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The Intrinsic Water Use Efficiency of Spruce Trees in China's Xinglong Mountains
Wang, W., Liu, X., An, W., Xu, G. and Zeng, X. 2012. Increased intrinsic water-use efficiency during a period with persistent decreased tree radial growth in northwestern China: Causes and implications. Forest Ecology and Management 275: 14-22.

The authors say that intrinsic water-use efficiency (iWUE) "represents the ratio of photosynthetic assimilation (A) to stomatal conductance (gw)," while noting that "higher iWUE can result from reducing gw, increasing A, or a combination of the two responses." They also say that "empirical evidence from lab studies with a controlled CO2 concentration and from free-air CO2 enrichment (FACE) experiments have revealed significantly increased iWUE in response to rising CO2," as demonstrated by the studies of Luo et al. (1996), Ainsworth and Rogers (2007) and Niu et al. (2011). And they indicate that "tree-ring stable carbon isotope ratios (δ13C) have proven to be an effective tool for evaluating variations in iWUE around the world," citing Farquhar et al. (1989), Saurer et al. (2004), Liu et al. (2007) and Andreu et al. (2011). What is more, they report that "during the past 100-200 years, most of the sampled forests demonstrated a trend of increasing iWUE, which paralleled the increasing atmospheric CO2," citing Penuelas et al. (2011) and other references therein.

What was done
Working at a site in the Xinglong Mountains in the eastern part of northwestern China (35°40'N, 104°02'E), Wang et al. extracted two cores from the trunks of each of 17 dominant living Qinghai spruce (Picea crassifolia) trees in November 2009, from which they obtained precise ring-width measurements that they used to calculate yearly mean basal area growth increments, after which they used subsamples of the cores to conduct the stable carbon isotope analyses needed to obtain the δ13C data required to calculate iWUE over the course of their study period: 1800-2009. Also, by calibrating the δ13C data against climatic data obtained at the nearest weather station over the period 1954-2009, they were able to extend the histories of major meteorological parameters all the way back to 1800. And by comparing these weather data with the tree growth and water use efficiency data, they were able to interpret the impacts of climate change and atmospheric CO2 enrichment on spruce tree growth and water use efficiency.

What was learned
For the arid region of northwestern China in which the spruce trees they studied were growing, the five Chinese researchers determined that iWUE increased by approximately 40% between 1800 and 2009, rising very slowly for the first 150 years, but then more rapidly to about 1975, and then faster still until 1998, whereupon it leveled off for the remaining eleven years of the record. And in commenting on the main cause of the increasing trend in iWUE from 1800 to 1998, they say it "is likely to be the increase in atmospheric CO2," because "regression analysis suggested that increasing atmospheric CO2 explained 83.0% of the variation in iWUE from 1800 to 1998 (p<0.001)." Thereafter, however, they note that a substantial drought at the end of the record is probably what caused the leveling off of iWUE, which was also strong enough to cause a decline in yearly basal area growth increment, much as what occurred between 1923 and 1934, which period they described as "the most severe drought since 1800," citing Fang et al. (2009).

What it means
The ultimate knowledge gained from of Wang et al.'s study would appear to be that the historical increase in the air's CO2 content over the course of the Industrial Revolution gradually but greatly enhanced the intrinsic water use efficiency of Qinghai spruce trees in northwest China, as well as their growth rates. However, during times of very severe drought stress, even this added help can fall short of what is needed to keep the trees from maintaining an exemplary rate of growth. Nevertheless, the continually rising atmospheric CO2 concentration sees them through their times of real distress to when they can once again grow like gangbusters once the drought is past.

Ainsworth, E.A. and Rogers, A. 2007. The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions. Plant, Cell and Environment 30: 258-270.

Andreu, L., Planells, O., Gutierrez, E., Muntan, E., Helle, G., Anchukaitis, K.J. and Schleser, G.H. 2011. Long tree-ring chronologies reveal 20th century increases in water-use efficiency but no enhancement of tree growth at five Iberian pine forests. Global Change Biology 17: 2095-2112.

Fang, K., Gou, X., Chen, F., Yang, M., Li, J., He, M., Zhang, Y., Tian, Q. and Peng, J. 2009. Drought variations in the eastern part of Northwest China over the past two centuries: evidence from tree rings. Climate Research 38: 129-135.

Farquhar, G.D., Ehleringer, J.R. and Hubick, K.T. 1989. Carbon isotope discrimination and photosynthesis. Annual Reviews of Plant Physiology and Plant Molecular Biology 40: 503-537.

Liu, X.H., Shao, X.M., Liang, E.Y., Zhao, L.J., Chen, T., Qin, D.H. and Ren, J.W. 2007. Species-dependent responses of juniper and spruce to increasing CO2 concentration and to climate in semi-arid and arid areas of northwestern China. Plant Ecology 193: 195-209.

Luo, Y., Sims, D.A., Thomas, R.B., Tissue, D.T. and Ball, J.T. 1996. Sensitivity of leaf photosynthesis to CO2 concentration is an invariant function for C3 plants: a test with experimental data and global applications. Global Biogeochemical Cycles 10: 209-222.

Niu, S., Xing, X., Zhang, Z., Xia, J., Zhou, X., Song, B., Li, L. and Wan, S. 2011. Water-use efficiency in response to climate change: from leaf to ecosystem in a temperate steppe. Global Change Biology 17: 1073-1082.

Penuelas, J., Canadell, J.G. and Ogaya, R. 2011. Increased water-use efficiency during the 20th century did not translate into enhanced tree growth. Global Ecology and Biogeography 20: 597-608.

Saurer, M., Siegwolf, R.T.W. and Schweingruber, F.H. 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years. Global Change Biology 10: 2109-2120.

Reviewed 14 November 2012