Growth increases but regeneration declines in response to warming and drying at Arctic treeline in white spruce (Picea glauca)

As a temperature-delineated boundary, the Arctic treeline is predicted to shift northward in response to warming. However, the evidence for northward movement is mixed, with some sections of the treeline advancing while others remain stationary or even retreat. To identify the drivers of this variation, we need a landscape-level understanding of the interactions occurring between climate, tree growth, and population regeneration. In this study, we assessed regeneration alongside annual tree growth and climate during the 20th century. We used an ageheight model combined with tree height from aerial lidar to predict the age structure of 38,652 white spruce trees across 250 ha of Arctic treeline in the central Brooks Range, Alaska, USA. We then used age structure analysis to interpret the trends in regeneration and tree-ring analysis to interpret changes in annual tree growth. The climate became significantly warmer and drier circa 1975, coinciding with divergent responses of regeneration and tree growth. After 1975, regeneration of saplings (trees ≤ 2m tall) decreased compared to previous decades whereas annual growth in mature trees (trees >2m tall) increased by 54% (p<0.0001, Wilcoxon test). Tree-ring width was positively correlated with May-August temperature (p<0.01, Pearson coefficient) during the 20th century. However, after circa 1950, the positive correlation between temperature and growth weakened (i.e., temperature divergence) while the positive correlation with July precipitation strengthened (p<0.01, Pearson coefficient), suggesting that continued drying may limit future growth at this section of Arctic treeline. We conclude that while warmer temperatures appear to benefit annual growth in mature trees, the warmer and drier environmental conditions in spring and summer inhibit regeneration and therefore may be inhibiting the northward advance at this Arctic treeline site. Researchers should consider the interactions between temperature, water availability, and tree age when examining the future of treeline and boreal forest in a changing climate. ### Competing Interest Statement The authors have declared no competing interest.