Induced defensive response of myrtle oak to foliar insect herbivory in ambient and elevated CO2

The rising level of atmospheric CO2 has stimulated several recent studies attempting to predict the effects of increased CO2 on ecological communities. However, most of these studies have been conducted in the benign conditions of the laboratory and in the absence of herbivores. In the current study, we utilized large octagonal chambers, which enclosed portions of an intact scrub-oak community to investigate the interactive effects of CO2 and insect herbivory on myrtle oak, Quercus myrtifolia. Specifically, we assessed the effects of ambient and elevated CO2 (2 x current concentrations) on percent foliar nitrogen, C:N ratio, total relative foliar tannin content, and the presence of leaf damage caused by leaf mining and leaf chewing insects that feed on myrtle oak. Total foliar N declined and C: N ratios increased significantly in oaks in elevated CO2 chambers. The percentages of leaves damaged by either leafminers or leaf chewers tended to be lower in elevated compared to ambient chambers, but they co-occurred on leaves less than expected, regardless of CO2 treatment. Leaves that had been either mined or chewed exhibited a similar wounding or defensive response; they had an average of 25 and 21% higher protein binding ability, which is correlated with tannin concentration, compared to nondamaged control leaves, respectively. While the protein-binding ability (expressed as total percent tannin) of leaves from elevated CO2 was slightly higher than from leaves grown in ambient chambers, this difference was not significant.