Temperature-photoperiod interactions improve simulations of early xylem phenology: Refining the Vaganov-Shashkin growth model

Wood formation plays a crucial role in forest productivity and carbon sequestration. Changes in early xylem phenology influence wood development (xylogenesis) and tree growth, thereby affecting carbon uptake by forests. However, a deeper understanding of long-term shifts in spring growth phenology in response to climate warming is lacking. Process-based simulation models, such as the Vaganov-Shashkin (VS) growth model, could be used to simulate and trace shifts in spring growth phenology. We propose upgrading the VS model with a new function that considers the temperature-photoperiod interaction and improves the simulation of growth onset timings. To assess this refined VS model, we compared the early growing xylogenesis phases of two conifers (Pinus pinaster and Pinus sylvestris) and two ring-porous oaks (Quercus faginea and Quercus pyrenacia) coexisting in a Mediterranean continental site located in Soria, central Spain. The improved VS model successfully simulated early growing season xylogenesis during the warm-dry 2012 year, which can be considered analogous to the forecasted warmer scenarios in the 21st century. The upgraded VS model improved its ability to simulate growth onset, demonstrating the importance of considering both temperature and photoperiod. The model was then used to simulate spring phenology considering two Representative Concentration Pathways scenarios (RCP4.5 and RCP8.5) with different warming rates. An earlier, but species-specific, xylem onset was forecasted for the four tree species in response to warmer spring conditions in the late 21st century.