Background
Plant intrinsic water-use efficiency (iWUE) is generally defined as the ratio of the photosynthetic uptake of CO2 through leaf stomata to the simultaneous transpirational loss of water vapor to the atmosphere through the same biological portals; and as atmospheric CO2 enrichment stimulates plant photosynthesis at the same time that it causes leaf stomata to partially close and slow the rate of water loss from plant leaves, a CO2-accreting atmosphere should tend to increase plant iWUE. That this is indeed the case has been demonstrated many times by reconstructions of the iWUE of various forests from analyses of δ¹³C measurements of cellulose found in their trees' yearly growth rings; but this approach presents a problem if one is studying a grassland that has to recreate itself each year. Such was the dilemma faced by Barbosa et al., who were involved in a study of an alpine grassland in Switzerland; but they devised an impressive way to resolve it.

What was done
Citing the principle expressed by DeNiro and Epstein (1978), who coined the phrase "you are what you eat isotopically," they decided to use the horns of numerous deceased alpine ibex (Capra ibex), since they are comprised of yearly growth layers that possess a temporal archive of the δ¹³C values of the alpine grassland plants the animals ate while they were alive; and, as fortune would have it, the researchers were given access to the horns of 24 such males that had lived in the grassland they were studying by the Natural History Museum of Bern, which horns covered the period from 1938 to 2006 and provided a total of 233 yearly δ¹³C data points.

What was learned
From information obtained from the ibex horns, Barbosa et al. determined that the iWUE of the alpine grassland plants had increased by approximately 18% over the 69-year period from 1938 to 2006, during which time interval the atmosphere's CO2 concentration had risen by about 23%. Between 1955 and 2006, however, meteorological data indicated that the vapor pressure deficit (or evaporative demand) of the air in their study area had risen by about 0.1 kPa, which was just enough to offset the iWUE benefit provided by the rise in the air's CO2 content.

What it means
Although the net effect of the increase in the air's CO2 content (which tended to reduce plant water loss) and the increase in the air's dryness (which tended to enhance plant water loss) resulted in no net change in plant iWUE, it can be appreciated that had the air's CO2 content not risen over the period in question, the alpine plants would have fared far worse than they did in reality.

Reference
DeNiro, M.J. and Epstein, S. 1978. Influence of diet on the distribution of carbon isotopes in animals. Geochimica et Cosmochimica Acta 42: 495-506.