What was done
By measuring carbon isotope ratios in the rings of coniferous trees from northern Eurasia, including the three genera Larix, Picea and Pinus, across a longitudinal transect covering the entire super-continent in the latitude range from 59 to 71°N, the authors were able to determine the change in intrinsic water use efficiency (Wi, the amount of carbon gain at the needle level per unit of water loss) that was experienced by the trees between the two 30-year periods 1861-1890 and 1961-1990.

What was learned
Saurer et al. report that the concomitant "increasing CO2 in the atmosphere resulted in improved intrinsic water-use efficiency," such that "125 out of 126 trees showed increasing Wi from 1861-1890 to 1961-1990, with an average improvement of 19.2 ± 0.9%."

What it means
The three Swiss scientists say their results suggest that the trees they studied "are able to produce the same biomass today [as they did 100 years ago] but with lower costs in terms of transpiration."  This finding is highly significant, because some data have indicated that recent warming in other longitudinal segments of the same latitude belt "may be accompanied by increased drought stress (Lloyd and Fastie, 2002)," and the historical increase in the air's CO2 content may be helping these trees to better cope with the newly established drought conditions.

Reference
Lloyd, A.H. and Fastie, C.L.  2002.  Spatial and temporal variability in the growth and climate response of treeline trees in Alaska.  Climatic Change 52: 481-509.