Contrasting responses of Norway spruce (Picea abies (L.) H.Karst) and European beech (Fagus sylvatica L.) seedlings to warming in the Western Carpathians: growth, root and leaf traits, physiology

• Soil warming affected spruce and beech seedlings differently. • Spruce showed reduced growth, slight pigment loss and impaired photochemistry. • Beech exhibited enhanced growth and stable physiological performance. • Leaf and needle shape traits responded strongly to warming. • Warming may shift seedling competition and forest regeneration patterns. Climate warming is expected to influence forest regeneration, particularly in mountain ecosystems. However, experimental evidence on how key European tree species respond to warming during the early stages of development remains limited. We conducted an in-situ experiment to assess the impact of warming on year-old seedling performance of Norway spruce ( Picea abies (L.) H.Karst.) and European beech ( Fagus sylvatica L.) in the Western Carpathians. One-year-old seedlings were grown for nearly 500 days under ambient and warmed conditions using open top chambers, which increased mean topsoil temperature by 0.25 °C on average. In total, 160 seedlings were assessed through comprehensive biometric and physiological analyses. Our results revealed contrasting species-specific responses. Spruce seedlings exhibited significantly reduced height (by 23 %) and root biomass (by 52 %) under warming, while beech maintained or slightly increased growth. Biomass allocation patterns also diverged: spruce allocated less to roots under warming, while beech remained stable. Warming caused beech leaves to become longer and wider, while spruce needles became shorter and narrower – patterns were derived from measurements of over 30,000 spruce needles and 2,300 beech leaves. Physiological assessments showed a slight decrease in chlorophyll content and reduced efficiency of the light phase of photosynthesis in spruce, while beech displayed stable pigment concentrations and photochemical performance. These findings highlight the vulnerability of cold-adapted spruce and the adaptive plasticity of more thermophilic beech under warming, suggesting potential shifts in competitive dynamics and forest composition, with implications for future management strategies.