The growth response of graminoid plants to goose grazing in a High Arctic environment

1 The response of plants to herbivory usually varies with the grazing regime experienced. We investigated (i) if the timing and frequency of grazing affected plant growth, (ii) if faeces deposition by herbivores stimulated plant growth, and (iii) if grazing affected the total nonstructural carbohydrate (TNC) reserves in the below-ground vegetation of two arctic graminoids, Dupontia fisheri and Eriophorum scheuchzeri. 2 This study was conducted in polygon fens exposed to intense summer grazing by greater snow geese (Chen caerulescens atlantica) on Bylot Island (73 degrees N) in the Canadian High Arctic. We manipulated the frequency (once or three times) and the timing (early, mid or late in the season) of grazing and faeces deposition in controlled grazing trials using captive goslings. 3 Although ungrazed plants were taller than grazed ones at the end of the season, data on cumulative tiller elongation (net above-ground height production) showed that plants grazed once or three times produced new foliage after each defoliation in both species. However, neither grazing (presence or absence) nor its frequency affected the net above-ground primary production (NAPP) or the number of tillers at the end of the summer. Nitrogen concentration was highest in plants grazed three times, intermediate in those grazed once, and lowest in ungrazed plants. 4 Timing of grazing and presence of goose faeces with or without grazing had no effect on plant growth. 5 Eriophorum plants grazed three times had less TNC in their below-ground tissues than ungrazed plants, and the trend was similar in Dupontia. 6 Dupontia and Eriophorum were able to compensate for leaves lost to grazing and to maintain production at a level similar to ungrazed plants, but at some cost (reduced below-ground reserves). The absence of an effect of faeces on plant growth may explain the absence of a positive effect of grazing on NAPP (i.e. overcompensation) in this ecosystem.