Background
The authors write that "with continued increases in global greenhouse gas emissions, climate models predict that, by the end of the 21st century, Minnesota [USA] summer temperature will increase by 4-9°C and summer precipitation will slightly decrease," citing in this regard the work of Kling et al. (2003) and Christensen et al. (2007), and they say that certain "forest models and extrapolations from the paleoecological record suggest that, in response to increased temperature and/or drought, forests may retreat to the extreme north-eastern parts of the state," citing the work of Pastor and Post (1998), Hamilton and Johnson (2002) and Galatowitsch et al. (2009).

What was done
Working with bur oak (Quercus macrocarpa) trees, Wyckoff and Bowers explored the likelihood of the latter of these two projections by: "(i) using tree rings to establish the relationship between drought and Q. macrocarpa growth for three sites along Minnesota's prairie-forest border, (ii) calculating the current relationship between growth and mortality for adult Q. macrocarpa and (iii) using the distributions of current growth rates for Q. macracarpa to predict the susceptibility of current populations to droughts of varying strength." In addition, they looked for "temporal trends in the correlation between Q. macrocarpa growth and climate, hypothesizing that increases in CO2 may lead to weaker relationships between drought and tree growth over time," because of the fact that atmospheric CO2 enrichment typically leads to increases in plant water use efficiency, which phenomenon generally makes them less susceptible to the deleterious impact of drought on growth.

What was learned
The two University of Minnesota researchers discovered that "the sensitivity of annual growth rates to drought has steadily declined over time as evidenced by increasing growth residuals and higher growth rates for a given PDSI [Palmer Drought Severity Index] value after 1950 [when the atmosphere's CO2 concentration rose by 57 ppm from 1950 to 2000] compared with the first half of the century [when the CO2 increase was only 10 ppm]."

What it means
Wyckoff and Bowers write that "for Q. macrocarpa, declining sensitivity of growth to drought translates into lower predicted mortality rates at all sites," and that "at one site, declining moisture sensitivity yields a 49% lower predicted mortality from a severe drought (PDSI = -8, on a par with the worst 1930s 'American Dust Bowl' droughts)." Hence, they conclude that "the decreasing drought sensitivity of established trees may act as a buffer and delay the movement of the prairie-forest ecotone for many decades even in the face of climate change [italics added]." Indeed, the bur oak forests may be so significantly benefited by continued increases in the air's CO2 content that they need never retreat, especially in light of the likely over-estimation of warming predicted by most climate models for the remainder of the 21st century.

References
Christensen, J.H., Hewitson, B., Bisuioc, A., Chen, A., Gao, X., Held, I. et al. 2007. Regional climate projections. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Avery, K.B., Tignor, M. and Miller, H.L. (Eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, Cambridge, UK/New York, New York, USA, pp. 847-940.

Galatowitsch, S., Frelich, L. and Phillips-Mao, L. 2009. Regional climate change adaptation strategies for biodiversity conservation in a mid-continental region of North America. Biological Conservation 142: 2012-2022.

Hamilton, J.D. and Johnson, S. 2002. Playing with Fire: Climate Change in Minnesota. Minnesotans for an Energy-Efficient Economy. St Paul, Minnesota, USA.

Kling, G.W., Hayhoe, K., Johnson, L.B., Magnuson, J.J., Polasky, S., Robinson, S.K. et al. 2003. Confronting Climate Change in the Great Lakes Region: Impacts on our Communities and Ecosystems. Union of Concerned Scientists and Ecological Society of America, Washington, DC, USA.

Pastor, J. and Post, W.M. 1988. Response of northern forests to CO2-induced climate change. Nature 334: 55-58.