| Summary: | The interactions between muskoxen (Ovibos moschatus) and graminoid meadows, their primary summer range, were examined in an area of high muskoxen density on north-central Banks Island, N.W.T., Canada (73°50$\sp\prime$N, 119°53$\sp\prime$W). The objective was to test the herbivore optimization hypothesis, i.e., moderate grazing intensities boost rates of net above-ground primary productivity (NAPP) above that of ungrazed, lightly grazed and heavily grazed levels.) Microplots (1 x 1m) were clipped once (1x) or twice (2x) per growing season, exposed to muskoxen, temporarily protected from muskoxen for one growing season, or permanently protected. These treatments removed from zero to 40% of shoot tissue each growing season. Contrary to the predictions of the herbivore optimization hypothesis, over-compensation did not occur anywhere across this gradient of tissue removal. Four methods of calculating net above-ground primary productivity (NAPP) consistently demonstrated that the more intense treatments (1x and 2x clipping) showed more under-compensation (64% of the time) while less intense treatments (grazing by muskoxen) show more exact-compensation (75% of the time). Clipping and grazing also decreased below-ground standing crop with less severe treatments showing exact-compensation. Mortality and fecundity of Eriophorum triste (Th. Fries) Hadac & Love populations were unaffected by grazing and clipping. Clipping (2x) or intense grazing treatments initially decreased rates of leaf extension, rates of increase in height and rates of increase in number of leaves per tiller, however within a year these parameters were boosted above or equaled control levels. Less intense treatments decreased population-level growth rates during the entire study suggesting that severe shoot tissue removal and a time lag of at least one year after the initial treatment are required to initiate compensatory growth in Eriophorum triste populations. The study site with the highest density of muskox fecal pats had the largest green standing crops, the most nutritious forage, the warmest microclimate, the highest soil NH<sub>4</sub> levels and the most favorable soil moisture regime for plant growth. High standing crops were the result of large quantities of overwintering green tissue rather than increased rates of NAPP. Although compensatory growth occurred in intensely clipped and grazed populations of Eriophorum triste, community-level data demonstrate that graminoid meadows in the study area are limited in their ability to tolerate high grazing and clipping intensities. Increased tiller growth in Eriophorum triste populations was not great enough to offset the community-level response. Increased forage quality and large green standing crops after grazing serve to ameliorate the potentially negative effects of reduced NAPP on muskoxen.
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