We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.

We characterized the structure of condensed tannins from 16 woody plant species (seven genera, six families) and determined their effects on six herbivorous insect species (four genera, two families). There were major differences in tannin structure, even between congeneric plant species. Condensed tannins differed markedly in their antiherbivore activity, averaged over these herbivores, and the herbivores differed in their sensitivity, averaged over these tannins. Furthermore, the same tannin can have different effects on different herbivores, presumably because of interactions between tannin structure and gut physiology. Results challenge the view that tannins provide an evolutionarily stable plant defense because of their uniform chemical properties. Condensed tannin can sometimes impact herbivore fitness through effects on survival and growth, but the largest effects in 45 insect-tannin combinations were less than that of many other plant metabolites at lower doses. Even at high doses, condensed tannins frequently had no strong antiherbivore activity, even against insects with no evolutionary history of encountering the tannin (Ͻ10% reduction in growth rate in 24 of 45 experiments). Most condensed tannins apparently do not have broad-spectrum antiherbivore activity. We doubt that selective pressures from folivorous insects can be the main explanation for the diversion of so much carbon, in so many plant species, into the synthesis of condensed tannins.

The transition from alder (Alnus tenuifolia) to balsam poplar (Populus balsamifera) is a critical turning point in primary succession on river floodplains in interior Alaska. Associated with the change in plant species are large changes in N cycling. N-fixation and nitrification decrease and the system becomes N-limited, with NH4+ dominating the inorganic N pool. Balsam poplar leaves contain large quantities of tannins and low molecular weight phenolic compounds. We evaluated the effect of these compounds on microbial respiration and N cycling in laboratory assays on soils from an alder-dominated site. Plant compounds were purified and applied to silica gel as an inert carrier. Both tannins and phenolics caused net N-immobilization over a 30-day assay. However, tannins inhibited respiration while phenolics stimulated it. There were no specific effects on nitrification. Thus, tannins acted as a general microbial inhibitor, while phenolics acted as a growth substrate. By inhibiting mineralization while stimulating immobilization, poplar secondary compounds may reduce soil N-availability during the transition betwen alder and poplar stages in succession. Keywords: respiration, mineralization, tannins, secondary chemicals, succession, plant–microbe interactions.

Mature growth-phase internodes of Alaska paper birch (Betula resinifera) are preferred by the snowshoe hare (Lepus americanus) over juvenile growth-phase internodes due to the low food value of the latter. While the mature over juvenile preferencec cannot be explained by the levels of inorganic nutrients or gross chemical fractions (resins or phenols), it can be explained by the striking differences in secondary metabolites of the two growth phases. The principle compound which renders the juvenile phase internodes unpalatable is papyriferic acid, a triterpene which is a demonstrated feeding deterrent to snowshoe hares and which is present in juvenile internodes at concentrations 25 times greater than those in mature internodes.

Plant carbon/nutrient balance has been implicated as an important factor in plant defensive chemistry and palatability to herbivores. We tested this hypothesis by fertilizing juvenile growth form Alaska paper birch and green alder with N, P and N-plus-P in a balanced 2x2 factorial experiment. Additionally, we shaded unfertilized plants of both species. Fertilization with N and N-plus-P increased growth of Alaska paper birch, reduced the concentration of papyriferic acid in internodes and increased the palatability of birch twigs to snowshoe hares. Shading decreased birch growth, decreased the concentration of papyriferic acid in internodes and increased twig palatability. These results indicate that the defensive chemistry and palatability of winter-dormant juvenile Alaska paper birch are sensitive to soil fertility and shade. Conversely the defensive chemistry and palatability of green alder twigs to snowshoe hares were not significantly affected by soil fertility or shade. The greater sensitivity of Alaska paper birch defensive chemistry and palatability to snowshoe hares in comparison to green alder is in agreement with the hypothesis that early successional woody plants that are adapted to high resource availability are more plastic in their chemical responses to the physical environment than are species from less favorable environments.

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The carbon/nutrient balance hypothesis fails to correctly predict effects of fertilization and shading on concentrations of defensive metabolites in Alaskan balsam poplar (Populus balsamifera). Of six metabolites analyzed, only one responded in the predicted fashion to fertilization and one to shading. These results and those of other similar studies suggest that while the carbon/nutrient balance hypothesis may correctly predict the effects of fertilization and shading on the concentrations of metabolic "end products", it fails for many metabolites because of the dynamics associated with their production and turnover. In metabolites that turn over, static concentration is a poor predictor of defensive investment.

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.