Warming Reduces Cold Hardiness of Boreal Plants but Damage Risk Varies by Species and Season

Winter warming is altering plant exposure to cold events, making it increasingly important to understand how cold hardiness dynamics respond across the dormant season. Using the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment in northern Minnesota, we measured bud cold hardiness across four dormant seasons (2021-2025). There, a boreal peatland community was exposed to continuous active whole ecosystem warming across five levels (+0.00°C to +9.00°C) combined with atmospheric and elevated CO₂ (+500 ppm) in open-top chambers. Cold hardiness of two overstory (Larix laricina and Picea mariana) and two understory (Chamaedaphne calyculata and Rhododendron groenlandicum) species was evaluated semi-regularly throughout seasons. Both understory and overstory species gained cold hardiness (acclimated) at warmer temperatures in the fall than those causing deacclimation in the spring, demonstrating different temperature responsiveness phases across the dormant season. Based on cold hardiness sensitivity, warming did not alter maximum midwinter cold hardiness (sensitivity~0°C change in cold hardiness/°C of warming) but delayed fall acclimation and, most importantly, accelerated spring deacclimation (sensitivity > 0°C). These changes resulted in increased cold damage risk in late winter and spring with warming (sensitivity > 1°C/°C) for L. laricina, C. calyculata and R. groenlandicum. Among overstory species, the deciduous L. laricina was more sensitive to warming than the evergreen P. mariana, indicating warming may disadvantage acquisitive species. For understory shrubs, reduced snow cover in the warmest enclosures that removed the insulating buffer and exposed buds to temperatures colder than in control plots associated with the high sensitivity of these species resulted in observed midwinter bud mortality with warming. Rather than uniformly changing cold damage risk, warming decreases risk of cold damage in fall and early winter for all species, but increases risk in late winter and spring for most species, suggesting species-specific cold hardiness dynamics and snow-driven microclimate may result in boreal forest composition changes.