Understanding the physiological adaptations of non-treeline trees to environmental stress is important to understand future shifts in species composition and distribution of current treeline ecotone. The aim of the present study was to elucidate the mechanisms of the formation of the upper elevation limit of non-treeline tree species, Picea jezoensis, and the carbon allocation strategies of the species on Changbai Mountain. We employed the ¹³C in situ pulse labeling technique to trace the distribution of photosynthetically assimilated carbon in Picea jezoensis at different elevational positions (tree species at its upper elevation limit (TSAUE, 1700 m a.s.l.) under treeline ecotone; tree species at a lower elevation position (TSALE, 1400 m a.s.l.). We analyzed ¹³C and the non-structural carbohydrate (NSC) concentrations in various tissues following labeling. Our findings revealed a significant shift in carbon allocation in TSAUE compared to TSALE. There was a pronounced increase in δ¹³C allocation to belowground components (roots, soil, soil respiration) in TSAUE compared to TSALE. Furthermore, the C flow rate within the plant-soil-atmosphere system was faster, and the C residence time in the plant was shorter in TSAUE. The trends indicate enhanced C sink activity in belowground tissues in TSAUE, with newly assimilated C being preferentially directed there, suggesting a more conservative C allocation strategy by P. jezoensis at higher elevations under harsher environments. Such a strategy, prioritizing C storage in roots, likely aids in withstanding winter cold stress at the expense of aboveground growth during the growing season, leading to reduced growth of TSAUE compared to TSALE. The results of the present study shed light on the adaptive mechanisms governing the upper elevation limits of non-treeline trees, and enhances our understanding of how non-treeline species might respond to ongoing climate change.