Background
The authors indicate that sessile oak (Quercus petraea Liebl.) is one of Central Europe's most important tree species in terms of its abundance in natural forests and its economic value, citing Ellenberg and Leuschner (2010). Yet they state there has been a lack of study of the climatic signals that may be present in the species' yearly growth rings, as well as the yearly isotopic signature of its wood, citing Weigl et al. (2008).

What was done
According to Hardtle et al., they "used a combined dendrochronological and dendrochemical approach to analyze the long-term climate responses of growth of Q. petraea (tree-ring width, basal area increment (BAI), δ¹³C signatures in the wood) at sites with ample and relatively low water availability (deeply weathered Cambisols vs. shallow Regosols, respectively) in Luxembourg, Central Europe."

What was learned
In the words of the thirteen researchers, "all stands showed a (more or less) continuous increase in BAI over 160 years," which finding "contradicts the sigmoid tree growth model according to which growth should plateau (and decline) with progressing tree ageing." However, they say their finding "is not unique" in this regard, and that it is "in agreement with other studies documenting long-term trends in BAI for other tree species during the last century," citing Johnson and Abrams (2009) and Kint et al. (2012). And they go on to suggest that "this trend is mainly influenced by enhanced CO2 levels," and that the trees responded to this stimulus "either directly with increasing carbon sequestration or indirectly through CO2 effects on intrinsic water use efficiency," citing Battipaglia et al. (2013). So in summation, they write that "Q. petraea showed little climate sensitivity and was able to recover from climate extreme events even at tree-ages beyond 200 years."

What it means
Hardtle et al. conclude that their findings "indicate the importance of Q. petraea as an adaptive tree species in forest ecosystems of Central Europe under shifting climatic conditions," while the similar findings of the others they cite, who worked with still other tree species, suggest that it was the increase in the air's CO2 concentration that enabled the various studied species to positively respond as observed.

References
Battipaglia, G., Saurer, M., Cherubini, P., Calfapietra, C., McCarthy, H.R., Norby, R.J. and Cotrufo, M.F. 2013. Elevated CO2 increases tree-level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites. New Phytologist 197: 544-554.

Ellenberg, H. and Leuschner, C. 2010. Vegetation Mitteleuropas mit den Alpen. Ulmer, Stuttgart, Germany.

Johnson, S.E. and Abrams, M.D. 2009. Basal area increment trends across age classes for two long-lived tree species in the Eastern U.S. In: Kaczka, R., Malik, I., Owczarek, P., Gartner, H., Helle, G., and Heinrich, I. (Eds.). Tree Rings in Archaeology, Climatology and Ecology. Volume 7. GFZ Potsdam, Scientific Technical Report 09/03, pp. 127-134.

Kint, V., Aertsen, W., Campioli, M., Vansteenkiste, D., Delcloo, A. and Muys, B. 2012. Radial growth change to altered regional climate and air quality in the period 1901-2008. Climatic Change 115: 343-363.

Weigl, M., Grabner, M., Helle, G., Schleser, G.H. and Wimmer, R. 2008. Characteristics of radial growth and stable isotopes in a single oak tree to be used in climate studies. Science of the Total Environment 393: 1554-161.