What was done
Defining intrinsic water-use efficiency (iWUE) as the ratio of the photosynthetic uptake of CO2 through leaf stomata to the simultaneous transpirational loss of water vapor through the stomata, the authors evaluated this parameter based on δ¹³C measurements of a-cellulose extracted from the wood of tree-ring cores taken from living Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) trees, focusing on the period AD 1850-2000 at time resolutions of three years for juniper from the semi-arid Qilian Mountains, two years for juniper from the arid Qaidam Basin and one year for spruce from both of the northwest China sites.

What was learned
Overall, based on means for the first and last decades of the study period, the seven Chinese researchers report that "the iWUE values of the two species both showed long-term increases, by 33.6 and 37.4% for spruce in the aird and semi-arid areas, respectively, versus increases of 24.7 and 22.5% for juniper," noting that "the main cause of this behavior is likely to be an increase in atmospheric CO2 concentration," which for the start and end decades of the study period rose from approximately 285 ppm to 362 ppm, or by about 27%.

What it means
In arid and semi-arid regions, increases in the water use efficiencies of trees must be considered a significant benefit. In the case of the two species studied by Liu et al., they report that Qinghai spruce, in particular, "plays an important role in preventing soil erosion, regulating climate, and retaining ecological stability," citing the work of Zhou and Li (1990) in this regard. This phenomenon is undoubtedly one of the chief reasons for the concomitant "greening of the earth" that has been so evident in many historical studies of China and other parts of Asia, as indicated by the materials archived under Greening of the Earth (Asia) in our Subject Index.

Reference
Zhou, Y. and Li, S. 1990. Forestry in China. Science Press, Beijing, China.