In recent decades, sugar maple (Acer saccharum) has suffered a well-documented decline in portions of the eastern forest of North America that experience high rates of sulfate and nitrate deposition. A key factor linked to mortality and declines of this sensitive tree species along the Allegheny Plateau is the co-occurrence of nutrient stress in acidified soils with outbreaks of defoliating insects. We investigated the interactive effects of acid soil stress and herbivory on sugar maple resistance to the invasive gypsy moth (Lymantria dispar) through feeding trials and chemical analyses of leaves.
Results/Conclusions
At established field sites with healthy and declining sugar maple stands, larvae grew at similar rates on undamaged sugar maple regardless of soil type. However, when trees had experienced prior herbivory, induced resistance was observed only in seedlings from healthy sites and not in those from acidic soil sites. A similar result was found in a common garden experiment using soil cores from the same sites. Furthermore, herbivory-induced accumulation of the signaling phytohormone jasmonic acid (JA), a key mediator of induced anti-herbivore defenses, was 51% lower in acid-soil stressed trees. Salicylic acid (SA), another signaling compound that has been shown to interfere with JA-regulated defenses, was significantly higher in declining stands. Finally, we also examined levels of tannins, compounds previously implicated in resistance to both abiotic and biotic stress. Condensed tannins were found to be significantly higher in acid-soil stressed seedlings following herbivore damage, though none of the phenolic groups measured correlated with growth rates of the gypsy moth larvae. Our results suggest that soil nutrient imbalances caused by acid rain can reduce the resistance of sugar maple against gypsy moth, effectively turning an unattractive food source into a suitable host plant for an invasive pest.