What was done
The author derived variations in plant intrinsic water-use efficiency over the last two centuries from 23 carbon isotope tree-ring chronologies, using data obtained from trees growing in natural forests located primarily in western North America.

What was learned
The tree-ring chronologies all exhibited downward trends that were mirror images of the historical trend in atmospheric CO2 concentration.  When converted to plant intrinsic water-use efficiencies, these data produced trends that look almost identical to the historical trend in the air's CO2 content, with plant intrinsic water-use efficiency rising by 10 to 25% from 1750 to 1970, during which period the CO2 concentration of the atmosphere rose by approximately 16%.

What it means
The author concluded that his results suggest that the responses of plants to elevated CO2 levels observed in short-term experimental studies also apply "to long-term physiological adjustments of natural trees."  Specifically, the CO2-induced increase in intrinsic plant water-use efficiency observed in this study implies an increase in the rate of CO2 assimilation and/or a decrease in stomatal conductance over the same period of time.  Hence, the author concludes that "in arid environments where moisture limits the tree growth, biomass may have increased with increasing transpiration efficiency," and that the enhanced growth of trees in arid environments may thus "have operated as a carbon sink for the anthropogenic CO2" emitted over the past two centuries.