Short and long term recovery of plant communities following intensive grazing by caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus) in the Low Arctic of Nunavut, Canada

Short and long term recovery of plant communities following intensive grazing by caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus) were examined in the Low Arctic of Nunavut, Canada. Animals stranded on small islands resulted in intensively grazed the plant communities, creating natural grazing "experiments." Field work was conducted during July and August of 1997, at four locations. Islands intensively grazed by muskoxen (in 1980, 1984, 1996 and 1997) and by caribou (in 1987) were measured for plant cover and biomass, and current grazing intensity. Regression was used to detect correlations between plant cover and biomass. For most species, cover was a poor predictor of biomass. Detrended Correspondence Analysis (DCA) clearly separated sites by latitude. Direct gradient ordination (Redundancy Analysis) of each location showed the importance of soil moisture and slope position to vascular plant cover, and current grazing pressure and previous intensive grazing to biomass. Intensive grazing affected cover and biomass for at least one year. After thirteen years recovery of vascular plants was complete. Analysis of Variance (ANOVA) of all locations showed that cover of all species and functional groups (forbs, graminoids, lichens, shrubs) differed significantly between intensively grazed island and adjacent mainland sites. Cover of all species differed significantly by location and by site. There was no evidence of overcompensation, rather equal or undercompensatory growth occurred. Significant differences were found among intensively grazed islands, adjacent islands and mainland sites. There were significant differences between the islands and between the adjacent island and the mainland sites, but no significant differences between the intensively grazed island and the mainland. Mainland sites had levels of grazing similar to those of the recently, intensively grazed islands. Thus, it is necessary to compare islands, rather than island and mainland sites, to detect grazing effects. No significant differences were found between locations after thirteen years recovery from intensive grazing by muskoxen. Intensive grazing by caribou had longer-term effects than that by muskoxen. Caribou eat lichen whereas muskoxen trample and leave thalli in situ. Ten years after grazing by caribou, non-crustose lichen cover on the intensively grazed island was lower than the mainland, whereas 1 month, 13 and 17 years after grazing by muskoxen, cover was equal or greater than adjacent sites. The fertilization effect of decomposing muskoxen carcasses was evident 13 years after 27 muskoxen were killed at Merkeley Lake. Growth in some species was nutrient limited; cover and biomass of surrounding graminoids and Dryas integrifolia and cover of mosses were greatly increased. Salix spp. were not nutrient limited, and their growth was unaffected by nutrient addition. Without stranded grazers, small islands were protected from summer grazing by ungulates, and grazing levels were lower than on the mainland. With stranded caribou or muskox, however, grazing impacts were greatest on these islands, intermediate on mainland sites, and least on adjacent islands. Upland sites experienced greatest grazing pressure. The recently, intensively grazed sites had temporarily lower species richness. Discrete episodes of intensive grazing on small islands do not appear to cause permanent plant community change.

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