Winter climate change in the boreal forest—what does it mean for the forest tree seedlings?

Winter physiological processes are often overlooked in climate change studies of boreal forests, despite their critical role in determining tree survival and carbon balance. Winter climate change is reshaping the environmental constraints that govern boreal forest regeneration, placing increasing pressure on the resilience of tree seedlings. The present mini-review summarises how changing non-growing season temperatures and snow conditions influence the seasonal survival of boreal tree seedlings and the carbon dynamics of both seedlings and soil, with implications for successful forest regeneration. Snow insulates boreal vegetation from extreme cold, regulates light and UV exposure, and shapes spring hydrology. Alterations in snow depth, structure, duration, and melt timing modify soil temperatures, gas exchange, and the snow-soil-root-microbe system, increasing root stress and mortality through deeper frost and hypoxia, ultimately resulting in impaired spring recovery. Winter remains one of the most uncertain parts of the boreal carbon budget, as soils, microbes, and roots continue emitting CO2 and climate change is altering these fluxes and their carryover effects. Boreal forests rely on tightly regulated annual cycles in which cold acclimation and frost hardiness are essential for winter survival, yet warming can delay acclimation, increase vulnerability to warm spells, and advance spring phenology, thereby raising frost-damage risk for seedlings. The post-planting resilience of seedlings can be strengthened by targeted silvicultural planning, nursery, and planting practices that better prepare seedlings to withstand increasingly variable and challenging winter conditions. More research is needed on how boreal forest tree seedlings physiologically and phenologically adapt to changing winter conditions.