Canopy scale measurements of stomatal and cuticular 03 uptake by sitka spruce

Ozone (03) fluxes, 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. O3 fluxes increased from − 0.2 μg m−2 s−1 at night to between − 0.6 and − 1.0 μg 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 O3 were compared to those of water vapour and the rate of uptake of O3 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 O3 in strong sunlight conditions, a pathway observed by Rondón (1993). The toxic effects of this pathway remain unclear but this mechanism may have severe consequences for coniferous forests during summer-time anticyclonic conditions in Europe when O3 concentrations may be very high.