CANOPY SCALE MEASUREMENTS OF STOMATAL AND CUTICULAR O-3 UPTAKE BY SITKA SPRUCE

Ozone (O-3) duxes, together with detailed multi-height micrometeorological fluxes of water vapour, sensible heat and momentum, have been determined for a 4 d period above a Sitka spruce canopy using eddy correlation techniques. O-3 fluxes increased from -0.2 mu g m(-2) s(-1) at night to between -0.6 and -1.0 mu g m(-2) s(-1) in the early morning and then gradually reduced throughout the day. Deposition velocities behaved in a similar way, varying from nighttime minima of less than 2 mm s(-1) to maxima of between 7 and 14 mm s(-1). Canopy resistances for O-3 were compared to those of water vapour and the rate of uptake of O-3 was shown to be up to three times that of stomatal deposition. The enhanced uptake rates were only observed during daylight hours and reached a maximum around midday. These increased fluxes may be caused by increased transport; however, the leaf cuticles could provide a photochemical sink for O-3 in strong sunlight conditions, a pathway observed by Rondon (1993). The toxic effects of this pathway remain unclear but this mechanism may have severe consequences for coniferous forests during summertime anticyclonic conditions in Europe when O-3 concentrations may be very high.