Warm springs and dry summers weaken radial stem growth dependence on primary growth by increasing phenological synchrony between primary and secondary meristems in Pinus taiwanensis

Abstract A complete integration of the phenology of primary and secondary meristems is essential for understanding the tree growth trajectories under changing climates and the consequences for carbon sequestration in forest ecosystems. Nevertheless, the extent to which the organs of trees may respond differently to climate change, and whether such divergence will alter their functional linkages, remain insufficiently understood. We conducted weekly observations of primary and secondary growth in Taiwan pine (Pinus taiwanensis Hayata) across an elevational gradient in the subtropical forests of southeastern China during 2017-2019, including one year with an extreme drought event. Needle unfolding and shoot elongation began between late March and early April at high elevations, and earlier in mid-to-late March at low elevations. Shoot and needle growth ceased by late May and late August, respectively. Stem growth started in late March regardless of site and year, but its cessation varied considerably, from August to November, with earlier cessation being observed in late July under extreme drought conditions. Primary growth was more responsive to spring temperature but less sensitive to summer-autumn drought than secondary growth. These organ-specific and seasonally asymmetric phenological responses systematically shifted the relative timings of primary and secondary growth, increasing the synchrony between needle and stem activity under warmer and drier conditions and the potential competition for carbon resources among organs. In the long run, the ongoing climate change could amplify the phenological synchrony between needle and stem growth through organ-specific responses, which may alter carbon allocation strategies and constrain radial stem growth in subtropical forest ecosystems.