What was done
The authors examined the effects of ambient and elevated concentrations of atmospheric CO2 (360 ppm and 560 ppm) and O3 (35-60 ppb and 52-90 ppb) on the foliar chemistry of birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259) trees at the Aspen Free Air CO2 Enrichment facility near Rhinelander, Wisconsin, USA, plus the consequences of these effects for host plant preferences of forest tent caterpillar (Malacosoma disstria Hbn.) larvae.

What was learned
Agrell et al. report that "the only chemical component showing a somewhat consistent covariation with larval preferences was condensed tannins," noting that "the tree becoming relatively less preferred as a result of CO2 or O3 treatment was in general also the one for which average levels of condensed tannins were most positively (or least negatively) affected by that treatment."  In this regard, it is also of interest to note that the mean condensed tannin concentration of birch leaves was 18% higher in the elevated CO2 and O3 treatment, while for the aspen 216 and 259 genotypes it was 25% and 57% higher, respectively.

What it means
As atmospheric concentrations of CO2 and O3 continue to rise as they have in the past, the increases in condensed tannin concentrations likely to occur in the foliage of birch and aspen trees should lead to their leaves becoming less preferred for consumption by the dreaded forest tent caterpillar, which according to Agrell et al. is "an eruptive generalist defoliator in North American hardwood forests, causing extensive damage during outbreak years (Fitzgerald, 1995)."  Also, because the amount of methane expelled in the breath of ruminants is an inverse function of the condensed tannin concentration of the foliage they consume (see Tannins in our Subject Index), the increased foliage tannin concentrations likely to exist in a high-CO2 world of the future should result in less methane being released to the atmosphere via ruminants ingesting such foliage, which phenomenon would tend to decrease the impetus for methane-induced global warming.

Reference
Fitzgerald, T.D.  1995.  The Tent Caterpillars.  Comstock Publishing, Ithaca, New York, USA.