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The Competing Effects of Rising Atmospheric CO2 and O3 Concentrations
Reference
King, J.S., Kubiske, M.E., Pregitzer, K.S., Hendrey, G.R., McDonald, E.P., Giardina, C.P., Quinn, V.S. and Karnosky, D.F. 2005. Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist 168: 623-636.

Background
The authors write that "pre-industrial concentrations of tropospheric O3 [an air pollutant that causes great damage to plants] are estimated to have been less than 10 ppb, and have risen to 30-40 ppb background levels today (Levy et al., 1997)." In addition, they note that the rising boundary-layer O3 concentration caused by increasing industrialization around the globe has had negative continent-scale implications for carbon sequestration for some time (Felzer et al., 2004). Hence, they felt a need to evaluate the net effect of the positive CO2 and negative O3 impacts of possible future increases in these trace atmospheric gases on the productivity of the most widespread tree species found in North America, i.e., trembling aspen (Populus tremuloides Michx.), as well as two-member mixed communities of trembling aspen-paper birch (Betula papyrifera Marsh.) and trembling aspen-sugar maple (Acer saccharum Marsh.).

What was done
At the Aspen FACE site (Dickson et al., 2000) near Rhinelander, Wisconsin, USA, pure stands of aspen and mixed stands of aspen-birch and aspen-maple were allowed to grow for seven years in either ambient air or air enriched with an extra 200 ppm of CO2 or air enriched with and extra 50% O3 or air thus enriched by both CO2 and O3, after which King et al. evaluated the effect of CO2 enrichment alone, O3 enrichment alone, and the net effect of both CO2 and O3 enrichment together.

What was learned
The eight researchers report that relative to the ambient-air control treatment, elevated CO2 "increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively," while elevated O3 "caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities." Of most interest of all, the combination of elevated CO2 and O3 "resulted in total biomass responses of -7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspen-sugar maple communities, respectively."

What it means
King et al. conclude from the results of their study 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 suggest that 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.

References
Dickson, R.E., Lewin, K.F., Isebrands, J.G., Coleman, M.D., Heilman, W.E., Riemenschneider, D.E., Sober, J, Host, G.E., Zak, D.R., Hendrey, G.R., Pregitzer, K.S. and Karnosky, D.F. 2000. Forest Atmosphere Carbon Transfer and Storage (FACTS-II): The Aspen Free-Air CO2 and O3 Enrichment (FACE) Project: An Overview. USDA Forest Service NCRS, St. Paul, Minnesota, USA.

Felzer, B., Kicklighter, D., Mellilo, J., Wang, C., Zhuang, Q. and Prinn, R. 2004. Effects of ozone on net primary production and carbon sequestration in the conterminous United States using a biogeochemistry model. Tellus 56B: 230-248.

Levy, H.I.I., Kasibhatla, P.S., Moxim, W.J., Klonecki, A.A., Hirsch, A.I., Oltmans, S.J. and Chameides, W.L. 1997. The global impact of human activity on tropospheric ozone. Geophysical Research Letters 24: 791-794.

Reviewed 29 March 2006