Do defoliation and subsequent phytochemical responses reduce future herbivory on oak trees?

Perennial plants are thought to respond to partial or complete defoliation by producing new foliage that is less susceptible to herbivores because of induction of allelochemicals. Here, I tested this hypothesis by manually removing primary foliage from branches ofQuercus emoryi (Fagaceae) at two different times in the season and monitoring changes in protein and tannin levels and the amount of herbivory relative to control branches. New, secondary leaves had 2.5 × greater hydrolyzable tannin content than mature foliage of control branches. Condensed tannins, which constitute a relatively low fraction of leaf mass, were lower, while protein content was temporarily greater, in new secondary leaves relative to mature leaves. Despite large increases in hydrolyzable tannins, herbivory levels were greater on refoliated branches than on control branches. New foliage is susceptible to herbivory regardless of when it is produced in the season, possibly because lower toughness and higher water content override any induced or developmentally related changes in allelochemistry. My results do not support the hypothesis that postherbivore changes in phytochemistry protect perennial plants from future herbivory, at least within a growing season.