Fluxes of trichloroacetic acid through a conifer forest canopy
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Date
2004Author
Stidson, R T
Heal, Kate V
Dickey, Catherine A
Cape, Neil
Heal, Mathew R
Metadata
Abstract
Controlled-dosing experiments with conifer seedlings have demonstrated an aboveground
route of uptake for trichloroacetic acid (TCA) from aqueous solution into the
canopy, in addition to uptake from the soil. The aim of this work was to investigate
the loss of TCA to the canopy in a mature conifer forest exposed only to
environmental concentrations of TCA by analysing above- and below-canopy fluxes
of TCA and within-canopy instantaneous reservoir of TCA. Concentrations and fluxes
of TCA were quantified for one year in dry deposition, rainwater, cloudwater,
throughfall, stemflow and litterfall in a 37-year-old Sitka spruce and larch plantation
in SW Scotland. Above-canopy TCA deposition was dominated by rainfall (86%),
compared with cloudwater (13%) and dry deposition (1%). On average only 66% of
the TCA deposition passed through the canopy in throughfall and stemflow (95% and
5%, respectively), compared with 47% of the wet precipitation depth. Consequently,
throughfall concentration of TCA was, on average, 1.4 × rainwater concentration.
There was no significant difference in below-canopy fluxes between Sitka spruce and
larch, or at a forest-edge site. Annual TCA deposited from the canopy in litterfall was
only 1–2% of above-canopy deposition. On average, 800 μg m−2 of deposited TCA
was lost to the canopy per year, compared with estimates of above-ground TCA
storage of 400 and 300 μg m−2 for Sitka spruce and larch, respectively. Taking into
account likely uncertainties in these values (±50%), these data yield an estimate for
the half-life of within-canopy elimination of TCA in the range 50–200 days, assuming
steady-state conditions and that all TCA lost to the canopy is transferred into the
canopy material, rather than degraded externally. The observations provide strong
indication that an above-ground route is important for uptake of TCA specifically of
atmospheric origin into mature forest canopies, as has been shown for seedlings (in
addition to uptake from soil via transpiration), and that annualized within-canopy
elimination is similar to that in controlled-dosing experiments.