What was done
The authors used a process-based model (BIOME-BGC) "to investigate the response of Picea schrenkiana forest to future climate changes and atmospheric carbon dioxide concentration increases in the Tianshan Mountains of northwestern China," which they "validated by comparing simulated net primary productivity (NPP) under current climatic conditions with independent field-measured data." The specific climate change scenario employed in this endeavor was a double-CO2-induced temperature increase of 2.6°C and precipitation increase of 25%.

What was learned
When the predicted precipitation increase was considered by itself, the NPP of the P. schrenkiana forest increased by 14.5%; while the predicted temperature increase by itself increased forest NPP by 6.4%, and the CO2 increase by itself boosted NPP by only 2.7%. When the predicted increases in precipitation and temperature occurred together, forest NPP increased by a larger 18.6%, which is just slightly less than the sum of the two individual effects; but when the CO2 concentration increase was added to the mix and all three factors increased together, the Chinese researchers report that forest NPP "increased dramatically [our italics], with an average increase of about 30.4%."

What it means
Su et al. conclude that comparison of the results derived from the various scenarios of their study indicates that "the effects of precipitation and temperature change were simply additive, but that the synergy between the effects of climate change and doubled CO2 was important," as it made the whole response much larger than the sum of its separate responses, due to the fact that "feedback loops associated with the water and nitrogen cycles [which may be influenced significantly by atmospheric CO2 enrichment] ultimately influence the carbon assimilation response."