The influence of goose grazing on the growth of Poa arctica: overestimation of overcompensation

The effect of grazing by barnacle geese (Branta leucopsis) on above-ground production was investigated on the Arctic tundra grass Poa arctica at Spitsbergen during the growing season of 1996. We applied three treatments: orated; whole season ungrazed and temporary ungrazed during the second half of the season. In each treatment individual shoots were marked and their leaves regularly measured to calculate production, Production was calculated in three different ways: Increment in Standing Crop (ISC), Above-ground Biomass Production (ABP) and leafbirths. ISC in temporary and permanent exclosures is the most commonly used method, but it neglects senescence and disappearance of biomass during the experiment. In our study, ISC of grazed Poa arctica was significantly higher than of ungrazed swards. However, the flow of senescing biomass was much lai ger in ungrazed swards compared to grazed swards. Thereby an overestimation of overcompensation was introduced. Calculation of ABP takes this flow of senescing biomass into account. In terms of ABP, Poa arctica did not overcompensate the losses due to goose grazing. Accordingly, the number of leafbirths did not differ between grazed and ungrazed swards. In several other studies, where senescence and disappearance of leaves were neglected, inclusion of these fluxes would at least decrease overcompensation. Therefore, we advocate regarding senescence and disappearance of leaves while measuring overcompensation.