Herbivory affects developmental instability of stone oak, Quercus rotundifolia

The effects of herbivory on plant fitness have traditionally been assessed by reductions in performance measured in terms of physiological, growth, and reproductive parameters. We investigated the direct effect of herbivory on a measure of foliar developmental instability (fluctuating asymmetry in leaf width) in evergreen stone oak. Quercus rotundifolia. Since leaves from more than a single growth season are present simultaneously, it is possible to compare measures of developmental instability of newly formed leaves in relation to the extent of herbivory in the previous season as assessed from damage to old leaves. We found significantly increased asymmetry in leaf width when leaves From the previous year showed signs of herbivory. This result was confirmed experimentally by using leaf removal to mimic herbivory. Oak trees where many old leaves were removed had increased foliar asymmetry compared to oak trees where only few leaves had been removed and untreated control trees. Using foliar asymmetry in old leaves from the previous year as a baseline for these comparisons confirmed that foliar asymmetry of individual trees increased as a consequence of herbivory. Oak trees with high degrees of asymmetry were more likely to die than oak trees with low asymmetry. These results suggest that foliar asymmetry is symptomatic of a stressed plant. Herbivores can both produce such a stress, leading to increased asymmetry, and attack differentially more asymmetric plants. In the extreme, the most asymmetric plants are those most likely to die.