FERTILIZATION MITIGATES CHEMICAL INDUCTION AND HERBIVORE RESPONSES WITHIN DAMAGED OAK TREES

Previous work has shown that fertilization can mitigate across-year induced resistance among individual Alaska paper birch, supporting the hypothesis that induction results from nutrient deficiency rather than active defense. The present study suggests that fertilization can prevent induction responses in oak within one season, and among leaves within individual trees. Saplings of two oak species, divided among fertilizer treatments within a plantation, were defoliated to varying degrees by a gypsy moth outbreak during spring. We monitored foliage chemistry and insect herbivore distributions during summer and fall of the same year. Damaged leaves within unfertilized Quercus prinus saplings showed increases in foliar astringency and proanthocyanidins. Within fertilized trees, damage did not induce increases in either astringency or proanthocyanidins. Defoliation also induced increases in proanthocyanidins and astringency within Q. rubra saplings, but induction was unaffected by fertilization. On both tree species, the distributions of certain insect guilds were skewed away from damaged leaves on unfertilized trees, but independent of damage on fertilized trees. We suggest that the effects of fertilization on induction can vary among tree species, and can occur at more than one spatial and temporal scale. Our data support the view that nutrient availability may affect phytochemical induction and herbivore responses.