McDonald et al. (1999) grew paper birch 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 doing so, they 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 tannin increase was a whopping 175%.

Peltonen et al. (2005) studied the impacts of doubled atmospheric CO2 and O3 concentrations on the accumulation of 27 phenolic compounds, including soluble condensed tannins, in the leaves of two European silver birch clones in seven-year-old soil-grown trees that were exposed in open-top chambers for three growing seasons to ambient and twice-ambient atmospheric CO2 and O3 concentrations singly and in combination. This work, which was carried out in central Finland, revealed that elevated CO2 increased the concentration of soluble condensed tannins in the leaves of the trees by 19%. In addition, they found that the elevated CO2 protected the leaves from elevated O3 because, as they describe it, "all the O3-derived effects on the leaf phenolics and traits were prevented by elevated CO2."

Kuokkanen et al. (2003) grew two-year-old silver birch seedlings in ambient air of 350 ppm CO2 or air enriched to a CO2 concentration of 700 ppm under conditions of either ambient temperature or ambient temperature plus 3°C for one full growing season in the field in closed-top chambers at the Mekrijarvi Research Station of the University of Joensuu in eastern Finland. Then, during the middle of the summer, when carbon-based secondary compounds of birch leaves are fairly stable, they picked several leaves from each tree and determined their condensed tannin concentrations, along with the concentrations of a number of other physiologically-important substances. This work revealed that the concentration of total phenolics, condensed tannins and their derivatives significantly increased in the leaves produced in the CO2-enriched air, as has also been observed by Lavola and Julkunen-Titto (1994), Williams et al. (1994), Kinney et al. (1997), Bezemer and Jones (1998) and Kuokkanen et al. (2001). In fact, the extra 350 ppm of CO2 nearly tripled condensed tannin concentrations in the ambient-temperature air, while it increased their concentrations in the elevated-temperature air by a factor in excess of 3.5.

In a study of roots, Parsons et al. (2003) grew two-year-old paper birch saplings in well-watered and fertilized 16-L pots from early May until late August in glasshouse rooms maintained at either 400 or 700 ppm CO2. This procedure revealed that the concentration of condensed tannins in the fine roots of the saplings was increased by 27% in the CO2-enriched treatment; and in regard to this finding, the researchers say "the higher condensed tannin concentrations that were present in the birch fine roots may offer these tissues greater protection against soil-borne pathogens and herbivores."

Parsons et al. (2004) collected leaf litter samples from early September to mid-October beneath paper birch trees growing in ambient and CO2-enriched (to 200 ppm above ambient) FACE plots in northern Wisconsin, USA, which were also maintained under ambient and O3-enriched (to 19 ppb above ambient) conditions, after which the leaf mass produced in each treatment was determined, sub-samples of the leaves were assessed for a number of chemical constituents (including nitrogen, which hastens leaf decay, and condensed tannins, which retard decay), and the remaining leaves were placed in 1-mm-aperture litterbags made of fiberglass cloth and left to decay upon the ground for the next twelve months under the same atmospheric conditions in which they were produced. Then, at the conclusion of the one-year litter-exposure period, the mass of remaining litter was measured and the time required to achieve 95% mass loss determined.

In following this protocol, the researchers learned that under ambient O3 conditions, the nitrogen concentrations of the leaves in the CO2-enriched plots at the time of litterfall were 31% less than those of the leaves in the ambient-CO2 plots, while condensed tannin concentrations were 64% greater in the CO2-enriched plots. Similarly, under the O3-enriched conditions, leaf nitrogen concentrations were a nearly-identical 32% less, while condensed tannins concentrations a much-larger 99% greater.

These observations suggest that leaf decay rates in the CO2-enriched plots should be lower than those in the ambient-CO2 plots; and the mass-loss rates determined at the end of the one-year exposure period bore out this expectation, with Parsons et al. reporting that "for control litter, 5% of mass remained after 3.6 years, while CO2-enriched litter took ~4.5 years to turn over 95% of its mass." Hence, it could well take 25% more time (4.5 years / 3.6 years) to lose an equivalent percentage of paper birch leaf litter from CO2-enriched forests, independent of the air's O3 concentration. And combining this fact with the facts that the CO2-enriched trees, in the words of the researchers, "attained greater size, and a greater degree of canopy closure, and contributed more litterfall to the development of [the] forest floor than did trees in the control rings," it is clear that the ongoing rise in the atmosphere's CO2 concentration should greatly augment the sequestration of carbon by paper birch tree stands as the air's CO2 content climbs ever higher.

In another study conducted at the Wisconsin FACE site, Agrell et al. (2005) 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 paper birch trees, as well as the impacts of these effects on the host plant preferences of forest tent caterpillar larvae, finding that the mean condensed tannin concentration of the birch tree leaves was 18% greater in the elevated CO2 and O3 treatment than in the ambient CO2 and O3 treatment. In addition, they state 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 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 birch 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 higher birch-foliage tannin concentrations likely to prevail in a high-CO2 world of the future should result in less methane being released to the atmosphere via ruminants browsing on the foliage of birch trees, which phenomenon should act to decrease the impetus for methane-induced global warming.

In conclusion, it appears that elevated concentrations of atmospheric CO2 tend to increase leaf and fine-root tannin concentrations of birch trees, and that this phenomenon tends to (1) protect the trees' foliage from predation by voracious insect herbivores, (2) protect the trees' roots from soil-borne pathogens and herbivores, (3) enhance the sequestration of carbon in forest soils, and (4) reduce methane emissions from ruminants that might nibble on the trees' foliage.

References
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Bezemer, T.M. and Jones, T.H. 1998. Plant-insect herbivore interactions in elevated atmospheric CO2, quantitative analyses and guild effects. Oikos 82: 212-222.

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

Kinney, K.K., Lindroth, R.L., Jung, S.M. and Nordheim, E.V. 1997. Effects of CO2 and NO3 availability on deciduous trees, phytochemistry and insect performance. Ecology 78: 215-230.

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Lavola, A. and Julkunen-Titto, R. 1994. The effect of elevated carbon dioxide and fertilization on primary and secondary metabolites in birch, Betula pendula (Roth). Oecologia 99: 315-321.

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Parsons, W.F.J., Kopper, B.J. and Lindroth, R.L. 2003. Altered growth and fine root chemistry of Betula papyrifera and Acer saccharum under elevated CO2. Canadian Journal of Forest Research 33: 842-846.

Parsons, W.F.J., Lindroth, R.L. and Bockheim, J.G. 2004. Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3. Global Change Biology 10: 1666-1677.

Peltonen, P.A., Vapaavuori, E. and Julkunen-Tiitto, R. 2005. Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone. Global Change Biology 11: 1305-1324.

Williams, R.S., Lincoln, D.E. and Thomas, R.B. 1994. Loblolly pine grown under elevated CO2 affects early instar pine sawfly performance. Oecologia 98: 64-71.