Effects of Mineral Nutrition on Delayed Inducible Resistance in Alaska Paper Birch

In subarctic forests, birch (Betula) trees respond to severe (50—100%) manual defoliation by delayed inducible resistance (DIR). This plant response to defoliation is characterized by a decline in the nutritional quality of leaves for immature insects for several years after defoliation events, and concomitant changes in leaf chemistry that may be detrimental to insect nutrition, that is, a decline in leaf nitrogen and an increase in leaf phenols. Two explanations of delayed inducible resistance have been proposed. (1) The active defense response hypothesis claims that delayed inducible resistance is an active response to defoliation per se rather than merely a passive consequence of recovery from the stress of severe defoliation. (2) In contrast, the carbon—nutrient balance (CNB) hypothesis claims that delayed inducible resistance is caused by nutritional stress resulting from severe defoliation. We used two experiments to test these hypotheses. (1) In a three—way factorial field experiment we reared spear—marked black moth (Rheumaptera hastata) larvae on Alaska paper birch (B. resinifera) saplings that had experienced combinations of 100% manual defoliation and fertilization with N and P in previous years, and measured larval survival and pupal mass. In association with these measurements of larval performance, we assayed leaf condensed tannin levels and the concentrations of N and P in leaves, and correlated the results of these assays with larval performance. (2) In a laboratory experiment we tested the biological activity of condensed tannin and linalool, the major secondary metabolites of Alaska paper birch leaves, by treating leaves collected from previously undefoliated Alaska paper birch saplings with combinations of condensed tannin and linalool, and measuring the performance of spear—marked black moth larvae reared on these leaves. Our results supported predictions of the carbon—nutrient balance hypothesis. We found that fertilization with nitrogen, the nutrient limiting growth of Alaska paper birch in our study site, mitigated delayed inducible resistance, and that condensed tannin is likely to be the major chemical cause of delayed inducible resistance in Alaska paper birch. In our field experiment we also found that fertilization of Alaska paper birch with phosphorus, a nutrient that does not limit the growth of Alaska paper birch in our study site, affected levels of condensed tannin in leaves and the sapling's defoliation history influenced this effect. Thus, future studies of the effects of mineral nutrition on secondary metabolite production by woody plants and their responses to herbivory should consider nutrients that do not limit growth in addition to those that do limit growth.