What was done
Tree-ring width and density chronologies (both earlywood and latewood) from 21 stands of Aleppo pine (Pinus halepensis Mill.) in the Provence region of southeast France were used to calibrate the BIOME3 biogeochemistry model of forest productivity in terms of growth responses to known historical changes in atmospheric temperature, precipitation and CO2 concentration, after which the BIOME3 model was used to calculate changes in the mean productivity of the same forest stands that could be expected to result from changes in these parameters driven by a doubling of the air's CO2 content, as calculated by Meteo-France's ARPEGE atmospheric general circulation model when downscaled to that specific part of the country.

What was learned
In response to the predicted changes in climate, forest productivity increased moderately for all stands (17% to 24%); while in response to the aerial fertilization effect of the doubling of the air's CO2 content, it increased considerably more (72% to 86%).  Even more impressively, when the climatic changes and atmospheric CO2 increase were considered together, forest productivity increased still more (107% to 141%), which response is greater than that provided by the sum of their individual contributions, due to the amplifying synergy that exists among these factors with respect to their combined impact on basic plant physiological processes.

What it means
In the words of the authors, "although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests."  Indeed, their study suggests, based on real-world-derived relationships, that a doubling of the air's CO2 content could well more than double the growth of Aleppo pine forests in southeast France.