Terminal bud size, spring and summer temperatures regulate the timing of height-growth cessation of Smith fir on the southeastern Tibetan Plateau

The cessation of height growth, a key event of autumn phenology, plays a vital role for tree growth and forest productivity. Despite its importance, our knowledge of environmental and biotic factors affecting autumnal events in plants remains incomplete. In particular, questions remain whether thermal conditions and previous growth influence the cessation of height growth. Here, we assessed the effects of spring and summer temperatures and terminal bud size on the cessation in height growth of Smith fir (Abies georgei var. smithii) using phenological observations performed in 2013-2019 at four sites located along an altitudinal gradient between 3800 and 4400 m a.s.l. on the southeastern Tibetan Plateau. The cessation of height growth mainly occurred from late–July to early–August with a weak delay along altitude gradient (1.5 days 100 m⁻¹). Structural equation modelling demonstrated that higher summer temperature (June–July) directly advanced the cessation of height growth, whereas warmer spring temperature (March–May) advanced the height-growth cessation mainly through inducing earlier bud swelling (i.e. the onset of height growth). Larger terminal buds delayed the cessation of height growth by increasing growth rates. However, bud swelling was more sensitive to the change in annual mean temperature than height-growth cessation, resulting in an extended duration of height growth under warmer conditions. The warmer conditions expected in a context of climate change would lengthen the period of tree growth and enhance forest productivity on the southeastern Tibetan Plateau.