Background
The authors write that "arthropod herbivory can reduce plant productivity by removing photosynthetic leaf area," and they note that the studies of Zangerl et al. (2002), Aldea et al. (2005, 2006) and Patankar et al. (2011) indicate that, in some cases, "damage to leaf surfaces causes a reduction in the quantum efficiency of photosystem II fluorescence, which is highly correlated with the rate of carbon assimilation."

What was done
Working at the aspen free-air CO2 enrichment (Aspen FACE) site in north-central Wisconsin (USA), Nabity et al. investigated how different herbivore damage types (leaf-chewing, gall-forming and leaf-folding) alter component processes of photosynthesis under both ambient and elevated (ambient + 200 ppm) atmospheric CO2 concentrations in aspen (Populus tremuloides, genotype 216) trees, as well as how leaf-chewing insects impact photosynthesis in birch (Betula papyrifera) trees.

What was learned
In the words of the four researchers who conducted the work, "growth under elevated CO2 reduced the distance that herbivore-induced reductions in photosynthesis propagated away from the point of damage in aspen and birch."

What it means
Nabity et al. conclude that their results suggest that "at least for these species," as they put it, elevated CO2 "may reduce the impact of herbivory on photosynthesis," which would be a very positive development indeed.

References
Aldea, M., Hamilton, J.G., Resti, J.P., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H. 2005. Indirect effects of insect herbivory on leaf gas exchange in soybean. Plant, Cell and Environment 28: 402-411.

Aldea, M., Hamilton, J.G., Resti, J.P., Zangerl, A.R., Berenbaum, M.R., Frank, T.D. and DeLucia, E.H. 2006. Comparison of photosynthetic damage from arthropod herbivory and pathogen infection in understory hardwood samplings. Oecologia 149: 221-232.

Patankar, R., Thomas,S.C. and Smith, S.M. 2011. A gall-inducing arthropod drives declines in canopy photosynthesis. Oecologia 167: 701-709.

Zangerl, A.R., Hamilton, J.G., Miller,T.J., Crofts, A.R., Oxborough, K., Berenbaum, M.R. and DeLucia, E.H. 2002. Impact of folivory on photosynthesis is greater than the sum of its holes. Proceedings of the National Academy of Sciences USA 99: 1088-1091.