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Keywords:

  • autocorrelation;
  • carbohydrates;
  • deciduous;
  • evergreen;
  • labelling;
  • lipids;
  • stable isotopes;
  • allocation;
  • climate reconstruction

ABSTRACT

Stable C isotope signals in plant tissues became a key tool in explaining growth responses to the environment. The technique is based on the fundamental assumption that the isotopic composition of a given unit of tissue (e.g. a tree ring) reflects the specific C uptake conditions in the leaf at a given time. Beyond the methodological implications of any deviation from this assumption, it is of physiological interest whether new C is transferred directly from sources (a photosynthesizing leaf) to structural sinks (e.g. adjacent stem tissue), or inherently passes through existing (mobile) C pools, which may be of variable (older) age. Here, we explore the fate of 13C-labelled photosynthates in the crowns of a 30–35 m tall, mixed forest using a canopy crane. In all nine study species labelled C reached woody tissue within 2–9 h after labelling. Four months later, very small signals were left in branch wood of Tilia suggesting that low mixing of new, labelled C with old C had taken place. In contrast, signals in Fagus and Quercus had increased, indicating more intense mixing. This species-specific mixing of new with old C pools is likely to mask year- or season-specific linkages between tree ring formation and climate and has considerable implications for climate reconstruction using stable isotopes as proxies for past climatic conditions.