Temporal and spatial patterns of carbon allocation in the canopy of white oak

The fate of photoassimilated 14C was followed by measuring 14C incorporation into leaf and branch tissues (≤ 5 years old) of two forest-grown white oak trees. Fate of 14C-labelled photosynthate was examined 7 days after 14CO2 uptake on live dates (April–October) during the growing season. Both upper and lower canopy positions were sampled. Incorporation of 14C into foliage was significant throughout the growing season. It ranged from 95% of the total 14C retained in April to 50% in October. Incorporation of 14C-labelled photosynthate into the canopy was highest in June and averaged 33% of gross photosynthetic production over the entire growing season. Higher retention of photosynthate in branches versus leaves was noted in the upper canopy than in the lower canopy during the middle and late growing season. Activity levels in tissues indicated that within-canopy sink strength was in the order acorns ≈ buds > leaves > branches. Translocation of initial 14C-labelled photosynthate from both leaves and branches was calculated based on 14C retention and estimated respiratory losses of leaf and branch tissues. These calculations indicate that leaves were still importing significant amounts of photosynthate in April when expansion was two-thirds complete. Translocation of 14C from the canopy during the remaining growing season amounted to 25–45% of gross photosynthesis. Our data indicate that seasonal demands of growth and maintenance of the forest canopy constitute a substantial sink for photosynthate allocation by white oak.