What was done
The authors studied the long-term effects of naturally occurring elevated CO2 concentrations on two species of mature oak trees growing in a Mediterranean climate in central Italy by measuring rates of photosynthesis, respiration, and stomatal conductance, calculating water-use-efficiencies and wood densities, and determining the amounts of nonstructural carbohydrates in leaves.  One species was deciduous (Quercus pubescens), which losses its leaves in the fall, and the other was an evergreen (Quercus ilex), which retains its leaves year-round.  Both types of oak had developed for 15 to 25 years in a basin containing CO2 springs that raised the CO2 content of the surrounding air to levels of 500 to 1000 ppm.

What was learned
In the summer, trees growing in elevated CO2 displayed photosynthetic rates that were, on average, 2.75 times and 6.1 times higher than rates observed in trees growing in normal air when measured in the early morning and afternoon, respectively.  Increased rates of dark respiration were also reported for both species with atmospheric CO2 enrichment, which was coincident with an observed increase in nonstructural carbohydrates in the CO2-enriched trees.  However, the amount of excess carbon lost via dark respiration from the trees exposed to elevated CO2 was much less than that gained from photosynthesis; and the net positive retention of this carbon was used, in part, to increase wood densities by 3% and 6% for the deciduous and evergreen oak species, respectively, in the high CO2 environment.

Elevated CO2 was also found to significantly reduce stomatal conductance relative to control trees, which resulted in transpiration reductions and higher water-use-efficiencies for both species.  The improvement in water-use-efficiency was so significant, in fact, that it prompted the authors to state that "such marked increases in water-use-efficiency under elevated CO2 might be of great importance in Mediterranean environments in the perspective of global climate change."

What it means
There is little evidence of down regulation of photosynthesis in these Italian oak trees that have grown for 15 to 25 years in air that has been naturally enriched with CO2.  On the other hand, the trees clearly experience enhanced water-use-efficiencies that should allow them to survive in their Mediterranean climate even if moisture becomes less available due to global climate change.  Furthermore, they should be able to significantly expand their ranges with the increased water-use-efficiencies they will experience as the CO2 content of the air climbs ever higher.