Warming aggravates physiological drought in Betula platyphylla during the winter–spring transitional period in Greater Khingan Mountains

Tree growth is highly susceptible to hydrothermal changes during winter-spring transition, particularly in cold-temperate regions while short-term climatic fluctuations could play a primary role in inter-annual growth dynamics. To unravel this mechanism, we focused on Betula platyphylla, a key companion and pioneer species in Larix–Betula forests in the Greater Khingan Mountains. Using dendrochronology methods, we investigated warming impacts on its radial growth during winter-spring transition (February–April) based on short-term 10-day climate data. Here, we showed warming exacerbated physiological drought by intensifying snowmelt and soil freeze-thaw cycles, disrupting water balance and therefore suppressing tree growth. The temperature threshold (∼4.0°C) for triggering tree growth advanced by 0.4 days/year since 1990, which in turn intensified pre-monsoon drought stress. Although freeze-thaw water dominated pre-monsoon growth, the tree growth was threatened by warming-induced soil hydrological changes. Our study highlights that winter-spring warming may constrain boreal forest carbon sequestration through intensified water stress, urging attention to non-growing season climatic controls on tree growth.