HERBACEOUS VEGETATION IN THE UNDERSTOREY OF THE BOREAL FOREST - DOES NUTRIENT SUPPLY OR SNOWSHOE HARE HERBIVORY REGULATE SPECIES COMPOSITION AND ABUNDANCE

1 The impact of nutrient addition and mammalian exclosures on the above-ground biomass and species composition of the understorey vegetation of the boreal forest were investigated in three field experiments. 2 Experiment 1 was run from 1990-92 during which time the major herbivore, the snowshoe hare, 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 8-week period in 1991, and tested the effects of exclosures on aboveground plant biomass at a range of snowshoe hare densities. Experiment 3 examined the effects of longer-term (6-year) exclosures, erected in 1987, on understorey species composition. 3 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 and others decreasing. 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. 4 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, at least during the period of the experiments when hare numbers were declining naturally.