Western Palaearctic breeding geese can alter carbon cycling in their winter habitat

Changes in land use, implementation of protective measures and a warming climate have improved the survival rate of geese, resulting in considerable increases in the majority of Western Palaearctic goose populations during recent decades. To the best of our knowledge, this is the first study aiming to understand the impact of goose grazing on carbon cycling in their winter habitat. To this end, the impact of goose grazing pressure on biomass, litter decomposition and CO2fluxes (net CO2exchange partitioned into photosynthesis and ecosystem respiration) was studied in the coastal polders of Belgium, a wintering habitat for geese of international importance. Experimentally manipulated grazing byAnser anser(Greylag Geese) in grassland mimicked four different grazing pressures, including a control treatment from which geese were excluded. We found that grazing pressure by geese has a significant, but variable effect on carbon fluxes during the entire year. In winter, at the end of the grazing season, both plants' carbon assimilation and total ecosystem respiration were decreased with increasing grazing pressure, resulting in less carbon taken up during day time. Total ecosystem respiration was also reduced due to goose grazing in spring and autumn (i.e., outside the grazing season), while no significant difference in ecosystem CO2fluxes was detected in summer. These grazing effects on CO2fluxes can partly be explained by the effect of goose grazing on standing biomass. Decomposition rates were significantly reduced by higher grazing pressure during the winter season when geese were present, but on the long term grazing accelerated decomposition rates. Our data suggest that the rising numbers of Western Palaearctic breeding geese can alter the carbon balance of their winter habitat. The differences between short‐ and long‐term effects observed in our study demonstrate the complexity of goose grazing effects on carbon cycling and indicate directions for future studies.