Effects of light and nutrient availability on aspen: Growth, phytochemistry, and insect performance

This study explored the effect of resource availability on plant phytochemical composition within the framework of carbon-nutrient balance (CNB) theory. We grew quaking aspen (Populus tremuloides) under two levels of light and three levels of nutrient availability and measured photosynthesis, productivity, and foliar chemistry [water, total nonstructural carbohydrates (TNC), condensed tannins, and phenolic glycosides]. Gypsy moths (Lymantria dispar) and forest tent caterpillars (Malacosoma disstria) were reared on foliage from each of the treatments to determine effects on insect performance. Photosynthetic rates increased under high light, but were not influenced by nutrient availability. Tree growth increased in response to both the direct and interactive effects of light and nutrient availability. Increasing light reduced foliar nitrogen, while increasing nutrient availability increased foliar nitrogen. TNC levels were elevated under high light conditions, but were not influenced by nutrient availability. Starch and condensed tannins responded to changes in resource availability in a manner consistent with CNB theory; levels were highest under conditions where tree growth was limited more than photosynthesis (i.e., high light-low nutrient availability). Concentrations of phenolic glycosides, however, were only moderately influenced by resource availability. In general, insect performance varied relatively little among treatments. Both species performed most poorly on the high light-low nutrient availability treatment. Because phenolic glycosides are the primary factor determining aspen quality for these insects, and because levels of these compounds were minimally affected by the treatments, the limited response of the insects was not surprising. Thus, the ability of CNB theory to accurately predict allocation to defense compounds depends on the response of specific allelochemicals to changes in resource availability. Moreover, whether allelochemicals serve to defend the plant depends on the response of insects to specific allelochemicals. Finally, in contrast to predictions of CNB theory, we found substantial allocation to storage and defense compounds under conditions in which growth was carbon-limited (e.g., low light), suggesting a cost to defense in terms of reduced growth.