What was done
The authors studied the effects of long-term atmospheric CO2 enrichment on gas exchange in mature oak (Quercus pubescens and Quercus ilex) trees growing naturally near CO2-emitting springs in central Italy.  Physiological and biochemical measurements were made on trees that had been exposed to atmospheric CO2 concentrations of approximately 370 and 700 ppm for their entire lifetimes, which ranged from 30 to 50 years in duration.  Q. pubescens is a deciduous less drought-tolerant species than
Q. ilex, which is a strongly drought-tolerant evergreen species.

What was learned
The CO2-enriched Q. pubescens and Q. ilex trees exhibited net photosynthetic rates that were 69 and 26% greater, respectively, than those displayed by control trees exposed to ambient CO2 concentrations, in spite of CO2-induced decreases of 30 and 15% in their respective foliar rubisco concentrations.  In addition, stomatal conductances of CO2-enriched Q. pubescens trees were approximately 23% lower than those of controls, while stomatal conductances of Q. ilex trees displayed no CO2-sensitivity.  Nonetheless, both species exhibited increased water use efficiencies as a consequence of elevated CO2.

What it means
As the atmospheric CO2 concentration continues to rise, Mediterranean oaks will likely exhibit enhanced rates of photosynthesis, leading to greater biomass production and carbon sequestration.  Future increases in the air's CO2 content may also help to alleviate water stress in the less drought tolerant Q. pubescens, due to significant CO2-induced reductions in its stomatal conductance.  In addition, elevated CO2 should greatly enhance the water use efficiencies of both species.  These results demonstrate that mature trees can continue to exhibit enhanced rates of photosynthesis and increases in water use efficiency even after decades of exposure to elevated atmospheric CO2 concentrations.