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In order to understand the main driving factors of ozone (O-3) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and dear sky) modulate O-3 flux in forest ecosystems via stomatal regulation. The hypothesis is based on the fact that complex microclimate conditions under cloudy sky usually stimulate stomatal conductance.

O-3 fluxes were inferred from a concentration gradient in a mountainous Norway spruce forest in the Czech Republic (Central Europe) for years 2012-2016 and measured directly by eddy-covariance during the summer of 2017. Daily and seasonal O-3 depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions.

The data show unequivocally that more O-3 is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O-3 flux. Though there are other mechanisms and pathways involved in the transport of O-3 to the plant-soil system, the highest O-3 deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O-3 ecosystem fluxes were observed under clear sky conditions despite the highest O-3 concentrations at this time. These findings suggest that forests growing at sites where conditions are predominantly cloudy are expected to deposit higher extent of O-3 than less-cloudy forests being thus more threatened by phytotoxic O-3. (C) 2019 Elsevier B.V. All rights reserved.