Background
Trees grown for long periods of time in elevated CO2 environments often, but not always, exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by modestly reduced rates of photosynthesis (compared to what might be expected on the basis of short-term exposure to CO2-enriched air) that result from a long-term decrease in the activity and/or amount of the primary plant carboxylating enzyme rubisco.

What was done
The authors measured many plant physiological processes and properties throughout the fourteenth year of a long-term study of the effects of a 75% increase in the air's CO2 concentration on the growth and development of sour orange trees that had been grown from the seedling stage to maturity under well-watered and fertilized conditions out-of-doors at Phoenix, Arizona, USA in clear-plastic-wall open-top enclosures, after which they compared certain of their results with those of similar measurements made in earlier years of the study.

What was learned
In the second year of the experiment, net photosynthesis rates were 2.84 times greater in the CO2-enriched enclosures than in the ambient-air enclosures.  By the sixth year of the study, however, this enhancement ratio had declined to 1.75, while in the fourteenth year it had dropped to 1.45.  Plotting similarly-declining above-ground woody biomass ratios against these net photosynthesis ratios, Adam et al. derived a linear relationship with an r² value of 0.997 that yielded a CO2-induced woody biomass enhancement ratio of 1.78 at the 14-year point of the study.  This value for the woody biomass ratio had previously been found by Idso and Kimball (2001) to have been essentially constant from year 10 to year 14, leading Adam et al. to conclude that the CO2-induced net photosynthesis ratio they derived (1.45) had likely also been essentially constant over this period, indicative of the final equilibrium level at which it had apparently stabilized.

Other evidence for the down-regulation of photosynthesis in the CO2-enriched sour orange tree leaves was provided by the observation that in year 14 the pooled mean of the large subunit of Rubisco in the CO2-enriched leaves was only 78% of that observed in the ambient-air leaves, while the small subunit of Rubisco was reduced by 34% in the CO2-enriched leaves compared to the ambient-air leaves.  In addition, the full and initial activities of Rubisco under CO2 enrichment were reduced, as were leaf chlorophyll a and total nitrogen concentrations.

What it means
Adam et al. conclude that "long-term CO2 enrichment can result in photosynthetic down-regulation in leaves of trees, even under nonlimiting nitrogen conditions."  It should also be pointed out, however, that at the final equilibrium level of acclimation experienced in the sour orange trees of this study, the 75% enhancement of the air's CO2 concentration still produced an equivalent percentage increase (or possibly slightly more) in both wood and fruit production (78 and 80%, respectively).