Seasonal warming alters spring needle unfolding and lammas shoot development in Pinus densiflora S. & Z. seedlings

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 & Zucc. seedlings to seasonal warming in an open-field experiment in South Korea. Methods: 108 one-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 but slowed completion. 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 (EDD >24–27°C) 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 phenological patterns in P. densiflora .