The Key Role of Xylem Hydraulic Vulnerability in Influencing Interannual Growth Variability of Earlywood and Latewood Across the Northern Hemisphere

Estimating interannual variability of earlywood and latewood growth is crucial for assessing the fluctuations in forest carbon sequestration. However, with amplified climate variations, it remains unclear how earlywood and latewood growth vary across space and over time, as well as the key driving factors that influence these patterns. Here, we quantified the interannual variability of earlywood (EW_CV) and latewood (LW_CV) from 1901 to 2013 based on 596 tree-ring chronologies across the Northern Hemisphere and investigated their spatial-temporal patterns. Using Boosted Regression Tree models, we assessed which factors related to climate, hydraulic status, plant structure, and stand properties determine EW_CV and LW_CV. We found that both EW_CV and LW_CV were higher in drier regions, but they exhibited divergent spatial distribution patterns. This may be because earlywood is more affected by local long-term aridity, whereas latewood appears to be more prone to within-season water availability. Besides, xylem hydraulic vulnerability (P₅₀, xylem water potential at 50% loss of hydraulic conductance) emerged as the key influencing factor of growth fluctuation in both earlywood and latewood. These findings suggest that differences in hydraulic functions between earlywood and latewood play a key role in how trees adapt to water stress. Our results illuminate the mechanisms of how earlywood and latewood respond to changing climatic conditions and highlight the crucial role of hydraulic functions in vegetation model prediction of tree growth.