A Seven-Decade History of the Water-Use Efficiency of a Swiss Alpine Grassland

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.

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.

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.

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.

Additional References
Bates, N.R. 2007. Interannual variability of the oceanic CO₂ sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades. Journal of Geophysical Research 112: 10.1029/2006JC003759.

Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO₂ and an outlook for the future. Oceanography 22: 26-35.

Wei, G., McCulloch, M. T., Mortimer, G., Deng, W. and Xie, L. 2009. Evidence for ocean acidification in the Great Barrier Reef of Australia. Geochimica et Cosmochimica Acta 73: 2332-2346.

Yates, K.K. and Halley, R.B. 2006. CO₃² concentration and pCO₂ thresholds for calcification and dissolution on the Molokai reef flat, Hawaii. Biogeosciences 3: 357-369.