Relationship of Temperature and Light Ring Formation at Subarctic Treeline and Implications for Climate Reconstruction

Abstract

During the past 8 centuries, light rings (LRs) have occasionally formed in black spruce ( Picea mariana) at treeline near Bush Lake, northern Quebec (L. Filion, S. Payette, L. Gauthier, and Y. Boutin, 1986, Quaternary Research 26, 272-279; A. Delwaide, L. Filion, and S. Fayette, 1991, Canadian Journal of Forest Research 21, 1828-1832). New analyses of climate data compiled during the period of overlapping tree-ring and instrumental records show that years of LR formation at Bush Lake have unusually cool May, June, August, and September temperatures. The analyses also show that August-September temperatures strongly correlate with May-July temperatures. Thus, late spring and entire growing-season temperatures influence LR formation at subarctic treeline. LRs formed in at least 5% of the trees at Bush Lake when May-September mean temperatures at Inukjuak fell below 4.2°C and August-September mean temperatures fell below 6.7°C. These threshold temperature/LR relationships can be used to infer limiting summer temperatures during the period preceding instrumental records. For example, the LR record suggests that May-September temperatures at northern Quebec treeline dropped below 4.2°C in A.D. 1601 after a major volcanic eruption of unknown source. Visual assessments of LR occurrence provide a new approach for extracting quantitative paleoclimatic information from tree rings.