Insect herbivory in an intact forest understory under experimental CO ₂ enrichment

Human-induced increases in atmospheric CO₂ concentration have the potential to alter the chemical composition of plant tissue, and thereby affect the amount of tissue consumed by herbivorous arthropods. At the Duke Forest free-air concentration enrichment (FACE) facility in North Carolina (FACTS-1 research facility), we measured the amount of leaf tissue damaged by insects and other herbivorous arthropods during two growing seasons in a deciduous forest understory continuously exposed to ambient (360 μl l ^-1) and elevated (∼560 μl l^-1) CO₂ conditions. In 1999, there was a significant interaction between CO₂ and species such that winged elm (Ulmus alata) showed lower herbivory in elevated CO₂ plots, whereas red maple (Acer rubra) and sweetgum (Liquidambar styraciflua) did not. In 2000, our results did not achieve statistical significance but the magnitude of the result was consistent with the 1999 results. In 1999 and 2000, we found a decline (10-46%) in community-level herbivory in elevated CO₂ plots driven primarily by reductions in herbivory on elm. The major contribution to total leaf damage was from missing tissue (66% of the damaged tissue), with galls, skeletonized, and discolored tissue making smaller contributions. It is unclear whether the decline in leaf damage is a result of altered insect populations, altered feeding, or a combination. We were not able to quantify insect populations, and our measurements did not resolve an effect of elevated CO₂ on leaf chemical composition (total nitrogen, carbon, C/N, sugars, phenolics, starch). Despite predictions from a large number of single-species studies that herbivory may increase under elevated CO₂, we have found a decrease in herbivory in a naturally established forest understory exposed to a full suite of insect herbivores and their predators.