C-14 ALLOCATION IN TREE SOIL SYSTEMS

We studied whole-tree C allocation with special emphasis on the quanti fication of C allocations to roots and root respiration. To document s easonal patterns of C allocation, 2-year-old hybrid poplar trees great er than 3 m tall were labeled with CO(2)C-14 in a large Plexiglas cham ber in the field, in July and September. Climate and CO2 concentration were controlled to track ambient conditions during labeling. Individu al tree canopy CO2 assimilation averaged 3.8 mumol CO2 m-2 s-1 (12.9 g C day-1 tree-1) in July and 6.2 mumol CO2 m-2 s-1 (9.8 g C day-1 tree -1) in September. Aboveground dark respiration was 12% of net daytime C fixation in July and 15% in September. Specific activity of root-soi l respiration peaked 2 days after labeling and stabilized to less than 5% of maximum 2 weeks later. Low specific activity of root-soil respi ration and a labeled pool of root C demonstrated that current photosyn thate was the primary source of C for root growth and maintenance duri ng the growing season. Root respiration averaged 20% of total soil res piration in both July and September based on the proportion of labeled C respired to labeled C fixed. In July, 80% of the recovered C-14 was found above ground and closely resembled the weight distribution of t he growing shoot. By September, 51% of the recovered C-14 was in the r oot system and closely resembled the weight distribution of different size classes of roots. The finding that the distribution of biomass an d C-14 were similar verified that the C introduced during labeling fol lowed normal seasonal translocation pathways. Results are compared to smaller scale labeling studies and the suitability of the approach for studying long-term C fluxes is discussed.