Reconstruction of Subarctic Lake Levels over Past Centuries using Tree Rings

Abstract

This paper presents the dendrochronological markers used to date the erosive activity of ice and waves on the shores of two large lakes in Northern Quebec. Damages to trees are dated by detailed analysis of anomalous tree rings in response to the shore geomorphic activity associated with extreme water levels over the last 2 centuries. Spring floods are dated using ice scars on trees and shrubs. Lake ice breakup occurs 4–6 weeks after snowmelt. Marks on trees made by ice displaced by the wind indicate the flood levels. Once ice breakup is complete, waves erode forest edges. Tree tilting, dated by counting rings in compression wood, mark high lake levels at the beginning of summer. At submerged sites that are not subject to waves and ice, stress related to prolonged submersion results in narrow tree rings. Trees located in the immersion zone show narrow rings compared to wider rings in unsubmerged trees. However, tree response is not immediate; they can primarily respond to sequences of several consecutive years of stress, reducing the resolution of dating to 2–3 years. Rings of submerged trees are denser than average. These high water level markers, as well as population recruitment showed by the age structure of shoreline trees and shrubs, which establish during intercalary years of low water levels, were used to date the extreme levels of two large lakes at the interface between the upper boreal and subarctic regions in northeastern Canada. During the past 200 years, the annual extreme levels have been increasing, becoming even more pronounced in the 1930s, and may be due to a century-long increase in snow precipitation. The summers seem to be drier. The 1930s, 1950s, and 1970s were marked by a high frequency of high water levels. Intercalary periods of low water levels allowed regeneration of shoreline forests and recovery of trees damaged during these events. It appears that episodes similar to the recent period of low waters experienced in the American northeast since 1982 may have occurred in the past during the 1920s, 1940s, and 1960s.