Oaks and ice: Linking Continental North American and North Atlantic Climate Proxies Between 9.5 and 14 k yr BP

Climate and plant growth data from agriculturally important continental regions is often limited in time series length and resolution. Recent work on the dendrochronology of buried sub-fossil oak (Quercus macrocarpa, Quercus bicolor) collected from streams in Missouri and Iowa, USA has yielded climate-sensitive tree-ring data within the last 14 k yr BP. We present the development of the American Long Oak Chronology (ALOC) and report on variations in tree growth and North Atlantic climate between 9.5 and 14 k yr BP. Oaks dated using 14C and density dating methods show decreased growth and depleted δ 13C coincident with North Atlantic climate proxies. The growth (basal area increment) of oak trees was significantly (p = 0.01) less (6.5 cm2 yr-1) during the Younger Dryas compared to growth (11.2 cm2 yr-1) before and after. Mean maximum annual basal area increment (17.0 cm2 yr-1) was reduced by 54 percent during the Younger Dryas. Between-tree growth variance is about 75 percent less during the Younger Dryas than it is during the post glacial period. This variance reduction within a tree population likely reflects limits on growth switching from highly variable endogenous factors (forest competition) to regional exogenous factors (climate). Three point moving averages of the mean ring width of the oak trees and temporally paired δ 18O values from Greenland ice cores (GRIP, GISP2) are correlated (r = 0.57) during the post glacial climate period. Values of δ 13C in oak wood are found to be positively and significantly correlated (r = 0.78) with temporally paired δ 18O values. The termination of the Younger Dryas in mid-continental North America oak was marked by an expected and abrupt increase in the climate sensitive stable isotope δ 13C at about 11,550 yr BP. These results indicate that climate changes during the glacial-Holocene transition in mid- continental North America were approximately synchronous with those in the North Atlantic (~ 4,500 km northeast of the study site) and suggests that post glacial climate variations had marked effects on the growth rates of trees.