The effect of long-term atmospheric CO₂ enrichment on the intrinsic water-use efficiency of sour orange trees

Every two months of 1992, as well as on three occasions in 1994-1995, we obtained leaf samples together with samples of surrounding air from eight well-watered and fertilized sour orange (Citrus aurantium L.) trees that were growing out-of-doors at Phoenix, Arizona, USA. These trees had been planted in the ground as small seedlings in July of 1987 and enclosed in pairs by four clear-plastic-wall open-top chambers of which two have been continuously maintained since November of that year at a CO₂ concentration of 400 μmol mol^-1 and two have been maintained at 700 μmol mol^-1. In September 2000, we also extracted north-south and east-west oriented wood cores that passed through the center of each tree's trunk at a height of 45 cm above the ground. Stable-carbon isotope ratios (¹³C/¹²C) derived from these leaf, wood and air samples were used to evaluate each tree's intrinsic water-use efficiency (iWUE). The grand-average result was an 80% increase in this important plant parameter in response to the 300 μmol mol^-1 increase in atmospheric CO₂ concentration employed in the study. This increase in sour orange tree iWUE is identical to the long-term CO₂-induced increase in the trees' production of wood and fruit biomass, which suggests there could be little to no change in total water-use per unit land area for this species as the air's CO₂ content continues to rise. It is also identical to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO₂ content that occurred between 1800 and 1985. Published by Elsevier Science Ltd.