What was done
The authors used a modified version of the Sheffield Dynamic Global Vegetation Model described by Woodward and Lomas (2004) to study changes in the structure, composition and carbon storage of vegetation and soils throughout all of China in response to changes in climate and atmospheric CO2 concentration that occurred between 1901 and 2000.

What was learned
Mao et al. report that their modeling exercise suggested that "during the past 100 years a combination of increasing CO2 with historical temperature and precipitation variability in continental China have caused the total vegetation carbon storage to increase by 2.04 Pg C, with 2.07 Pg C gained in the vegetation biomass but 0.03 Pg C lost from the organic soil carbon matter." They also found that "the increasing CO2 concentration in the 20th century is primarily responsible for the increase of the total potential vegetation carbon." However, because the biological effects of temperature and precipitation were negative, the historical increase in the air's CO2 content was actually totally responsible for the net storage of carbon in China's terrestrial vegetation and soil over the 20th century, since without the aerial fertilization effect of CO2, there would have been a net loss of carbon that exceeded in absolute value the net gain that ultimately prevailed.

What it means
China has helped both its people and their country's natural ecosystems -- as well as the people and lands of the rest of the world -- more than they ever knew they were doing, as a result of their mining and burning massive amounts of coal over the latter part of the 20th century, which provided a goodly portion of the increase in the atmosphere's CO2 concentration that fueled the great Greening of the Earth, which has enabled the planet to sustain an ever greater presence of life upon its surface.

Reference
Woodward, F.I. and Lomas, M.R. 2004. Vegetation dynamics -- Simulation responses to climatic change. Biological Reviews 79: 643-670.