What was done
Open-top chambers were established around 30-year-old mature oak (Quercus ilex) trees growing naturally in perennial evergreen stands located near central Italy.  Trees were continually exposed to atmospheric CO2 concentrations of 350 and 700 ppm for five years to study the long-term effects of elevated CO2 on photosynthesis within this important tree species.

What was learned
Elevated CO2 increased rates of net photosynthesis in sun-exposed and shaded leaves by 68 and 59%, respectively, in comparison with control rates measured on leaves of trees exposed to ambient air.  In addition, after measuring short-term photosynthetic rates at various atmospheric CO2 concentrations, the authors reported that photosynthetic acclimation was not apparent in leaves of these mature trees exposed to long-term atmospheric CO2 enrichment.  Interestingly, the authors also determined that the CO2 light compensation point - the light level at which photosynthetic carbon uptake is equivalent to respiratory carbon loss - was 24 and 30% lower in sun-exposed and shaded leaves, respectively, of CO2-enriched trees than what was measured in corresponding control leaves of ambiently-growing trees.

What it means
As the air's CO2 content increases, it is likely that the stimulatory effect of elevated CO2 on photosynthesis in oak seedlings, which is well-documented, will persist in the long-term within mature trees, without showing any signs of photosynthetic acclimation.  In addition, because elevated CO2 significantly lowered the light compensation point in mature oak trees, which would allow them to exhibit net carbon gains earlier in the mornings and maintain them later into the evenings, the stimulatory effect of elevated CO2 on daily carbon uptake would be further enhanced.  Together, these two observations suggest that carbon sequestration by this and perhaps other tree species will likely be much greater and more substantial in future CO2-enriched atmospheres than what is currently being projected.