Tissue et al. (1997) grew loblolly pine tree seedlings for a period of four years in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 650 ppm in a study of the long-term effects of elevated CO2 on the photosynthetic rates of this abundant species of pine. This experiment indicated that the trees growing in CO2-enriched air exhibited photosynthetic rates that were 60-130% greater than those exhibited by seedlings growing in ambient air during the warmer summer months, while during the colder winter months the extra CO2 boosted seedling photosynthetic rates by a lesser 14 to 44%.

Maier et al. (2002) constructed open-top chambers around loblolly pine trees that had been growing on an infertile sandy soil for 13 years, after which they fumigated them for two additional years with air containing atmospheric CO2 concentrations of either 350 or 550 ppm. This study indicated that the elevated CO2 enhanced the trees' net photosynthesis rates by 82%, with the trees showing no signs of photosynthetic acclimation over the two-year duration of the study.

Crous and Ellsworth (2004) made photosynthetic measurements of different-age needles at different crown positions on 19-year-old (in 2002) loblolly pine trees at the Duke Forest FACE facility -- where the CO2-enriched trees were exposed to air containing an extra 200 ppm of CO2 -- in the sixth year of a long-term study, after which the results were compared with the results of similar measurements made over the prior five years. In doing so, they found "some evidence of moderate photosynthetic down-regulation ... in 1-year-old needles across the fifth to sixth year of CO2 exposure." However, they report that "strong photosynthetic enhancement in response to elevated CO2 (e.g., +60% across age classes and canopy locations) was observed across the years." Hence, there is reason to believe that the positive effects of atmospheric CO2 enrichment on earth's woody plants may perhaps be maintained indefinitely, which bodes well indeed for all components of the biosphere that will live in the high-CO2 world of the future that grows ever closer with each passing day.

References
Crous, K.Y. and Ellsworth, D.S. 2004. Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest. Tree Physiology 24: 961-970.

Maier, C.A., Johnsen, K.H., Butnor, J., Kress, L.W. and Anderson, P.H. 2002. Branch growth and gas exchange in 13-year-old loblolly pine (Pinus taeda) trees in response to elevated carbon dioxide concentration and fertilization. Tree Physiology 22: 1093-1106.

Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134.