Background
The authors describe Araucaria angustifolia as "an indigenous conifer tree restricted to the southern region of South America that plays a key role in the dynamics of regional ecosystems where forest expansion over grasslands has been observed [italics added]."

What was done
Working with various types of tree-ring data obtained from A. angustifolia trees growing in both forest and grassland sites in southern Brazil, Silva et al. compared changes in intrinsic water use efficiency - iWUE, defined as the ratio of the rate of CO2 assimilation by the trees' needles to their stomatal conductance - with concomitant historical changes in temperature, precipitation and atmospheric CO2 concentration that occurred over the past century.

What was learned
The four researchers report that during the past several decades, "iWUE increased over 30% in both habitats," and that "this increase was highly correlated with increasing levels of CO2 in the atmosphere." Over this latter period, however, tree growth remained rather stable, due to lower-than-normal precipitation and higher-than-normal temperatures, which would normally tend to depress the growth of this species, as Katinas and Crisci (2008) describe A. angustifolia as being "intolerant of dry seasons and requiring cool temperatures."

What it means
Silva et al. conclude that the "climatic fluctuations during the past few decades," which would normally be expected to have been deleterious to the growth of A. angustifolia, seem to have had their growth-retarding effects "compensated by increases in atmospheric CO2 and changes [i.e., increases] in iWUE." And it would appear that this phenomenon has helped to sustain the historical-and-still-ongoing greening of the earth that is transforming the terrestrial face of the planet.

Reference
Katinas, L. and Crisci, J.V. 2008. Reconstructing the biogeographical history of two plant genera with different dispersion capabilities. Journal of Biogeography 35: 1374-1384.