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

Abstract

Winter warming is altering plant exposure to cold events, and its effects on seasonal cold hardiness dynamics remain poorly understood. Here we quantified the effect of whole-ecosystem warming (+0.00{degrees}C to +9.00{degrees}C) on bud cold hardiness of two overstory (Larix laricina and Picea mariana) and two understory (Chamaedaphne calyculata and Rhododendron groenlandicum) boreal peatland forest species across four dormant seasons. Warming reduced cold hardiness in fall and spring by delaying acclimation and advancing deacclimation, yet damage risk increased only in late winter and spring for three species. Warming also reduced snow cover, increasing temperature variability and cold damage for understory shrubs. Together, these results show that warming restructures rather than uniformly increasing cold hardiness risk, concentrating vulnerability at transitional seasons through species-specific dynamics and microclimate exposure.