Light rings as bioindicators of climate change in Interior North America

In light of the recent cool summers observed in Interior North America (INA), it is important to put these events into a longer term context of climate variability and change. Due to the limited coverage of meteorological stations and to the near absence of temperature-sensitive bio-environmental indicators in INA, we conducted studies of temperature-sensitive light-ring (LR) chronologies. Five LR chronologies were developed from 1167 jack pine (Pinus banksiana Lamb.) trees growing along a ca. 675 km latitudinal transect in INA. Each chronology represents the annual variability in the proportion (ρ) of tree-ring samples identified as LR (hereafter ρLR) and covers the last two to three centuries. Years recording high ρLR were synchronous across the transect with southern and northern regions showing common variability. In each region and for the instrumental period, both late spring and late summer temperatures (particularly daytime maximum temperatures) were significantly cooler (1 to 2 °C) in years with high ρLR compared with low ρLR years. These temperature anomalies were associated with increased cyclonic activity over much of central Canada. Changes in the occurrence rate of LR years (ρ ≥ 2%) were analyzed using kernel-density functions and bootstrap confidence bands. The distribution of LRs in the south provided little evidence of reduced occurrence in the last 250 years, suggesting there were no long-term linear changes in years marked by cool late springs and late summers. In contrast, the northern chronology provided evidence of a decreasing occurrence rate of LR years from 1830 to the present. These results support prior evidence of a response of trees to ongoing environmental changes in northern regions. However, they also illustrate the spatial complexity in these responses.