Warming winters can increase the risk of root frost damage in boreal tree seedlings

Boreal winters are warming rapidly, but associated changes in snow cover, precipitation phase, and freeze-thaw conditions vary geographically. Where warming reduces or interrupts snow insulation while freezing conditions persist, root-zone temperatures may become more strongly coupled to atmospheric variability, potentially increasing episodic exposure of tree-seedling roots to damaging temperatures despite higher mean air temperatures. This review synthesizes our current understanding of snow-soil thermal processes and root physiological responses, evaluates the consequences of changing winter conditions for boreal forest regeneration, and assesses the capacity of empirical, forest growth, and land-surface modelling approaches to represent winter root frost risk. Evidence from boreal conifers shows that roots are generally less frost hardy than above-ground tissues and that root frost hardiness (FH) varies seasonally with root-zone temperature and thermal history. Experimental studies further demonstrate that root freezing injury can impair root regeneration, physiological function, shoot growth, and subsequent survival, with effects sometimes becoming apparent only after budburst or during later growing seasons. However, these responses depend strongly on species, injury severity, recovery capacity, and post-injury conditions. Altered winter environments can also impose interacting stresses, including ice encasement, hypoxia, waterlogging, and frost heaving. Ectomycorrhizal fungi show substantial taxon-specific freezing tolerance, but the consequences of repeated winter freezing and thawing for functioning of intact root-fungus symbioses remain poorly resolved. Existing modelling frameworks increasingly represent snow, frozen soils, plant hydraulics, and in some cases dynamic FH or frost mortality, but these components are rarely integrated with root-specific FH, root injury, and delayed seedling performance. Building on this synthesis, we propose a process-based framework linking winter climate forcing, snowpack and root-zone thermal dynamics, dynamic root FH, cellular and hydraulic injury pathways, and subsequent seedling growth and survival. The framework highlights that root frost damage risk emerges from the interaction between environmental exposure and dynamic physiological vulnerability rather than from air temperature extremes alone, and identifies key processes for improving prediction of boreal forest regeneration under changing winter conditions.