What was done
In a two-hectare scrub-oak community at the Kennedy Space Center, Florida, USA, eight open-top chambers have been enriched with an extra 350 ppm of CO2 since May of 1996, while eight similar chambers that serve as controls have been maintained at ambient CO2 concentrations.  Four species dominate this community and are present in every chamber, including three oaks (Quercus myrtifolia Willd, Q. chapmanii Sargent and Q. geminata Small) plus the nitrogen-fixing legume Galactia elliottii Nuthall.  At three-month intervals from May 2001 to May 2003, samples of undamaged leaves were removed from each of the four species in all chambers and analyzed for various chemical constituents, while 200 randomly selected leaves of each species in each chamber were scored for the presence of six types of herbivore damage.

What was learned
There were no significant differences between the CO2-enriched and ambient-treatment leaves of any single species in terms of either condensed tannins, hydrolyzable tannins, total phenolics or lignin.  However, in all four species there were always greater concentrations of all four leaf constituents in the CO2-enriched leaves, with across-species mean increases of 6.8% for condensed tannins, 6.1% for hydrolyzable tannins, 5.1% for total phenolics and 4.3% for lignin.  In addition, there were large and often significant CO2-induced decreases in all leaf damage categories among all species: chewing (-48%, P < 0.001), mines (-37%, P = 0.001), eye spot gall (-45%, P < 0.001), leaf tier (-52%, P = 0.012), leaf mite (-23%, P = 0.477) and leaf gall (-16%, P = 0.480).

What it means
Hall et al. conclude that the changes they observed in leaf chemical constituents and herbivore damage "suggest that damage to plants may decline as atmospheric CO2 levels continue to rise."