A 5-year study of the effects of nutrient availability and herbivory on two boreal forest herbs

1) The responses of populations of Mertensia paniculata (bluebells or lungwort) and Anemone parviflora (small-flowered anemone) to herbivore exclosure and fertilization in a factorial experiment were monitored over a 5-year period beginning at peak herbivore (hare) densities during the snowshoe hare population cycle. For each species the population density, number and size of leaves and the number of flowers were measured. 2) Both species responded more strongly to fertilizer addition than to the exclusion of herbivores. Mertensia produced more flowering stems, more leaves per stem, and stem density increased in the fertilized plots. Fertilizing increased total leaf length per plant for non-flowering stems but this was not observed for flowering stems. However, the net effect of having more flowering stems and having greater leaf area on nonflowering stems was to increase the total leaf area of the population. The responses of Mertensia make it likely to become a stronger competitor in a more productive plant community. 3) Anemone showed contrasting responses at both individual and population levels. While individual stems produced slightly more leaves in fertilized plots, the density of stems declined. There were no strong effects on either leaf size or flowering. There was evidence of higher leaf turnover in fertilized plants. Meanwhile, control and exclosed unfertilized plots showed an increase in population density. 4) The weak responses to herbivory may be explained by the timing; this part of the experiment was run during a period of declining herbivore activity. However, observed interaction effects suggest that those herbivores remaining in the system may be attracted to fertilized plots. It is planned to continue the experiment for at least another 5 years through, and beyond, the next hare peak.