What was done
Cuttings, taken from 4-year-old Monterey Pine (Pinus radiata D. Don) trees that had experienced lifetime exposure to either ambient (360 ppm) or elevated (650 ppm) atmospheric CO2 concentrations, were grown for one year in their respective CO2 treatments.  The study was conducted at Christchurch, New Zealand in open-top chambers, where the trees were irrigated daily and fertilized every three months.

What was learned
In the words of the authors, "photosynthetic rate in young needles during summer, autumn and spring was 34, 43 and 38% higher, respectively, in trees grown at elevated CO2 than in trees grown at ambient CO2," while "in older needles, the corresponding photosynthetic rate increases were 26, 47 and 49%."  In addition, water use efficiency was 49% higher in the foliage of the elevated CO2 treatment, although there was no change in needle stomatal conductance.  These responses were comparable to those observed in the parent trees when they were approximately the same size and age as the second-generation trees.  However, there were signs of photosynthetic acclimation in the older needles of the parent trees when they were larger (in their 3rd and 4th years) but physiologically younger than the second-generation trees, which by the time of this study (having been derived from 4-year-old trees and grown for an additional year) were in their fifth year of life.

What it means
The authors suggest that the down-regulation of photosynthesis observed in the parent trees in their third and fourth years "was a result of a shift in the proportion of young to old needles as the trees increased in size."  They hypothesize that "in small trees, close proximity of active sinks, such as developing buds, to a proportionally small reservoir of source tissue (mature foliage) would increase the overall sink strength and reduce the extent of photosynthetic acclimation (down-regulation) at elevated CO2."  They note, for example, that "within an immature P. radiata canopy, young needles may represent as much as 4 to 10 times the biomass of older needles," but that "as the canopy matures this ratio approaches equality (Turnbull et al., 1998)."  Hence, they conclude that "down-regulation of photosynthesis at elevated CO2 is related to tree size rather than tree age or duration of exposure," and that "the capacity for enhanced photosynthesis in trees growing in elevated CO2 is unlikely to be lost in subsequent generations."

References
Turnbull, M.H., Tissue, D.T., Griffin, K.L., Rogers, G.N.D. and Whitehead, D.  1998.  Photosynthetic acclimation to long-term exposure to elevated CO2 concentration in Pinus radiata D. Don is related to age of needles.  Plant, Cell and Environment 21: 1019-1028.