This latter finding is of special importance to New Zealanders, because the methane expelled in the breath of cattle and sheep - which is a by-product of the fermentation of feed in the rumen of these animals - accounts for close to 90% of the country's methane emissions. Consequently, a significant reduction in such a large national source of one of the atmosphere's most potent greenhouse gases would go a long way towards helping the country reduce its emissions of climate-altering substances, which would be, according to the press release that described this development, "very welcome." The press release also states that tannins "have a variety of other animal related benefits, such as improved milk yield, increased liveweight gain, decreased internal parasite burden and reduced occurrence of bloat, dags and fly strike."

In this regard, it is important to note that in addition to sheep and cattle, ruminants, as they are called, comprise a great group of animals (four-footed, hoofed, even-toed, cud-chewing mammals that have a stomach consisting of four divisions or chambers) that includes antelope, bison, buffalo, camel, deer, giraffe, goat, llama, etc., and that these animals eat a number of other types of plants, which may also experience increases in leaf tannin production as the air's CO2 content rises. In this Summary, therefore, we begin an investigation of this subject as it applies to aspen (Populus tremuloides) trees.

King et al. (2001) grew aspen seedlings for five months in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. At the end of this period, naturally-senesced leaf litter was collected and analyzed; and it was found that the elevated CO2 of this particular study had no effect on leaf litter tannin concentration.

A substantially different result was obtained in an earlier study of aspen leaves that was conducted by McDonald et al. (1999), who grew aspen seedlings in controlled environment greenhouses that were maintained at either ambient (387 ppm) or elevated (696 ppm) CO2 concentrations under conditions of either low or high light availability (half and full sunlight, respectively) for 31 days after the mean date of bud break. In this case it was determined that under low light conditions, the CO2-enriched seedlings exhibited an increase of approximately 15% in leaf condensed tannin concentration, while under high light conditions the CO2-induced increase in leaf condensed tannin concentration was a whopping 175%.

In a much more complex study than either of the two preceding ones, Agrell et al. (2005) examined the effects of ambient and elevated concentrations of atmospheric CO2 (360 ppm and 560 ppm, respectively) and O3 (35-60 ppb and 52-90 ppb, respectively) on the foliar chemistry of more mature aspen trees of two different genotypes (216 and 259) growing out-of-doors at the Aspen Free Air CO2 Enrichment (FACE) facility near Rhinelander, Wisconsin, USA, as well as the impacts of these effects on the host plant preferences of forest tent caterpillar larvae.

In reporting the results of the study, Agrell et al. say 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 of interest to note that the mean condensed tannin concentrations of the aspen 216 and 259 genotypes were 25% and 57% higher, respectively, under the elevated CO2 and O3 combination treatment compared to the ambient CO2 and O3 combination treatment.

In light of these findings, it is logical to presume that as atmospheric concentrations of CO2 and O3 continue to rise, the increase in condensed tannin concentration likely to occur in the foliage of 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, the increased aspen 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 browsing on aspen foliage, which phenomenon should act to decrease the impetus for methane-induced global warming.

References
Agrell, J., Kopper, B., McDonald, E.P. and Lindroth, R.L. 2005. CO2 and O3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria). Global Change Biology 11: 588-599.

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

King, J.S., Pregitzer, K.S., Zak, D.R., Kubiske, M.E., Ashby, J.A. and Holmes, W.E. 2001. Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability. Global Change Biology 7: 65-74.

McDonald, E.P., Agrell, J., and Lindroth, R.L. 1999. CO2 and light effects on deciduous trees: growth, foliar chemistry, and insect performance. Oecologia 119: 389-399.