5. Carbon allocation in branch wood

Taking advantage of the d¹³C-depleted CO₂ used for the CO₂ enrichment, we assessed the relative contribution of stored and current carbon compounds to new shoot growth in larch and pine. Wood and needle material from trees exposed to both ambient and elevated CO₂ concentrations was collected and analysed for the ¹³C/¹²C-ratio. The deciduous larch was found to supply new shoot growth (structural woody part) by 46% from storage. Surprisingly, the evergreen pine which we expected to use current-year photosynthates for new wood growth, also considerably depended on storage (42%). In contrast, storage made only a very small contribution to needle growth (more decreased ¹³C/¹²C-ratio under elevated CO₂ conditions for needles than for wood). In wood, the ¹³C/¹²C-ratio of different wood fractions was determined. However, the allocation of new carbon to different fractions of the wood was similar. We conclude that storage plays an important role for new shoot growth in these treeline conifers. In particular, stored carbon compounds apparently are more relevant for current-year growth in the evergreen pine than has previously been thought.