In many areas of the world, there exist natural vents and springs that have emitted concentrated amounts of CO2 into the air for centuries, thereby exposing surrounding vegetation to elevated concentrations of atmospheric CO2 for their entire lifetimes.  Several researchers have taken advantage of these natural settings to determine the long-term effects of atmospheric CO2 enrichment on trees and other woody plants.  Stylinski et al. (2000), for example, observed that net photosynthetic rates in mature oak trees subjected to 700 ppm CO2 for approximately 40 to 50 years were 36 to 77% greater than those measured in control trees exposed to normal ambient CO2 concentrations for the same time period.  Similarly, 30-year old Arbutus unedo trees exposed to a lifetime atmospheric CO2 concentration of 465 ppm displayed net photosynthetic rates that were 110 to 140% greater than those observed in trees continuously exposed to normal non-CO2-enriched air (Bartak et al., 1999).  With respect to other parameters, Tognetti et al. (2000) found that leaves of three different shrubs exposed to lifetime atmospheric CO2 concentrations of 700 ppm exhibited significantly greater turgor pressures than leaves of control shrubs, particularly during the warmer summer months; and Paoletti et al. (1998) observed a 1.5-fold reduction in the stomatal frequency of leaves of mature white oak trees exposed to 750 ppm CO2 for several decades.

In shorter open-top chamber studies of pine trees, some interesting discoveries have been made concerning tree growth and wood density.  Walker et al. (2000) reported that ponderosa pine seedlings exposed to twice-ambient levels of atmospheric CO2 for five years displayed average heights and trunk diameters that were 44 and 39% greater, respectively, than those exhibited by their ambiently-grown counterparts; while Telewski et al. (1999) reported that annual growth-ring widths in trees subjected to an atmospheric CO2 concentration of 650 ppm were 93, 29, 15 and 37% greater than those observed in ambient controls for the first four years of the study.  Similarly, the average ring densities in CO2-enriched trees for each of the four study years were 60, 4, 3 and 5% greater than those measured in control trees subjected to 350 ppm CO2.

Finally, in a review of 180 different tree experiments, Idso (1999) showed that almost any conceivable growth response to atmospheric CO2 enrichment could be obtained if the trees were grown in pots or other types of root-restricting containers for periods of less than a year or two.  On the other hand, he demonstrated that if trees were rooted in the ground and exposed to elevated atmospheric CO2 concentrations for longer time periods, much more reliable data could be obtained.  Based on such studies, Idso reported that the mean growth enhancement in four such experiments involving three tree species exposed to twice-ambient levels of atmospheric CO2 was 90% after five years, which is consistent with data reported for mature trees growing near CO2-emmitting springs and vents.

In summary, it is evident that as the air's CO2 content continues to rise, earth's woody shrubs and trees will likely respond by enhancing their photosynthetic rates and biomass production.  Moreover, these species may reduce their leaf stomatal frequencies, thus reducing water loss to the atmosphere and consequently increasing their water-use efficiencies.  With all these good things happening to them, woody plants will likely sequester increasingly larger amounts of carbon in their tissues as time progresses, thereby slowing the rate of rise of the air's CO2 content.

References
Bartak, M., Raschi, A. and Tognetti, R.  1999.  Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2.  Photosynthetica 37: 1-16.

Idso, S.B.  1999.  The long-term response of trees to atmospheric CO2 enrichment.  Global Change Biology 5: 493-495.

Paoletti, E., Nourrisson, G., Garrec, J.P. and Raschi, A.  1998.  Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments.  Plant, Cell and Environment 21: 1071-1075.

Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A.  2000.  Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques.  Plant, Cell and Environment 23: 1353-1362.

Telewski, F.W., Swanson, R.T., Strain, B.R. and Burns, J.M.  1999.  Wood properties and ring width responses to long-term atmospheric CO2 enrichment in field-grown loblolly pine (Pinus taeda L.).  Plant, Cell and Environment 22: 213-219.

Tognetti, R., Rashi, A. and Jones, M.B.  2000.  Seasonal patterns of tissue water relations in three Mediterranean shrubs co-occurring at a natural CO2 spring.  Plant, Cell and Environment 23: 1341-1351.

Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T.  2000.  Growth, nutrition, and water relations of ponderosa pine in a field soil as influenced by long-term exposure to elevated atmospheric CO2.  Forest Ecology and Management 137: 1-11.