Abstract.  Human‐induced climate changes threaten the health of forest ecosystems. In particular, carbon dioxide (CO2) and tropospheric ozone (O3) will likely have significant but opposing impacts on forests and their associated insect communities. Compared with other animal groups, insect communities are expected to be especially sensitive to changes in global climate. This study examined the effects of elevated CO2 and O3 (eCO2 and eO2) individually and in combination on the abundance, diversity and composition of forest insect communities. Insects were sampled using yellow pan traps in an aggrading aspen‐birch forest at the Aspen Free Air CO2 Enrichment (FACE) site in northern Wisconsin, USA. We trapped for 24 h every 10–15 days throughout the summers (June to September) of 2000–2003. We examined 47 415 insects from 4 orders and 83 families. Elevated CO2 reduced abundance of phloem‐feeding herbivores and increased abundance of chewing herbivores, although results were not statistically significant. Enriched CO2 increased numbers of some parasitoids. The effects of eO3 on insect abundance were generally opposite those of eCO2. No significant differences in arthropod family richness were found among treatments. However, eCO2, eO3, or both significantly affected insect community composition in all years. Carbon dioxide and tropospheric ozone have the potential to alter significantly forest insect communities. Feeding guild may strongly influence insect response to environmental change and may provide the best opportunity to generalise for conservation efforts. Because insect communities influence forest health and ecosystem services, continued research on their response to global change is critically important to forest management and conservation.

Plant adaptations for defense against herbivory vary both among species and among genotypes. Moreover, numerous forms of within-plant variation in defense, including ontogeny, induction, and seasonal gradients, allow plants to avoid expending resources on defense when herbivores are absent. We used an 18-year-old cottonwood common garden composed of Populus fremontii, Populus angustifolia, and their naturally occurring F(1) hybrids (collectively referred to as "cross types") to quantify and compare the relative influences of three hierarchical levels of variation (between cross types, among genotypes, and within individual genotypes) on univariate and multivariate phytochemical defense traits. Within genotypes, we evaluated ontogeny, induction (following cottonwood leaf beetle herbivory), and seasonal variation. We compared the effect sizes of each of these sources of variation on the plant defense phenotype. Three major patterns emerged. First, we observed significant differences in concentrations of defense phytochemicals among cross types, and/or among genotypes within cross types. Second, we found significant genetic variation for within-plant differences in phytochemical defenses: (a) based on ontogeny, levels of constitutive phenolic glycosides were nearly three times greater in the mature zone than in the juvenile zone within one cottonwood cross type, but did not significantly differ within another cross type; (b) induced levels of condensed tannins increased up to 65 % following herbivore damage within one cottonwood cross type, but were not significantly altered in another cross type; and (c) concentrations of condensed tannins tended to increase across the season, but did not do so across all cross types. Third, our estimates of effect size demonstrate that the magnitude of within-plant variation in a phytochemical defense can rival the magnitude of differences in defense among genotypes and/or cross types. We conclude that, in cottonwood and likely other plant species, multiple forms of within-individual variation have the potential to substantially influence ecological and evolutionary processes.

Elevated concentrations of carbon dioxide and tropospheric ozone pose important threats to the abundance, diversity, and composition of forest arthropod communities. In turn, modification of arthropod communities may alter forest health, productivity, and ecosystem services. We studied the independent and interactive effects of elevated CO2 (eCO2) and elevated O3 (eO3) on the abundance, species richness, and community composition of herbivorous arthropods in stands of trembling aspen and paper birch at the Aspen Free Air CO2 Enrichment (FACE) site in northern Wisconsin, USA. We conducted timed, visual surveys of canopy arthropods during each of the summers of 2005, 2006, and 2007. We examined 26 983 arthropods on aspen and 8344 arthropods on birch across the fumigation treatments. Elevated CO2 and eO3 had species‐specific and temporally variable (i.e. idiosyncratic) effects on aspen and birch arthropod abundance and species richness. Weak, idiosyncratic effects of eCO2 and eO3 on herbivorous arthropod abundance and species richness did not significantly alter aspen arthropod community composition but occasionally altered birch insect community composition. Few interactive effects of CO2 and O3 were observed. Growing evidence suggests that the effects of eCO2 and eO3 on communities of insects are difficult to predict because responses are generally weak and species‐ and time‐specific. Although studies to date suggest that impacts of future atmospheres on insect community metrics are likely to be minimal, the possibility remains that effects on particularly important or susceptible species may cascade to alter trophic interactions and, ultimately, ecosystem processes.

Herbivory can influence ecosystem productivity, but recent evidence suggests that damage by herbivores modulates potential productivity specific to damage type. Because productivity is linked to photosynthesis at the leaf level, which in turn is influenced by atmospheric CO(2) concentrations, we investigated how different herbivore damage types alter component processes of photosynthesis under ambient and elevated atmospheric CO(2). We examined spatial patterns in chlorophyll fluorescence and the temperature of leaves damaged by leaf-chewing, gall-forming, and leaf-folding insects in aspen trees as well as by leaf-chewing insects in birch trees under ambient and elevated CO(2) at the aspen free-air CO(2) enrichment (FACE) site in Wisconsin. Both defoliation and gall damage suppressed the operating efficiency of photosystem II (ΦPSII) in remaining leaf tissue, and the distance that damage propagated into visibly undamaged tissue was marginally attenuated under elevated CO(2). Elevated CO(2) increased leaf temperatures, which reduced the cooling effect of gall formation and freshly chewed leaf tissue. These results provide mechanistic insight into how different damage types influence the remaining, visibly undamaged leaf tissue, and suggest that elevated CO(2) may reduce the effects of herbivory on the primary photochemistry controlling photosynthesis.