Assessing tree-ring carbon and oxygen stable isotopes for climate reconstruction in the Canadian northeastern boreal forest

In this study, we analyzed stable isotopes of oxygen (delta O-18) and carbon (delta C-13) of five black spruce (Picea mariana (Mill.) BSP) trees growing in the Quebec boreal forest. We then produced the first dendroisotopic series covering the last two centuries (1800-2003) for this region where climatic records are particularly scarce and of short duration. Our aim was to evaluate the isotope sensitivity to climate and their potential as proxies for past climate conditions. The delta O-18 results show a strong coherence between all trees indicating that these values express a homogenous response at the site scale. For the delta C-13 series, slight inter-tree differences suggest the influence of micro-site conditions. Isotopic values were compared to climatic data for the period 1944-2003. These statistical analyses indicate that the delta C-13 and delta O-18 ratios are directly influenced by summer maximum temperatures but also correlate with parameters that integrate temperatures and moisture status. In all cases, the climate effects on delta C-13 values are weaker than the ones recorded by the delta O-18 series. Moreover, because the delta O-18 and delta C-13 values are sensitive to climatic variables that are linked and commonly associated in typical subarctic climate ambiances (warm-dry, cold-wet), the patterns of their joint response show stronger correlations with climatic parameters. Then, a temperature reconstruction was developed based on a regression model that calibrates the combined delta C-13 and delta O-18 series against summer maximum temperature (TmaxjjA). The reconstructed summer temperatures indicates that the first half of the 19th century was the coldest period in the past 200 years and that a steadily change throughout milder conditions started in the early 1940's. These trends are consistent with other reconstructed values from independent proxies available for the same region. This research confirms that C and O dendroisotopic series of black spruce trees in high latitudes can document adequately past climatic conditions. (C) 2015 Elsevier B.V. All rights reserved.