Temperature-Driven onset and light quality-linked senescence in Fagus sylvatica phenology

Understanding phenology is essential to assessing forest responses to climate change, yet most research focuses on leaf development and the onset of the growing season. Less is known about the end-of-season dynamics and their physiological underpinnings. Here, we examined the seasonal timing and coordination of leaf phenology, cambial activity, xylem and phloem formation, sap flow, and gross primary production (GPP) in European beech (Fagus sylvatica L.) from 2018 to 2022 in the Czech Republic. Spring phenology, including budbreak, cambial reactivation, and GPP onset, were driven by air temperatures approaching 10 °C, with a consistent initiation around DOY 112. In European beech, light appears to function as a regulatory cue, acting as a safeguard against premature budburst, whereas temperature operates as the principal driver of phenological development. Sap flow followed shortly thereafter. In contrast, autumn phenology showed strong sensitivity to solar radiation quality—particularly the clearness index—highlighting the role of spectral light composition in driving senescence. Specifically, 10 % leaf colouring and the onset of phloem compression exhibited peak correlations with late-summer cloudiness (r = 0.97–0.99). Stem growth initiation, measured by dendrometers, lagged two weeks behind xylem enlargement, but the cessation of radial growth (90 %) precisely coincided with the end of secondary wall thickening. These findings reveal distinct spring and autumn triggers—thermal versus radiative—shaping phenology and underline the need to include ecophysiological indicators in phenology modelling to better represent forest functioning and carbon cycling under climate change.