What was done
Cores were obtained from 8 to 20 black pine (Pinus nigra) trees growing at each of 28 sites within the Vienna basin of Austria in the European Eastern Alps during the summers of 1996 and 1997, with the authors focusing on trees possessing umbrella-like crowns (indicative of water-limited conditions) growing on shallow and poor soils, in order to maximize their ring-width response to moisture availability.

What was learned
Leal et al. discovered "a very clear change in the sensitivity of the growth rate of tree stems to water availability in the late 20th century," noting that "trees previously sensitive to spring-summer drought show a lack of response to this climatic parameter in recent decades." That is to say-- as they actually did say -- that "tree-ring indices were larger in the second half of the 20th century than predicted given prevailing spring-summer drought conditions and the previous sensitivity of growth to these conditions." In addition, they found "a decrease in correspondence between the occurrence of extreme events in precipitation and rate of change of growth," such that "in the second half of the century this correspondence was not significant," and that "recent extreme droughts did not result in the formation of very narrow rings, which means the droughts were not as limiting to tree growth as they had been in the past."

What it means
The five researchers concluded their paper by suggesting that the greater atmospheric CO2 concentrations of the latter decades of the 20th century "induced improved water-use efficiency enabling P. nigra growing in the Vienna basin to avoid the impact of recurrent dry conditions," which phenomenon has also been observed in many other parts of the world in a number of different tree species (see Water Use Efficiency (Trees) in our Subject Index), which is but another indication of the propensity of the ongoing rise in the air's CO2 content to promote the greening of the earth.