What was done
The authors studied long-term effects of elevated CO2 on the photosynthetic rates of young silver birch trees that were rooted in the ground and exposed to 700 ppm CO2 over four growing seasons in open-top chambers located in England.  Some of the parameters they measured were net photosynthesis, stomatal conductance, leaf nitrogen and starch concentrations.

What was learned
Over the four years of the study, leaf assimilation rates in the CO2-enriched trees were consistently higher than those observed in trees growing in normal air.  This physiological advantage was maintained in spite of the fact that partial photosynthetic acclimation had occurred in the elevated CO2 treatment, as evidenced by an average decrease in leaf rubisco activity of 27% and a 13% reduction in leaf nitrogen per unit dry mass.  Some of this acclimation may have occurred as a result of a 100% increase in starch accumulation that was observed in trees grown at 700 ppm CO2, which probably provided a signal to down regulate photosynthesis.  Additionally, elevated CO2 caused an average decrease in stomatal conductance of 21%, which led to increased water-use-efficiencies for trees in this treatment.

What it means
Down regulation of photosynthesis occurs in this species without fully negating the photosynthetic stimulation brought about by atmospheric CO2 enrichment.  This finding suggests that silver birch trees will continue to increase their photosynthetic rates as the CO2 content of the air continues to rise; and the resulting increase in photosynthetic products will likely be used to facilitate further growth and development.  With parallel increases in water-use-efficiency, silver birch may thus be able to significantly expand the boundaries of its current habitat and better cope with droughts that could occur.  Clearly, then, down regulation is not a negative phenomenon.  In fact, the authors conclude that "it is possible that down regulation is a positive response to maximize resources, allowing the tree to use nutrients for more limiting processes without large effects on the net assimilation of the whole tree."