Abstract
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 CO2 fluxes (net CO2 exchange 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 by Anser 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 CO2 fluxes was detected in summer. These grazing effects on CO2 fluxes 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.
Highlights
Global temperatures have increased and recent simulations suggest that the global average surface temperature will increase even more in the coming decades (IPCC 2007)
In a field experiment with manipulated grazing by Greylag Geese imposing different grazing pressures, information was gathered on the biomass, decomposition rates and carbon fluxes of temperate grasslands to assess the consequences of winter grazing by geese for two key processes that contribute to the carbon balance of a temperate grassland ecosystem: photosynthesis and ecosystem respiration, and this in different seasons
This study demonstrated that grazing pressure by geese in their wintering habitat can significantly impact carbon cycling in these temperate grasslands
Summary
Global temperatures have increased and recent simulations suggest that the global average surface temperature will increase even more in the coming decades (IPCC 2007). Recent research in the Arctic breeding sites has revealed that these birds influence the dynamics of CO2 exchange between the biosphere and atmosphere by reducing carbon stocks and ecosystem sink strength (Sjogersten et al 2008, Speed et al 2010, Sjogersten et al 2011), which in turn feed back to the climate (Chapin et al 2008). Geese are migratory birds and likely to impact on the carbon balance of their wintering habitats: temperate grasslands, which usually act as sinks for greenhouse gases (Soussana et al 2007, Ostle et al 2009, Schulze et al 2009)
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