Response of the prairie–forest border to climate change: impacts of increasing drought may be mitigated by increasing CO 2

Summary 1.Quercus macrocarpa (bur oak) is the dominant tree species along much of the prairie–forest border in the northern‐central United States, and movement of Q. macrocarpa in response to climate change may determine the rate at which the prairie–forest ecotone shifts. To investigate likely controls over Q. macrocarpa performance at the edge of its range, we used tree rings to establish the links between drought, growth‐rate and mortality for three sites spanning the prairie–forest border in Minnesota. 2.Quercus macrocarpa growth during the 20th century correlates strongly with the Palmer Drought Severity Index (PDSI) and more weakly with raw temperature and precipitation values for all three sites. However, 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 value after 1950 compared with the first half of the century. We hypothesize that increased atmospheric carbon dioxide concentration may lead to increased water‐use efficiency, although we cannot rule out other environmental factors. 3. Because growth is an excellent predictor of Q. macrocarpa mortality, growth–climate relationships provide information on whether oak forests will contract, because of individual tree death, when climate changes. For Q. macrocarpa , declining sensitivity of growth to drought translates into lower predicted mortality rates at all sites. At one site, declining moisture sensitivity yields a 49% lower predicted mortality from a severe drought (PDSI = −8, on par with the worst 1930s ‘American Dust Bowl’ droughts in our study region). 4. Unless the changing relationship between growth and climate is incorporated into forest simulation models, the predicted rate of established tree dieback in a warmer, drier climate may be exaggerated. 5.Synthesis . Adult Quercus macrocarpa trees appear to be increasingly insensitive to drought‐induced mortality. Because the species is dominant at the prairie–forest ecotone in the northern‐central United States, movement of the ecotone in response to climate change may be delayed for decades.