Divergent Growth and Changing Climate Relationships of Boreal and Subalpine Spruce in Southern Yukon, Canada

Tree-ring data for 1942-2013 were used to determine if boreal and subalpine Picea albertiana S. Brown emend. Strong & Hills radial growth rates changed after 1950 in southern Yukon, northwest Canada. The latter year represented the beginning of accelerated atmospheric greenhouse gas accumulation, which has subsequently caused global climate changes. Ring-width chronologies for both boreal and subalpine trees were constructed using 80 trees. Individual ring-width sequences were crossdated, age-detrended, and converted to tree ring-width indices (RWI). In 2013, boreal and subalpine RWI were 41% and 169% greater than in 1950, respectively. In contrast, elevational treeline chronologies in the region typically decreased 20%-70%. During 1942-2013, boreal and subalpine chronologies were significantly, at best modestly (r < vertical bar 0.36 vertical bar), and exclusively correlated with either moisture or temperature variables, respectively. Within this same time frame, RWI-climate variable correlates differed between the early and late phases of each chronology. Average pre-1969 or early-phase boreal correlations based on a 19-year moving window reached a maximum of -0.56. Late-phase chronology correlations typically shifted towards weaker associations with climate variables (typically +0.04 to +0.30). For boreal RWI, 25 of 68 tested climate variables shifted as much as 0.40-0.88 correlation coefficient units between the early and late phases of the chronologies. Subalpine RWI were correlated only with July-August precipitation during the early phase of its chronology. These changes suggest responses by spruce to climate have shifted since 1970. The modest 1942-2013 RWI-climate variable correlations likely reflect the result of these changing ecological relationships.