Whether a photosynthesis-related low temperature limitation (source-limitation hypothesis) or a direct inhibition of meristematic activity (sink-limitation hypothesis) causing the treeline formation is still in larger debate. For unifying globally valid theory of treeline formation, we analysed growth and carbon source-sink balance in different tissues of deciduous Betula ermanii to test the sink-limitation hypothesis on Mt.Changbai, Northeast China. The mean 10-cm soil temperature of 8.2°C across the 131-day growing season, matched the threshold temperature found at global treelines positions. However, its previous position implied temperature may be not the only limited factor here. Numerous seedlings at the uppermost site suggested that it was highly unlikely that a lack of seedlings determined the treeline. Altitudinal trends of NSC increasing significantly in tissues revealed no depletion of carbon reserves, suggesting direct low temperature constraints of growth. However, the pronounced variation of leaf δ¹³C indicated that trees above the treeline may be confronted with the certain physiological drought, due to low soil water availability in winter or early growing season. Hence, we conclude that the treeline formation on Mt.Changbai is most likely the result of comprehensive effects of low temperature and water stress, which drove its actual position.