Investigating the short-term dynamics of trees’ carbon allocation is critical for understanding the mechanisms underlying annual patterns of carbon allocation in forest ecosystems, influenced by tree phenology and environmental conditions. Pulse-labelling with stable carbon isotope (13C) enables to trace the fate of labelled carbon into tree and its release to the atmosphere by respiration. This study aimed at characterizing the short-term allocation pattern of the recently assimilated carbon in beech leaves, from its assimilation to its partitioning among the several carbon-containing metabolic pools, and its evolution across seasons. In situ whole-tree crown 13CO2-pulse labelling experiments were performed on 10 adult beeches (20-year-old Fagus sylvatica L.) in a natural regeneration mixed stand located in the state forest of Hesse in Northeastern France. Pulse-labelling campaigns were conducted four times during the growing season, in May, July, August, and September, in order to cover the different beech growing phases. For each labelled tree, leaf samples were harvested over a 6-day chase period. Purification and quantification of C-containing metabolic pools - including the isolation of soluble carbohydrates (including soluble sugars, amino and organic acids), starch and structural compounds - were performed. Total leaf organic matter (bulk) and metabolic pools δ13C were determined by IRMS. In addition, the isotope composition of leaf CO2 efflux was monitored by incubating leaves and analysing respired CO2 by IRMS. The labelled 13C was rapidly assimilated in leaves of each tree and recovered in starch, soluble carbohydrates, structural compounds, and in leaf CO2 efflux. Variations among the different labelling periods were observed and the implication of season vs environmental factors will be discussed. Finally, these results will also be used for feeding a leaf-scale compartmental model, which describes the fate of the recently-assimilated carbon among leaf respiration, growth, storage and export, giving the opportunity to isolates kinetics parameters of carbon allocation in leaves.