of La Pérouse Bay salt marsh were investigated. Goslings fed on swards of Puccinellia phryganodes; net above-ground primary production (NAPP)and forage quality (amounts of nitrogen and carbon) were measured over the course of the season.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 occurred at low to moderate levels of grazing compared to production in ungrazed swards. At higher levels of grazing production was reduced. Enhancement of NAPP following grazing was detected only early in the season; the ability of the plants to recover from the effects of grazing decreased later in the season.3.Repeated grazing episodes on the same swards of Puccinellia over the course of the summer demonstrated that swards could maintain high rates of production, and high nitrogen content of shoots even when they were grazed late into the season. Swards of Puccinellia grazed at approximately 24 day intervals throughout the summer and swards grazed at approcimately 12 day intervals on four occasions had the largest increase in NAPP compared to ungrazed swards. Increases in NAPP were achieved over a wide range of grazing periods and intervals between grazing episodes.4. NAPP was 30 to 40% less in swards of Puccinellia where faeces were removed compared to corresponding values for swards where the faeces remained. Faeces provided a source of soluble nitrogen that could be readily taken up by plants. However, the amount nitrogen contributed by the feaces to the total amount of nitrogen accumulated in the above-ground biomass of grazed swards of Puccinellia was less than 10%. Up to 44% of total nitrogen accumulated in the above-ground biomass of grazed swards could not be accounted for.5. Nitrogen mineralisation processes which occur in goose faeces may be important in maintaining the nitrogen balance of the salt marsh. Rates of microbial respiration and the net mineralization of nitrogen in fresh faeces from gosings grazing Carex subspathecea were higher than corresponding values for faeces derived from Puccinellia phryganodes and calamagrostisu deschampsiodes. However the mineralisation efficiency (net mineralisation rate / respiration rate) was similar in faeces derived from Carex and Puccinellia. The ratios were considerably higher than the corresponding rate in faeces derived from Calamagrostis. The digestibility of Calamagrostis shoots was considerably lower than that of carex and Puccinellia shoots based on analysis of acid-detergent fibre.6. The interactive and dynamic processes of nitrogen cycling between plants, geese, sediments and microbes lead to enhanced primary production in this grazed system. A suite of interactions exist which extend the influence of plant-herbivore interactions beyond the immediate participants, and involve all trophic levels in this grazing system.
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