title: Rapid mixing between old and new C pools in the canopy of mature forest trees.
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 (13)C-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.
authors.affiliation: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland. sonja.keel@sek.slu.se
authors.forename: Sonja G
authors.initials: SG
authors.lastname: Keel
authors.forename: Rolf T W
authors.initials: RT
authors.lastname: Siegwolf
authors.forename: Maya
authors.initials: M
authors.lastname: Jäggi
authors.forename: Christian
authors.initials: C
authors.lastname: Körner
mesh_terms: D001692:Biological Transport; D002244:Carbon / Q000032:analysis / Q000378:metabolism*; D002247:Carbon Isotopes; D010788:Photosynthesis / Q000502:physiology; D024301:Plant Bark / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D013045:Species Specificity; D014197:Trees / Q000378:metabolism*; D014934:Wood / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D002244:Carbon