Long-term effects of yearly grazing by moulting Greylag geese ( Anser anser) on reed ( Phragmites australis ) growth and nutrient dynamics

Abstract

In this study, the effects were quantified of grazing by moulting Greylag geese in spring on biomass and nutrient dynamics of monospecific stands of Phragmites australis. A comparison was made between a marsh which had been grazed for more than 15 years, and a site which had not been grazed for at least that period. Both marshes bordered the same lake. Net above-ground primary production (NAPP) and N and P allocation to above-ground parts were estimated for two growing seasons from regular measurements of the standing shoot biomass and its N and P content, and by using temporary exclosures during goose grazing. Annual NAPP was similar between grazed and ungrazed areas (1997) or higher in the grazed area (1999). Rhizome biomass was similar between areas but the timing of minimum biomass shifted from June to September with grazing. A higher spring shoot emergence due to lower standing dead mass in the grazed area and relocation of reserves from rhizomes to shoots were important in sustaining/increasing NAPP with grazing. Grazing by moulting Greylag geese in spring approximately doubled the annual allocation of N and P to shoots, and almost tripled the annual above-ground losses of these elements compared to an ungrazed reed vegetation. In the grazed area, 50% or more of the annual nutrient losses were due to goose activity, as no nutrients were resorbed from the nutrient-rich tissue that was consumed or torn and rejected. Differences in timing of shoot emergence in spring were important in affecting the extent of nutrient loss due to goose activity. The increased N and P losses as a result of grazing were associated with consistently lower N and P concentrations in the rhizomes, but only in 1999 were nutrient reserves in rhizomes over winter lower in the grazed reed stand. Resorption efficiency from senescent tissue was increased with grazing in 1997 but not affected in 1999. Our results indicate that grazing increased growth and nutrient dynamics of P. australis in this eutrophic reed marsh.