At the FACE facility near Rhinelander, Wisconsin, USA, King et al. (2001) grew a mix of paper birch and quaking aspen trees in 30-m diameter plots that were maintained at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times the ambient O3 concentration) for a period of two years. In their study of the belowground environment of the trees, they found that the extra O3 had no effect on the growth of fine roots over that time period, but that elevated O3 and CO2 together increased the fine-root biomass of the mixed stand by 83%.

One year later at the same FACE facility, Oksanen et al. (2001) observed O3-induced injuries in the thylokoid membranes of the chloroplasts of the birch trees' leaves; but the injuries were partially ameliorated in the elevated CO2 treatment. And in a study conducted two years later, Oksanen et al. (2003) say they "were able to visualize and locate ozone-induced H2O2 accumulation within leaf mesophyll cells, and relate oxidative stress with structural injuries." However, they report that "H2O2 accumulation was found only in ozone-exposed leaves and not in the presence of elevated CO2," adding that "CO2 enrichment appears to alleviate chloroplastic oxidative stress."

Across the Atlantic in Finland, Kull et al. (2003) constructed open-top chambers around two clones (V5952 and K1659) of silver birch saplings that were rooted in the ground and had been growing there for the past seven years. These chambers were fumigated with air containing 360 and 720 ppm CO2 in combination with 30 and 50 ppb O3 for two growing seasons, after which it was noted that the extra O3 had significantly decreased branching in the trees' crowns. This malady, however, was almost completely ameliorated by a doubling of the air's CO2 concentration. In addition, after one more year of study, Eichelmann et al. (2004) reported that, by itself, the increase in the air's CO2 content increased the average net photosynthetic rates of both clones by approximately 16%, while the increased O3 by itself caused a 10% decline in the average photosynthetic rate of clone V5952, but not of clone K1659. When both trace gases were simultaneously increased, however, the photosynthetic rate of clone V5952 once again experienced a 16% increase in net photosynthesis, as if the extra O3 had had no effect when applied in the presence of the extra CO2.

After working with the same trees for one additional year, Riikonen et al. (2004) harvested them and reported finding that "the negative effects of elevated O3 were found mainly in ambient CO2, not in elevated CO2." In fact, whereas doubling the air's O3 concentration decreased total biomass production by 13% across both clones, simultaneously doubling the air's CO2 concentration increased total biomass production by 30%, thereby more than compensating for the deleterious consequences of doubling the atmospheric ozone concentration.

In commenting on this ameliorating effect of elevated CO2, the team of Finnish scientists said it "may be associated with either increased detoxification capacity as a consequence of higher carbohydrate concentrations in leaves grown in elevated CO2, or decreased stomatal conductance and thus decreasing O3 uptake in elevated CO2 conditions (e.g., Rao et al., 1995)." They also noted that "the ameliorating effect of elevated CO2 is in accordance with the results of single-season open-top chamber and growth chamber studies on small saplings of various deciduous tree species (Mortensen 1995; Dickson et al., 1998; Loats and Rebbeck, 1999) and long-term open-field and OTC studies with aspen and yellow-poplar (Percy et al., 2002; Rebbeck and Scherzer, 2002)."

In another paper to come out of the Finnish silver birch study, Peltonen et al. (2005) evaluated the impacts of doubled atmospheric CO2 and O3 concentrations on the accumulation of 27 phenolic compounds in the leaves of the trees, finding that elevated CO2 increased the concentration of phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%). Elevated O3, on the other hand, increased the concentration of one glucoside by 22%, chlorogenic acid by 19%, and flavone aglycons by 4%. However, Peltonen et al. say that this latter O3-induced production of antioxidant phenolic compounds "did not seem to protect the birch leaves from detrimental O3 effects on leaf weight and area, but may have even exacerbated them." Last of all, in the combined elevated CO2 and O3 treatment, they found that "elevated CO2 did seem to protect the leaves from elevated O3 because all the O3-derived effects on the leaf phenolics and traits were prevented by elevated CO2."

Meanwhile, back at the FACE facility near Rhinelander, Wisconsin, USA, Agrell et al. (2005) examined the effects of ambient and elevated concentrations of atmospheric CO2 and O3 on the foliar chemistry of birch and aspen trees, plus the consequences of these effects for host plant preferences of forest tent caterpillar larvae. In doing so, they found that "the only chemical component showing a somewhat consistent co-variation with larval preferences was condensed tannins," and they discovered 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 concentration of birch leaves was 18% higher in the elevated CO2 and O3 treatment. Consequently, as atmospheric concentrations of CO2 and O3 continue to rise, the increases in condensed tannin concentrations 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 (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.

Concurrent with the work of Agrell et al., King et al. (2005) evaluated the effect of CO2 enrichment alone, O3 enrichment alone, and the net effect of both CO2 and O3 enrichment together on the growth of the Rhinelander birch trees, finding that relative to the ambient-air control treatment, elevated CO2 increased total biomass by 45% in the aspen-birch community, while elevated O3 caused a 13% reduction in total biomass relative to the control. Of most interest of all, the combination of elevated CO2 and O3 resulted in a total biomass increase of 8.4% relative to the control aspen-birch community. King et al. thus concluded that "exposure to even moderate levels of O3 significantly reduces the capacity of net primary productivity to respond to elevated CO2 in some forests." Consequently, they suggested it makes sense to move forward with technologies that reduce anthropogenic precursors to photochemical O3 formation, because the implementation of such a policy would decrease an important constraint on the degree to which forest ecosystems can positively respond to the ongoing rise in the air's CO2 concentration.

One final paper to come out of the Finnish silver birch study was that of Kostiainen et al. (2006), who studied the effects of elevated CO2 and O3 on various wood properties. Their work revealed that the elevated CO2 treatment had no effect on wood structure, but that it increased annual ring width by 21%, woody biomass by 23% and trunk starch concentration by 7%. Elevated O3, on the other hand, decreased stem vessel percentage in one of the clones by 10%; but it had no effect on vessel percentage in the presence of elevated CO2.

In discussing their results, Kostiainen et al. note that "in the xylem of angiosperms, water movement occurs principally in vessels (Kozlowski and Pallardy, 1997)," and that "the observed decrease in vessel percentage by elevated O3 may affect water transport," obviously lowering it. However, as they continue, "elevated CO2 ameliorated the O3-induced decrease in vessel percentage." In addition, they note that "the concentration of nonstructural carbohydrates (starch and soluble sugars) in tree tissues is considered a measure of carbon shortage or surplus for growth (Korner, 2003)." Hence, they conclude that "starch accumulation observed under elevated CO2 in this study indicates a surplus of carbohydrates produced by enhanced photosynthesis of the same trees (Riikonen et al., 2004)." In addition, they report that "during winter, starch reserves in the stem are gradually transformed to soluble carbohydrates involved in freezing tolerance (Bertrand et al., 1999; Piispanen and Saranpaa, 2001), so the increase in starch concentration may improve acclimation in winter." Considering these several responses, therefore, it can be appreciated that the ongoing rise in the air's CO2 content should be a boon to silver birch (and likely many other trees) in both summer and winter in both pristine and ozone-polluted air.

Rounding out the suite of Rhinelander FACE studies of paper birch is the report of Darbah et al. (2007), who found that the total number of trees that flowered increased by 139% under elevated CO2 but only 40% under elevated O3. Likewise, with respect to the quantity of flowers produced, they found that elevated CO2 led to a 262% increase, while elevated O3 led to only a 75% increase. They also determined that elevated CO2 had significant positive effects on birch catkin size, weight, and germination success rate, with elevated CO2 increasing the germination rate of birch by 110%, decreasing seedling mortality by 73%, increasing seed weight by 17% and increasing new seedling root length by 59%. On the other hand, they found that just the opposite was true of elevated O3, as it decreased the germination rate of birch by 62%, decreased seed weight by 25%, and increased new seedling root length by only 15%.

In discussing their findings, Darbah et al. additionally report that "the seeds produced under elevated O3 had much less stored carbohydrate, lipids, and proteins for the newly developing seedlings to depend on and, hence, the slow growth rate." As a result, they conclude that "seedling recruitment will be enhanced under elevated CO2 but reduced under elevated O3," which is another important reason to hope that the atmosphere's CO2 concentration continues to climb as long as the air's O3 content is in a significantly ascending mode.

In summary, from their crowns to their roots, birch trees are generally negatively affected by rising ozone concentrations. When the air's CO2 content is also rising, however, these negative consequences may often be totally eliminated and replaced by positive responses.

References
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