| Summary: | White spruce (Picea glauca, Voss) contains an antifeedant (camphor) which deters snowshoe hares (Lepus americanus Erxleben), a generalist herbivore, from feeding on it. Spruce was used as a model species to test the sometimes conflicting predictions of the Optimal Defense, and the Carbon: Nutrient Balance (CNB) theories of plant defense. The Optimal Defense theory predicts that plants will produce inducible defenses, this concentration being a function of the intensity of herbivore attack, and soil fertility. The CNB theory predicts that changes to the carbon: nutrient ratio will alter the relative amount of available carbon in the plant, and thus, the amount of defensive investment. Twig samples of white spruce were collected from small (0.5-1 m tall), medium (2-3m), and large (6-10m) trees growing in areas which have had Herbivore Exclusion, Fertilization, and Herbivore Exclusion + Fertilization treatments for 9 years. Twig samples were also collected from a group of medium-sized trees on Control and Fertilized areas that had been exposed to low and high levels of simulated herbivory. Medium and large trees were also sampled repeatedly during a period of fifteen months to examine yearly patterns of defensive investment. Medium and large trees on Herbivore Exclusion and Fertilization treatment areas had reduced defenses, with additively reduced defenses on the Herbivore Exclusion + Fertilization treatment area. Small tree defenses did not vary between treatment areas. Medium-sized trees exposed to low levels of simulated herbivory decreased defenses within the same year as treatment, and increased defenses the year after treatment. Medium-sized trees exposed to high levels of simulated herbivory had reduced defenses in both the same and the year following treatment. A yearly defense cycle was detected, with a minimum in June-July, and a maximum in December-January. Results suggest that spruce trees defend both optimally and as a response to carbon: nutrient limitation. A modified CNB theory is proposed that addresses problems in the original CNB theory, and has better predictive abilities. The defensive content (spartein) of arctic lupines {Lupinus arcticus S. Watts) was also investigated. No variation in defenses was found between lupines on different treatment areas. Spartein concentration was found to cycle on a daily basis, with a maximum at 2 am and a minimum from 10 am to 6 pm. The results of the lupine study are included in an appendix. Science, Faculty of Botany, Department of Graduate
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