Seasonal warming alters spring needle unfolding and lammas shoot development in Pinus densiflora Siebold and Zucc. seedlings

Key message Field experiments in South Korea show that warming alters the seedling phenology of Pinus densiflora Siebold and Zucc.. Spring needle unfolding was accelerated by spring warming and influenced by elevated daily maximum temperatures. Cumulative high-temperature exposure strongly accelerated lammas shoot development, highlighting extreme heat as a key driver of climate impacts on lammas shoot occurrence and needle unfolding. Context Climate warming is reshaping plant phenology worldwide; however, the specific effects of seasonal and extreme temperature accumulation on seedling development remain poorly understood. Aims We investigated the phenological responses of Pinus densiflora Siebold and Zucc. seedlings to seasonal warming in an open-field experiment in South Korea. Methods One hundred eight 1-year-old seedlings were subjected to four treatments: spring–fall warming (W SF ), summer warming (W S ), consistent warming (W), and control (C). We assessed spring needle unfolding and lammas shoot development, including occurrence and needle elongation stages. Results Elevated spring temperatures advanced spring needle unfolding in W SF and W with 80% and 86% of seedlings reaching needle elongation over 1 cm in May. First lammas shoots commenced development in July, with occurrence rates exceeding 95% across warming treatments by October. Rates were consistently higher in W S and W than in W SF and C, though W SF also increased occurrence by late autumn. Second lammas shoots emerged only in September, with greater frequency and more advanced stages in W S and W until November. Cumulative extreme heat exerted the greatest influence on both first and second lammas shoot development. Conclusion These findings indicate that extreme heat accumulation, rather than elevated mean temperatures alone, is the key determinant of altered lammas shoot formation in P. densiflora .