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Tree growth at northern treelines is generally temperature‐limited due to cold and short growing seasons. However, temperature‐induced drought stress was repeatedly reported for certain regions of the boreal forest in northwestern North America, provoked by a significant increase in temperature and possibly reinforced by a regime shift of the pacific decadal oscillation (PDO). The aim of this study is to better understand physiological growth reactions of white spruce, a dominant species of the North American boreal forest, to PDO regime shifts using quantitative wood anatomy and traditional tree‐ring width (TRW) analysis. We investigated white spruce growth at latitudinal treeline across a >1,000 km gradient in northwestern North America. Functionally important xylem anatomical traits (lumen area, cell‐wall thickness, cell number) and TRW were correlated with the drought‐sensitive standardized precipitation–evapotranspiration index of the growing season. Correlations were computed separately for complete phases of the PDO in the 20th century, representing alternating warm/dry (1925–1946), cool/wet (1947–1976) and again warm/dry (1977–1998) climate regimes. Xylem anatomical traits revealed water‐limiting conditions in both warm/dry PDO regimes, while no or spatially contrasting associations were found for the cool/wet regime, indicating a moisture‐driven shift in growth‐limiting factors between PDO periods. TRW reflected only the last shift of 1976/1977, suggesting different climate thresholds and a higher sensitivity to moisture availability of xylem anatomical traits compared to TRW. This high sensitivity of xylem anatomical traits permits to identify first signs of moisture‐driven growth in treeline white spruce at an early stage, suggesting quantitative wood anatomy being a powerful tool to study climate change effects in the northwestern North American treeline ecotone. Projected temperature increase might challenge growth performance of white spruce as a key component of the North American boreal forest biome in the future, when drier conditions are likely to occur with higher frequency and intensity. [ABSTRACT FROM AUTHOR]

High northern latitude regions are experiencing some of the most rapid environmentalchanges. Carbon sequestration might be enhanced by increased forest productivity due towarming temperatures and potentially higher atmospheric CO2, but declines in tree growthand mortality due to drought stress could counteract this enhancement by releasing CO2 tothe atmosphere. Observed changes in boreal forest dynamics are already altering theglobal carbon and hydrological cycles, albedo and land surface feedbacks. Here, wecombined ring-width data with annually resolved stable carbon (δ13C) and oxygenisotope (δ18O) records to investigate the physiological responses of white spruce(Picea glauca [Moench] Voss) trees to warming and increased atmospheric CO2concentrations at northern treeline sites. We developed an extensive network of δ13Cand δ18O tree-ring chronologies covering the 20th century derived from 10 sitesacross North America (NA) spanning from 60.5º to 68.7ºN and 104º to 162ºW. Weused a range of observational data and a climate model to interpret signals in theseannually-resolved tree-ring series. At most sites, the inter-tree correlations werehigher for δ18O than δ13C series. Both δ13C and δ18O were highly influenced bytemperature variations, although the dominant season was different: summer and spring,respectively. This suggests that different processes associated with temperature variationsmay be operating. Physiological processes occurring at leaf level during summermainly influenced δ13C. Spring climate had the strongest influence on tree-ringδ18O via temperature effects on the δ18O of source water, and to a lesser degree,in summer, physiological process at leaf level may also occur. Accordingly, wefound weak relationships between the tree-ring δ18O and atmospheric circulation(such as integrated moisture transport) during summer using the GISS ModelE2isotopically-equipped general circulation model. Calibration and simulations of tree-ring datain the process-based mechanistic model MAIDENiso will help us to elucidate thecomplex linkages between increases in temperature and CO2 driven tree responses.This tree-ring isotope network provides a unique opportunity to disentangle theclimatic and physiological controls driving growth patterns at the northern treeline. [ABSTRACT FROM AUTHOR]