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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 < |0.36|), 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. [ABSTRACT FROM AUTHOR]

Regional warming has led to increased productivity near the boreal forest margin in Alaska. To date, the effects of warming on seedling recruitment have received little attention, in spite of forecasted forest expansion. Here, we used stand structure and environmental data from 95 white spruce ( Picea glauca) plots sampled across a longitudinal gradient in southwest Alaska to explore factors influencing spruce establishment and recruitment near western treeline. We used total counts of live seedlings, saplings, and trees, representing five life stages, to evaluate whether geospatial, climate, and measured plot covariates predicted abundance, using current abundance distributions as a surrogate for climate conditions in the past. We used generalized linear models to test the null hypothesis that conditions favorable for recruitment were similar along the environmental gradient represented by longitude, by exploring relationships between per-plot counts of each life stage and the covariates hypothesized to affect abundance. We also examined the relationship between growing degree days ( GDD) and seedling establishment over a period of three decades using tree-ring chronologies obtained from cores taken at a subset of our sites ( n = 30). Our results indicated that seedling, sapling, and tree abundance were positively correlated with temperature across the study area. The response to longitude was mixed, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, and later life stages (trees) most abundant to the east. The differential relationship between longitude and life-stage abundance suggests a moving front of white spruce establishment through time, driven by changes in environmental conditions near the species' western range limit. Likewise, we found a positive relationship between periods of seedling establishment and GDD, suggesting that longer summers and/or greater heat accumulation might enhance establishment, consistent with the positive relationship we found between life-stage abundance and temperature. [ABSTRACT FROM AUTHOR]