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Late frost can destroy the photosynthetic apparatus of trees. We hypothesized that this can alter the normal cyclic dynamics of C-reserves in the wood. We measured soluble sugar concentrations and radiocarbon signatures (Delta C-14) of soluble nonstructural carbon (NSC) in woody tissues sampled from a Mediterranean beech forest that was completely defoliated by an exceptional late frost in 2016. We used the bomb radiocarbon approach to estimate the time elapsed since fixation of mobilized soluble sugars. During the leafless period after the frost event, soluble sugar concentrations declined sharply while Delta C-14 of NSC increased. This can be explained by the lack of fresh assimilate supply and a mobilization of C from reserve pools. Soluble NSC became increasingly older during the leafless period, with a maximum average age of 5 yr from samples collected 27 d before canopy recovery. Following leaf re-growth, soluble sugar concentrations increased and Delta C-14 of soluble NSC decreased, indicating the allocation of new assimilates to the stem soluble sugars pool. These data highlight that beech trees rapidly mobilize reserve C to survive strong source-sink imbalances, for example due to late frost, and show that NSC is a key trait for tree resilience under global change.