Several studies have documented the effects of elevated levels of atmospheric CO2 on photosynthesis in various aspen clones.  In the short-term study of Kruger et al. (1998), aspen seedlings grown for 70 days at atmospheric CO2 concentrations of 650 ppm exhibited photosynthetic rates that were approximately 10% greater than those displayed by seedlings maintained at ambient CO2 concentrations; while in the longer five-month study of Kubiske et al. (1998), atmospheric CO2 enrichment significantly increased photosynthetic rates in four aspen genotypes, regardless of soil nitrogen status.

In an even longer 2.5-year study, Wang and Curtis (2001) also observed significant CO2-induced photosynthetic increases in two male and two female aspen clones; and when six aspen genotypes were grown in open-top chambers for 2.5 years at atmospheric CO2 concentrations of 350 and 700 ppm, Curtis et al. (2000) reported that the elevated CO2 concentrations increased rates of net photosynthesis by 128 and 31% at high and low soil nitrogen contents, respectively.  In addition, in a study that only looked at air temperature effects that was conducted at ambient CO2 concentrations, King et al. (1999) determined that increasing the air temperature from 13 to 29°C enhanced photosynthetic rates in four different aspen clones by an average of 35%.

In a FACE study, where O3-sensitive and O3-tolerant clones were grown for six months in field plots receiving 360 and 560 ppm CO2 in combination with ambient and enriched (1.5 times ambient) O3 levels, Noormets et al. (2001) reported that CO2-induced increases in photosynthetic rates were at least maintained, and sometimes even increased, when clones were simultaneously exposed to elevated O3.  After an entire year of treatment exposure, in fact, Karnosky et al. (1999) noted that the powerful ameliorating effect of elevated CO2 on ozone-induced damage was still operating strongly in this system; for O3-induced foliar damages in O3-sensitive and O3-tolerant clones were reduced from 55 and 17%, respectively, at ambient CO2, to 38 and 3%, respectively, at elevated CO2.

With respect to biomass production, Pregitzer et al. (2000) reported that 2.5 years of exposure to twice-ambient concentrations of atmospheric CO2 increased fine-root biomass in six aspen genotypes by an average of 65 and 17% on nitrogen-rich and nitrogen-poor soils, respectively.  Using this same experimental system, Zak et al. (2000) determined that elevated CO2 enhanced total seedling biomass by 38% at high soil nitrogen and by 16% at low soil nitrogen.  Similar results were reported in the two-year open-top chamber study of Mikan et al. (2000), who observed 50 and 25% CO2-induced increases in total seedling biomass at high and low soil nitrogen levels, respectively.

In summary, as the air's CO2 content continues to increase, aspen seedlings will likely display enhanced rates of photosynthesis and biomass production, regardless of genotype, gender, O3-sensitvity, and soil nitrogen status.  Consequently, greater amounts of carbon will likely be sequestered in the tissues of this most abundant of North American tree species and in the soils in which they are rooted in the years and decades ahead.

For more information on aspen growth responses to atmospheric CO2 enrichment see Plant Growth Data: Bigtooth Aspen (dry weight, photosynthesis), and Quaking Aspen (dry weight, photosynthesis).

References
Curtis, P.S., Vogel, C.S., Wang, X.Z., Pregitzer, K.S., Zak, D.R., Lussenhop, J., Kubiske, M. and Teeri, J.A.  2000.  Gas exchange, leaf nitrogen, and growth efficiency of Populus tremuloides in a CO2-enriched atmosphere.  Ecological Applications 10: 3-17.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

King, J.S., Pregitzer, K.S. and Zak, D.R.  1999.  Clonal variation in above- and below-ground responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability.  Plant and Soil 217: 119-130.

Kruger, E.L., Volin, J.C. and Lindroth, R.L.  1998.  Influences of atmospheric CO2 enrichment on the responses of sugar maple and trembling aspen to defoliation.  New Phytologist 140: 85-94.

Kubiske, M.E., Pregitzer, K.S., Zak, D.R. and Mikan, C.J.  1998.  Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil N availability.  New Phytologist 140: 251-260.

Mikan, C.J., Zak, D.R., Kubiske, M.E. and Pregitzer, K.S.  2000.  Combined effects of atmospheric CO2 and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms.  Oecologia 124: 432-445.

Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F.  2001.  Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3.  Plant, Cell and Environment 24: 327-336.

Pregitzer, K.S., Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and Lussenhop, J.  2000.  Interactive effects of atmospheric CO2 and soil-N availability on fine roots of Populus tremuloides.  Ecological Applications 10: 18-33.

Wang, X. and Curtis, P.S.  2001.  Gender-specific responses of Populus tremuloides to atmospheric CO2 enrichment.  New Phytologist 150: 675-684.

Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J.  2000.  Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides.  Ecological Applications 10: 34-46.