Paper Reviewed
Meehan, T.D., Couture, J.J., Bennett, A.E. and Lindroth, R.L. 2014. Herbivore-mediated material fluxes in a northern deciduous forest under elevated carbon dioxide and ozone concentrations. New Phytologist 204: 397-407.

In a recent New Phytologist publication, Meehan et al. (2014) write that anthropogenic CO2 enrichment is "known to alter tree physiology and growth," but they say that "cascading effects on herbivore communities and herbivore-mediated nutrient cycling are poorly understood." And, therefore, working in early-succession temperate deciduous forest stands of aspen and birch at the Aspen FACE facility near Rhinelander, Wisconsin (USA), they measured herbivore frass, herbivore-mediated greenfall, and leaf-litter deposition in the soils of these young forests in both ambient and CO2-enriched (to 560 ppm) air, while they analyzed substrate chemical composition and compared the quality and quantity of fluxes under the two CO2 treatments, which were initiated in 1998 on saplings that had been planted the previous year, and which were concluded in 2010.

This work revealed, as the four U.S. scientists report, that "herbivore fluxes of dry matter, carbon, condensed tannins, and nitrogen increased under elevated CO2 (35, 32, 63, and 39%, respectively). And they note that these "changes in herbivore fluxes were also broadly consistent with changes in plant growth and soil respiration under altered atmospheric conditions." In addition, they say that "combined, these findings suggest that insect herbivory has the potential to reinforce other ecological processes expected to impact the productivity of, and carbon sequestration by, forests in future environments," citing in this regard "photosynthetic rate and fine-root production," the latter of which phenomena has been reviewed in depth by Madhu and Hatfield (2013).

And so it would appear that the outwardly-negative activity of insect herbivores chomping on tree leaves may have a number of hidden (below-ground) positive consequences, which become even more positive as the air's CO2 content continues its temporal upward trajectory.

Reference
Madhu, M. and Hatfield, J.L. 2013. Dynamics of plant root growth under increased atmospheric carbon dioxide. Agronomy Journal 105: 657-669.