The dynamics of carbon-nutrient balance: Effects of cottonwood acclimationto short-and long-term shade on beetle feeding preferences

The carbon-nutrient balance hypothesis (CNBH) predicts that shading should increase leaf palatability to herbivores by decreasing concentrations of ca rbon (C) -based chemical defenses and increasing nitrogen (N). We measured cottonwood (Populus deltoides) growth, leaf chemistry, and beetle (Plagiode ra versicolora) feeding preferences on saplings grown in either continuous high (HH) or low (LL) light, and saplings switched from high to low (HL) or low to high (LH) light for nine days. As expected, based on the CNBH, shad ing increased total N and decreased total phenol glycoside (C-based seconda ry metabolites) concentrations in plants from all shade treatments (LL, HL, and LH), relative to HH plants, with plant growth and gross leaf chemistry being affected by initial and final light regime. In contrast, while speci fic phenol glycoside concentrations were affected by the initial and final light regime, they also showed an initial x final light interaction. Beetle s tended to prefer LL to HH plants. Beetles unexpectedly preferred HH to ei ther HL or LH switched plants, most likely because high concentrations of a specific phenol glycoside - salicin - occurred in both switched treatments and inhibited beetle feeding. Plant chemical allocation during light accli mation led to unpredictable changes in specific C-based compounds, even tho ugh plant growth and gross chemistry conformed to expectations for shading effects and the CNBH. The response of this herbivore to altered concentrati ons of a specific compound confounded predictions based on average dynamics of suites of chemicals. Our findings may help explain why relationships be tween light availability and herbivory in field studies, where light varies on many time scales. can differ from those predicted by the CNBH. Understa nding both dynamic plant chemical responses to altered resource availabilit y and controls over allocation to specific compounds would likely enhance f uture predictability of specific environment-plant-herbivore interactions.