In the relatively short-term study of Wayne et al. (1998), yellow birch seedlings grown for two months at atmospheric CO2 concentrations of 800 ppm exhibited photosynthetic rates that were about 50% greater than those displayed by control seedlings fumigated with air containing 400 ppm CO2.  Similarly, in the three-month study of Tjoelker et al. (1998a), paper birch seedlings grown at 580 ppm CO2 displayed photosynthetic rates that were approximately 30% greater than those exhibited by seedlings exposed to 370 ppm CO2.  Likewise, Kellomaki and Wang (2001) reported that birch seedlings exposed to an atmospheric CO2 concentration of 700 ppm for five months displayed photosynthetic rates that were about 25% greater than seedlings grown at 350 ppm CO2.  Finally, in the much longer four-year study conducted by Wang et al. (1998), silver birch seedlings grown in open-top chambers receiving twice-ambient concentrations of atmospheric CO2 displayed photosynthetic rates that were fully 110% greater than rates displayed by their ambiently-grown counterparts.  Thus, short-term photosynthetic enhancements resulting from atmospheric CO2 enrichment appear to persist for several years or longer.

Because elevated CO2 enhances photosynthetic rates in birch trees, it likely will also lead to increased biomass production in these important deciduous trees, as it indeed has in several experiments.  In the three-month study of Tjoelker et al. (1998b), for example, a 57% increase in the air's CO2 content increased the biomass of paper birch seedlings by 50%.  When similar seedlings were grown at 700 ppm CO2 for four months, Catovsky and Bazzaz (1999) reported that elevated CO2 increased total seedling biomass by 27 and 130% under wet and dry soil moisture regimes, respectively.  In the interesting study of Godbold et al. (1997), paper birch seedlings grown at 700 ppm for six months not only increased their total biomass, but also increased the number of root tips per plant by over 50%.  In the longer two-year study of Berntson and Bazzaz (1998), twice-ambient levels of CO2 increased the biomass of a mixed yellow and white birch mesocosm by 31%; and in another two-year study, Wayne et al. (1998) reported that yellow birch seedlings grown at 800 ppm CO2 produced 60 and 227% more biomass than seedlings grown at 400 ppm CO2 at ambient and elevated air temperatures, respectively.  Finally, after exposing silver birch seedlings to twice-ambient CO2 concentrations for four years, Wang et al. (1998) noted that CO2-enriched seedlings produced 60% more biomass than ambiently-grown seedlings.  Hence, atmospheric CO2 enrichment clearly enhances birch biomass in both short- and medium-term experiments.

In some studies, elevated CO2 also reduced stomatal conductances in birch trees, thereby boosting their water-use efficiencies.  Tjoelker et al. (1998a), for example, reported that paper birch seedlings grown at 580 ppm CO2 for three months experienced 10-25% reductions in stomatal conductance, which contributed to 40-80% increases in water-use efficiency.  Similar CO2-induced reductions in stomatal conductance (21%) were reported in silver birch seedlings grown for four years at 700 ppm CO2 by Rey and Jarvis (1998).

The results of these several studies suggest that the ongoing rise in the air's CO2 content will likely increase rates of photosynthesis and biomass production in birch trees, as well as improve their water use efficiencies, irrespective of any concomitant changes in air temperature and/or soil moisture status that might occur.  Consequently, rates of carbon sequestration by this abundant temperate forest species should also increase in the years and decades ahead.

For more information on birch growth responses to atmospheric CO2 enrichment see Plant Growth Data: Bog Birch (dry weight, photosynthesis), Downy Birch (dry weight, photosynthesis), Gray Birch (dry weight), Paper Birch (dry weight, photosynthesis), and Yellow Birch (dry weight, photosynthesis).

References
Berntson, G.M. and Bazzaz, F.A.  1998.  Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling.  Oecologia 113: 115-125.

Catovsky, S. and Bazzaz, F.A.  1999.  Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure.  Global Change Biology 5: 507-518.

Godbold, D.L., Berntson, G.M. and Bazzaz, F.A.  1997.  Growth and mycorrhizal colonization of three North American tress species under elevated atmospheric CO2.  New Phytologist 137: 433-440.

Kellomaki, S. and Wang, K.-Y.  2001.  Growth and resource use of birch seedlings under elevated carbon dioxide and temperature.  Annals of Botany 87: 669-682.

Rey, A. and Jarvis, P.G.  1998.  Long-Term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees.  Tree Physiology 18: 441-450.

Tjoelker, M.G., Oleksyn, J. and Reich, P.B.  1998a.  Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature.  Tree Physiology 18: 715-726.

Tjoelker, M.G., Oleksyn, J. and Reich, P.B.  1998b.  Temperature and ontogeny mediate growth response to elevated CO2 in seedlings of five boreal tree species.  New Phytologist 140: 197-210.

Wang, Y.-P., Rey, A and Jarvis, P.G.  1998.  Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations.  Global Change Biology 4: 797-807.

Wayne, P.M., Reekie, E.G. and Bazzaz, F.A.  1998.  Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts.  Oecologia 114: 335-342.