Background
Water use efficiency is the ratio of photosynthesis (A) to transpiration (E), or the amount of carbon gained per unit of water used in the process of acquiring the carbon. Likewise, A/gs -- where gs is stomatal conductance -- is referred to as intrinsic water use efficiency (Wi); and the authors state that "an increase in Wi in response to increasing CO2 since the industrial revolution has been found in nearly all temperate trees that have been studied," citing the work of Feng (1999), Saurer et al. (2004) and Nock et al. (2010). Thus, they decided to see if such was also the case in tropical trees.

What was done
Noting that "increases in Wi have been observed in short-term experiments of tree responses to elevated CO2 (Norby et al., 1999), and over long-time periods using records of δ¹³C in tree rings that reflect the global increase in atmospheric CO2 (Feng, 1999; Waterhouse et al., 2004)," Brienen et al. "analyzed carbon isotope ratios over the last 40 years in tree rings of Mimosa acantholoba, a tropical dry forest pioneer species," in a study conducted "on the Pacific slope of the isthmus of Tehuantepec, close to the village of Nizanda in the state of Oaxaca, South Mexico (16°39'N, 95°00'W)."

What was learned
The three researchers, representing Austria, Mexico and the United Kingdom, report that the dry-forest tropical M. acantholoba trees "responded strongly to the increase in atmospheric CO2 over the last four decades," as their "Wi increased dramatically by 40%."

What it means
Brienen et al. say their results show that "δ¹³C in tree rings is a promising tool to evaluate long-term responses of tropical trees to increasing CO2," and that such studies "may be used to test DGVMs [dynamic global vegetation models] which predict large-scale die-off of the Amazon rainforest (Betts et al., 2004)," but which they say "remain highly simplistic due to lack of data and understanding of key processes."

References
Betts, R., Cox, P.M., Collins, M., Harris, P.P., Huntingford, C. and Jones, C.D. 2004. The role of ecosystem-atmosphere interactions in simulated Amazonian precipitation decrease and forest dieback under global climate warming. Theoretical and Applied Climatology 78: 157-175.

Feng, X.H. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: a response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Nock, C.A., Baker, P.J., Wanek, W., Leis, A., Grabner, M. Bunyavejchewin, S. and Hietz, P. 2010. Long-term increases in intrinsic water-use efficiency do not lead to increased stem growth in a tropical monsoon forest in western Thailand. Global Change Biology 17: 1049-1063.

Norby, R.J., Wullschleger, S.D., Gunderson, C.A., Johnson, D.W. and Ceulemans, R. 1999. Tree responses to rising CO2 in field experiments: implications for the future forest. Plant, Cell and Environment 22: 683-714.

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.

Waterhouse, J.S., Switsur, V.R., Barker, A.C., Carter, A.H.C., Hemming, D.L., Loader, N.J. and Robertson, I. 2004. Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations. Quaternary Science Reviews 23: 803-810.