Near-synchronous xylem width increase and biomass production in European beech revealed by X-ray micro-computed...

Understanding the intra-annual timing of carbon allocation into wood is critical for linking environmental variability to forest carbon sequestration. We quantified the seasonal dynamics of xylem width increase and xylem biomass production in European beech (Fagus sylvatica) in Belgium during two contrasting growing seasons, a year characterized by a severe spring drought and peak summer hot drought (2020) and a mesic year (2021), using high-resolution X-ray micro-computed tomography (XμCT). Point dendrometer and microtomy measurements were used for validation and comparison. Both xylem width increase and cumulated xylem biomass production followed a sigmoidal seasonal trajectory in both years. Contrary to patterns commonly reported for conifers with a clear early- to latewood transition, the temporal lag between xylem width increase and xylem biomass production was minimal (<3 days) and statistically insignificant, indicating near-synchronous development. XμCT-derived measurements of ring width, wood density and biomass production broadly matched dendrometer data, as well as independent regional biomass estimates from the literature. Seasonal onset and cessation of xylogenesis shifted between years, with earlier onset and cessation in the dry year, which was aligned to microtomy data. Our results demonstrate that, in semi diffuse- to ring-porous angiosperms like beech, xylem expansion and secondary wall formation are tightly coupled, even under contrasting water availability, and suggest that dendrometer data can serve as a proxy for intra-seasonal wood biomass production at the stand level. XμCT provides a reliable tool for quantifying xylogenesis and carbon allocation, offering new insights into wood formation dynamics under variable environmental conditions.