Lengthening height-growth duration in Smith fir as onset becomes more synchronous across elevations under climate warming scenarios

Accurate projections of growing-season duration of trees are crucial in evaluating the capacity of forests to mitigate climate warming through growth and CO₂ uptake. However, little is known on how and to what extent the growing season of tree height growth will change under future warming. Herein, we projected the height-growth phenology of Smith fir (Abies georgei var. smithii) along two altitudinal gradients on the southeastern Tibetan Plateau under CMIP6 climate scenarios by using process-based phenology models. The models performed well when simulating timings of height-growth onset and cessation on the plot level, with root mean square errors of less than 5.8 and 3.8 days, respectively. From the 2020s to 2090s, the onset of height growth (i.e. bud swelling date) was predicted to advance by 10 ± 6 days under four warming scenarios. The elevational gradients of onset of bud swelling would significantly decrease from 3–4 to 1–2 days 100m⁻¹ under the warmest climate scenario, suggesting future warming may result in more uniform onset of height growth along elevational gradients. Given the temporal variation of thermal requirements, the projected cessation of height growth was delayed by 3 ± 1 days in most plots. Overall, the height-growth duration was predicted to lengthen by 4–22 days by the end of the century. The extended duration of height growth predicted for Smith fir could enhance primary growth, forest productivity, and CO₂ sequestration on the southeastern Tibetan Plateau.