Benefits and costs of phytochemical defense in Aspen-insect interactions: Causes and consequences of variation

Optimal defense theories suggest that tradeoffs between benefits and costs maintain genetic variation in chemical defense traits within plant populations. A central focus of this research was to assess the independent and interactive effects of genetic and environment-based variation in aspen leaf chemistry on plant protection and to assess how these factors mediate the realization of defense benefits and costs. Aspen (Populus tremuloides) produces specific genetically determined chemicals (phenolic glycosides) that affect insect folivores. Foliar concentrations of these chemicals can differ markedly among clones and during outbreaks of defoliating Lepidoptera levels of defoliation differ among aspen clones. Chapters One and Two attempt to link these variables in controlled gypsy moth (Lymantria dispar) outbreak mesocosms and in the field during a forest tent caterpillar ( Malacosoma disstria) outbreak, respectively. Phenolic glycosides provided protection against defoliation in mesocosms and resource availability influenced the degree to which benefits occurred. While considerable phytochemical variation occurred among clones in the field, all clones had relatively low levels of phenolic glycosides and plant chemistry did not influence defoliation rates. Surveys of aspen in the Lake States region suggest that advanced stands generally produce lower levels of phenolic glycosides than expected and therefore, these compounds may not play as significant a defensive role in these stands during outbreaks as previously thought. Chapter Three suggests a mechanism by which variable resource availability and variable competitive interactions within a plant population may change the relative importance of the selective pressure exerted by defense costs. Significant trade-offs between growth and phenolic glycosides occurred, but only when resources were limiting. Condensed tannin levels increased when N was limiting, but contrary to predictions of substrate-based models of allocation, increased allocation to tannins came at a cost to growth. Chapter Four surveyed the effects of plant development on leaf chemistry in naturally occurring aspen clones. Young trees contained higher concentrations of phenolic glycosides and lower concentrations of tannins than did older trees. This pattern may explain the low levels of phenolic glycosides found in advanced aspen stands in the Lake States. Evolutionary and ecological implications are discussed.