What was done
The authors grew 20 oak (Quercus ilex) seedlings in 16- x 120-cm tall PVC pipes in two compartments of a controlled-environment greenhouse, one of which was maintained at an atmospheric CO2 concentration of 350 ppm and one of which was maintained at 700 ppm.  Air temperature and vapor pressure deficit were set to track outside ambient conditions.  The plants were exposed to natural sunlight and were watered every other week with 200 ml of water.  Various growth parameters and physiological responses of the seedlings were measured over a four-month period that began when the trees had been exposed to the two CO2 treatments for a total of ten months.

A unique and important aspect of this part of the study was the measurement of plant monoterpene emissions; because, in the words of the authors, "monoterpenes serve as carbon-based defensive compounds playing important roles in plant-parasite interactions."  In this regard, they also note that "an alteration of the plant's capacity to defend or to compete can be crucial in habitats where leaf growth is limited by environmental stresses such as drought," which is clearly the case for Mediterranean vegetation that is rich in monoterpene-producing plants.

What was learned
The extra CO2 increased the young oak trees' leaf area by 40% and their leaf biomass and volume by 50%.  The leaves of the CO2-enriched trees also, in the words of the authors, "had increased proportions of lignin and cellulose, and were thick and sclerophyllous, a characteristic that is positively related with leaf longevity and leaf resistance to low water potential."  Even more impressive was the fact that trunk and branch biomass increased by 90% in the doubled-CO2 treatment.  The authors note that this increased allocation of resources to woody tissues "would favor the capture of belowground resources."  Finally, and perhaps most impressive of all, were the results of the authors' monoterpene emission measurements.  "On average," they report, "plants grown in elevated CO2 had 1.8-fold higher emission capacities and released 2.8-fold more monoterpenes per plant than plants grown in ambient CO2."

What it means
The authors' results suggest that Mediterranean forests will become more resistant to insect pests as the air's CO2 content continues to climb.  They also suggest that the increased foliage of these trees will be longer-lived and better adapted to droughty conditions, and that the trees will be better able to acquire plant nutrients from the soil.  Hence, we could well expect Mediterranean forests to increase their productivity and well-being in the years ahead.  They will also likely expand their ranges into areas that are presently too dry or poor in nutrients to support them at the present time.