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Emerging leaves in evergreen tree species are supplied with carbon (C) from the previous year's foliage. In deciduous trees, no older leaves are present, and the early phase of leaf development must rely on C reserves from other tissues. How soon developing leaves become autotrophic and switch from being C sinks to sources has rarely been studied in mature forest trees, and simultaneous comparisons of species are scarce. Using a canopy crane and a simple  13 CO 2 -pulse-labelling technique, we demonstrate that young leaves of mature trees in three European deciduous species ( Fagus sylvatica  L.,  Quercus petraea  (Matt.) Liebl.,  Tilia platyphyllos  Scop.) start assimilating CO 2  at a very early stage of development (10–50% expanded). One month after labelling, all leaves were still strongly  13 C enriched, suggesting that recent photosynthates had been incorporated into slow turnover pools such as cellulose or lignin and thus had contributed to leaf growth. In line with previous studies performed at the same site, we found stronger incorporation of recent photosynthates into growing tissues of  T. platyphyllos  compared with  F. sylvatica  and  Q. petraea . Non-structural carbohydrate (NSC) concentrations analysed for one of the three study species ( F. sylvatica ) showed that sugar and starch pools rapidly increased during leaf development, suggesting that newly developed leaves soon produce more NSC than can be used for growth. In conclusion, our findings indicate that expanding leaves of mature deciduous trees become C autonomous at an early stage of development despite the presence of vast amounts of mobile carbohydrate reserves.

Expanding leaves of mature deciduous forest trees rapidly become autotrophic