What was done
The authors compared, synthesized and evaluated the scientific literature describing (1) atmospheric CO2 enrichment experiments conducted on trees and (2) empirical tree-ring studies designed to determine if the growth-promoting effects of rising atmospheric CO2 concentrations occur in natural forests.

What was learned
Huang et al. report that numerous CO2-enrichment experiments have "demonstrated significantly positive physiological and growth responses of trees to CO2, providing strong evidence to support the direct CO2 fertilization effect (increased photosynthesis, water use efficiency, above- and below-ground growth) and thus allowing prediction of which ecosystems might be most responsive to CO2," with their thoughts in this regard being "warm, moderately drought-stressed ecosystems with an ample nitrogen supply," because, as they continue, "drought-stressed trees could benefit from increased water use efficiency to enhance growth." They also note, however, that tree-ring studies on the cold and arid Tibetan Plateau "showed significant growth enhancements as well as increased water use efficiency (24.7% and 33.6% for each species, respectively) in Qilian juniper and Qinghai spruce since the 1850s," citing as evidence for this statement the studies of Zhang et al. (2003), Shao et al. (2005), Liang et al. (2006), Huang and Zhang (2007) and Zhang and Qiu (2007).

What it means
In light of the findings of this two-pronged review of the pertinent scientific literature, it is little wonder we continue to find numerous papers describing the historic and ongoing Greening of the Earth, which has been driven by the joint effects of the CO2 emitted to the air by the engines of the Industrial Revolution and the warming of the globe that brought about the demise of the Little Ice Age, as well as the positive synergism that exists between the latter two phenomena, whereby the optimum temperature for C3-plant photosynthesis rises ever higher as the air's CO2 concentration rises ever higher, as is also described in the review of Huang et al. plus the many reviews of papers we have archived under the heading of Growth Response to CO2 with Other Variables (Temperature) in our Subject Index.

Yes, the aerial-fertilization and transpiration-reducing effects of atmospheric CO2 enrichment are real, and they have been having a positive impact on earth's trees ever since humanity has been burning fossil fuels.

References
Huang, J.G. and Zhang, Q.B. 2007. Tree-rings and climate for the last 680 years in Wulan area of northeastern Qinghai-Tibetan Plateau. Climatic Change 80: 369-377.

Liang, E.Y., Shao, X.M., Eckstein, D., Huang, L. and Liu, X.H. 2006. Topography- and species-dependent growth response of Sabina przewalskii and Picea crassifolia to climate on the northeast Tibetan Plateau. For. Ecol. Manage. 236: 268-277.

Shao, X.M., Huang, L., Liu, H.B., Liang, E.Y., Fang, X.Q. and Wang, L.L. 2005. Reconstructions of precipitation variation from tree-rings in recent 1000 years in Delingha, Qinghai. Sci. China 48: 939-949.

Zhang, Q.B., Cheng, G.D., Yao, T.D., Kang, X.C. and Huang, J.G. 2003. A 2,326-year tree-ring record of climate variability on the northeastern Qinghai-Tibetan Plateau. Geophysical Research Letters 30: 10.1029/2003GL017425.

Zhang, Q.B. and Qiu, H.Y. 2007. A millennium-long tree-ring chronology of Sabina przewalskii on northeastern Qinghai-Tibetan Plateau. Dendrochronologia 24: 91-95.