Drought signals inferred from ring-width and stable carbon isotope chronologies from Thuja occidentalis trees growing at their northwestern distribution limit, central Canada

Stable carbon isotopes (delta C-13) fixed in tree rings are dependent upon environmental conditions. Old northern white-cedar (Thuja occidentalis L.) trees were sampled at their northwestern limit of distribution in central Canada. The objectives of the study were (i) to investigate the association between tree-ring delta C-13 values and radial growth in addition to the response of these variables to climate, (ii) to assess site differences between two sites varying in moisture regime, and (iii) to compare tree-ring delta C-13 of T. occidentalis with that of other boreal tree species growing at the northern limit of their distribution in central Canada. Over 2500 tree rings comprised of 15 T. occidentalis trees were analyzed for delta C-13. Annually resolved delta C-13 (1650-2006) and ring-width (1542-2006) chronologies were developed. During the year of ring formation, ring width was associated with spring and early-summer conditions, whereas delta C-13 was more indicative of overall summer conditions. However, compared with delta C-13 values, ring width was more often associated with climate conditions in the year prior to ring formation. Conditions conducive to moisture stress were important for both parameters. Although ring width and delta C-13 corresponded to the drought intervals of the 1790s, 1840s, 1890s, 1930s, and 1960-1970, ring width may be more responsive to prolonged drought than delta C-13. Tree-ring delta C-13 could, however, provide important information regarding physiological adaptations to drought.