John and Turkington investigated the impact of nutrient addition and mammalian exclosures on the above-ground biomass and species composition of the understory vegetation of the boreal forest of southern Yukon in three field experiments. Experiment 1 was run from 1990-1992 during which time the major herbivore, the snowshoe hare (Lepus americanus), declined dramatically in numbers. It combined the addition of nutrients with the exclusion of herbivores in a 2 X 2 factorial design. Experiment 2 was run over an eight-week period in 1991, and tested the effects of exclosures on above-ground plant biomass at a range of snowshoe hare densities. Experiment 3 examined the effects of long-term (6-yr) exclosures, erected in 1987, on understory species composition. At natural densities the impact of herbivores on vegetation is low compared with the effect of fertilizers. Fertilizer resulted in some species increasing in abundance while others decreased. Where herbivores were at artificially high densities their impact was greater. There is a natural dynamic to the system as some species changed in abundance in control plots during the experiment. The results suggest that both the composition and abundance of herbaceous vegetation in the boreal forest are determined more by the productivity of the site than the activities of mammalian herbivores.
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