What was done
Using data obtained from the website of the International Tree-Ring Data Bank, as well as from cores that had been collected previously and stored in their laboratory at The Pennsylvania State University (USA), the authors explored growth rate (basal area increment, BAI) relationships across age classes (from young to old) for eight tree species commonly found throughout the eastern United States, namely, bigtooth aspen (Populus grandidentata Michx.), blackgum (Nyssa sylvatica Marsh.), black oak (Quercus velutina Lam.), chestnut oak (Quercus Montana L.), hemlock (Tsuga canadensis L. Carr.), pitch pine (Pinus rigida Mill.), red oak (Quercus rubra) and white oak (Quercus alba L.).

What was learned
Johnson and Abrams say that "a remarkable finding of this study is that even the oldest trees of several species had slow but increasing BAI values, which continued throughout the life of most trees." The reason they characterize this finding as "remarkable," as they explain, is because it "contradicts the sigmoidal growth model that predicts growth rate should plateau and then decline, as middle age trees approach old age," citing the studies of Ryan and Yoder (1997) and Weiner and Thomas (2001). And they further report that "over the last 50-100 years, younger trees within a species grew faster than did the older trees when they were of the same respective age," which is what Knapp and Soule (2011) also found to be the case with ponderosa pine trees in the USA's northern Rocky Mountains.

What it means
In discussing their findings, the two researchers from The Pennsylvania State University's School of Forest Resources say "it seems reasonable to assume" that the greater growth rates of older trees of the current era compared to older trees of older times "may be due to a stimulatory effect of anthropogenic global change defined in the broadest sense," including "increased CO2 levels, warming temperatures, increased precipitation, and changes in precipitation chemistry," while noting that "yearly average temperatures, atmospheric CO2 and nitrogen levels have increased in the eastern US (as well as much of the rest of the world) over the last 50-100 years." And Knapp and Soule (2011) go even further, stating that "old-growth ponderosa pine forests of the northern Rockies have likely benefited from the effects of increased atmospheric CO2 since the mid-20th century," additionally noting that "the benefits increase with tree age."

References
Knapp, P.A. and Soule, P.T. 2011. Increasing water-use efficiency and age-specific growth responses of old-growth ponderosa pine trees in the Northern Rockies. Global Change Biology 17: 631-641.

Ryan, M.G. and Yoder, B.J. 1997. Hydraulic limits to tree height and tree growth. Bioscience 47: 235-242.

Weiner, J. and Thomas, S.C. 2001. The nature of tree growth and the age-related decline in forest productivity. Oikos 94: 374-376.