Background
Due to a phenomenon referred to as acclimation, the initial beneficial effects of atmospheric CO2 enrichment on plant CO2 exchange with the atmosphere (net photosynthesis and dark respiration) often decline over time, although they typically never become negligible. And thus it is that long term atmospheric CO2 enrichment experiments, conducted under as natural outdoor conditions as possible, assume a most important role in helping to discern the future growth potential of earth's plants in a substantially CO2-enriched world of the future.

What was done
In a study designed to acquire such knowledge, Lhotakova et al. exposed recently-transplanted and initially well-fertilized ten-year-old Norway spruce (Picea abies) trees to both ambient CO2 concentrations (365-377 ppm) and enriched CO2 concentrations (700 ppm) over the eight-year period 1996-2004 within two semi-open glass domes with adjustable windows at a site in the Beskydy Mountains of the Czech Republic, at the end of which period they measured, among a number of other things, needle light-saturated net photosynthesis (ANmax) and dark respiration (RD) rates for trees grown in both sunlit and shaded conditions.

What was learned
At the end of the eight-year study, the CO2-induced increase in the rate of light-saturated net photosynthesis, due to the approximate 330-ppm increase in the air's CO2 concentration, was about 113% in the sunlit trees and 54% in the shaded trees, as best we can determine from the graphical representation of the authors' results; while the CO2-induced reduction in the rate of dark respiration was about 22% in the sunlit trees and 39% in the shaded trees.

What it means
Clearly, these results are extremely positive because, as the nine Czech researchers indicate, "the observed stimulation of ANmax simultaneously with suppressed RD under elevated CO2 may lead to higher biomass accumulation." And they buttress this conclusion by noting that findings similar to what they found have also been shown "in other long-term studies on conifers," citing the work of Crous et al. (2008), Maier et al. (2008), Kosvancova et al. (2009) and Logan et al. (2009).

References
Crous, K.Y., Walters, M.B. and Ellsworth, D.S. 2008. Elevated CO2 concentration affects leaf photosynthesis-nitrogen relationships in Pinus taeda over nine years in FACE. Tree Physiology 28: 607-614.

Kosvancova, M., Urban, O., Sprtova, M., Hrstka, M., Kalina, J., Tomaskova, I., Spunda, V. and Marek, M.V. 2009. Photosynthetic induction in broadleaved Fagus sylvatica and coniferous Picea abies cultivated under ambient and elevated CO2 concentrations. Plant Science 177: 123-130.

Logan, B.A., Combs, A., Myers, K., Kent, R., Stanley, L. and Tissue, D.T. 2009. Seasonal response of photosynthetic electron transport and energy dissipation in the eighth year of exposure to elevated atmospheric CO2 (FACE) in Pinus taeda (loblolly pine). Tree Physiology 29: 789-797.

Maier, C.A., Palmroth, S. and Ward, E. 2008. Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO2 concentration. Tree Physiology 28: 597-606.