Resource matching explains the cause of masting events by simulating models, assuming that fruiting exhausts stored carbon (C) reserves. However, C source (reserves or current photoassimilates) for seed development has not been proved yet. Here we assessed the dependency of fruiting on stored C reserves in mature trees of two masting (Fagus sylvatica, Quercus petraea) and one non-masting tree species (Carpinus betulus) at the Swiss Canopy Crane research facility. After 8 years of CO₂ enrichment with highly deleted ¹³C, 14 mature trees were entirely ¹³C-labeled, with delta¹³C of bulk tissue being about 4‰ lower than the corresponding tissue of unlabeled control trees. In 2009, the year following the CO₂ enrichment, the ¹³C label of stored C reserves thus differed significantly from current-year photoassimilate. In order to identify the C sources for reproduction, we therefore tracked ¹³C-labels in different tissues including fruits. delta¹³C of flowers were 4‰ lower than in control trees, indicating the usage of stored C for their development. In contrast, ¹³C-labels were completely absent in developing and ripe fruit in all three investigated species. This striking result is the first experimental proof that even in masting trees, fruit development does not depend on C reserves, but is exclusively supplied from current-year photoassilimates.