Consequences of manipulations in carbon and nitrogen supply for concentration of anti-herbivore defence compounds in Salix polaris

The concentration of carbon-based anti-herbivore defence compounds is key to herbivore utilization of forage. Production of phenolics and condensed tannins in boreal woody plants is known to reduce grazing pressure. Their production depends, among other things, on the availability of nutrient resources, especially nitrogen, relative to the availability of assimilates. The carbon-nutrient balance (CNB) hypothesis (Bryant, Chapin & Klein, 1983) predicts a decrease in the concentration of carbon-based defence compounds with increased availability of nutrients. In a High Arctic heath, I manipulated the carbon-nitrogen balance of polar willow (Salix polaris) in a factorial, multi-level fertilizing and shading experiment. Other plots were subject to elevated temperature. After 2 years, shading and, to a lesser extent, fertilization had caused an increase in nitrogen concentrations in the leaves, which were highly and negatively correlated with lower total phenolic and condensed tannin concentrations. Elevated temperature caused no such effects, but increased growth significantly, while shading reduced shoot biomass, and fertilization had no detectable effect. These results are consistent with the CNB hypothesis. On the basis of this study it seems unlikely, however, that environmental modifications due to global climate change will have a major impact on defence chemistry, as very high shading (>70%) and fertilization levels (>1.5 g m–2 years–1) were needed to produce significant deviations from controls.