Evidence of advancing spring xylem phenology in Chinese forests under global warming

Phenological responses of vegetation to the ongoing warming trend impact current and future primary productivity. However, few studies focus on wood phenology because its observed data are much scarcer, which hinders the estimation and prediction of forest carbon budgets over large regions. Here, we use a physiological process-based tree-ring growth model (Vaganov-Shashkin model) to investigate the spatial and temporal variations of spring xylem phenology (start of the growing season of xylem, SOSx) in tree-ring sites of China during 1962–2016 CE. The model is calibrated on measured tree-ring width chronologies (70 tree-ring chronologies) and successfully validated with field observations of xylogenesis. We found that spring xylem phenology significantly advances during 1962–2016 CE period under global warming, with the rate of advance quickly increasing after the 1990s to an average of 0.25 days per year. The preseason daily mean temperature is the main climatic driver for spring xylem phenology as indicated by its significant correlations with SOSx at most sites (71%). Warmer preseason allows heat requirements for tree growth to be reached more quickly, with increase of 1°C in temperature of preseason anticipates SOSx by 6 to 7 days, which will benefit the radial growth of trees in the relatively cold-humid environments. In addition, the significant positive correlation between the simulated spring xylem phenology and remote sensing derived phenology highlights the primary and secondary growth may be governed by the same variable (temperature) and change in the same direction with global warming. This study provides the long-term perspective on the spring xylem phenology variations covering most of China.