Vertical variation in the transport and fate of radiocesium through the
canopy via branchflow and stemflow
Abstract
This study seeks to better understand the vertical variation in the
transport and fate of radiocesium via branchflow and stemflow in the
aftermath of the Fukushima nuclear power plant accident. Working in both
a coniferous forest (Cryptomeria japonica, young Japanese cedar stands,
average height is 14.0 m) and a mixed deciduous broadleaved forest
(Quercus serrata, Japanese oak stands, average height is 13.3 m), we
employed isotopic tracers to help determine the fate of radiocesium
transported by branchflow and stemflow from the upper and lower portions
of the canopy. Branchflow was harvested and examined from the upper
canopy layers (including younger foliage, dead foliage, and live
branches), whereas stemflow was collected in both the upper and lower
portions of the canopy (with varying portions of live and dead
branches). Particular attention was paid to the washoff, leaching,
adsorption, transport, and storage (stem and bark) of radiocesium. The
preliminary results showed radiocesium leaching (Cs-137 concentration)
was greater for branchflow that received washoff and leachate from the
dead foliage than the branchflow receiving radiocesium inputs from mixed
and young foliage. For the tree trunk, radiocesium leached more in
stemflow from the lower part of the canopy as compared to the upper
canopy. We also found that the isotopic composition of branchflow was
generally enriched in δ18O and δD compared to open rainfall and
throughfall, however, the differences in enrichment between branchflow
and stemflow remains unclear. Further work should examine the effect of
tree architecture on the cycling of radiocesium both stemflow and
branchflow.