The effects of simulated acid rain and heavy metal pollution on the mountain birch–autumnal moth interaction

The exposure of plants to abiotic stresses like air pollutants may increase their susceptibility to herbivores. However, abiotic stresses often induce the accumulation of phenolic compounds that may have adverse effects on plant-eating animals. In this study, we examined the effects of long-term (14 years) deposition of simulated acid rain (H2SO4) and heavy metals (CuNi) on the plant–herbivore interaction. The used species were well-known model species of plant–herbivore interaction, namely the mountain birch (Betula pubescens ssp. czerepanovii) and autumnal moth (Epirrita autumnata). The study set-up was factorial with four combinations of two treatments: −acid−CuNi, +acid−CuNi, −acid+CuNi, and +acid+CuNi. The deposition of pollutants had no marked effects on the growth, survival, or immune function of the autumnal moth although the chemistry of birch leaves was markedly affected. The concentrations of gallic acid, chlorogenic acid, three of hydrolyzable tannins (HTs), and quercetin glycosides were induced by the acid rain treatment when compared to leaves not treated with acid rain. We concluded that an increase in compounds with high redox capacity was a defense against oxidative stress caused by pollution treatments. We suggested that pollution might have increased the deposition of non-phenolic antioxidants like ascorbate that diminish the adverse effects of phenolics on herbivores. We also stated that abiotic stresses might play an equally strong role in the evolution of phenolics than herbivory if not larger.