Ozone flux and ozone deposition in a mountain spruce forest are modulated by sky conditions

In order to understand the main driving factors of ozone (O ) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and clear sky) modulate O 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 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 depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions. The data show unequivocally that more O is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O flux. Though there are other mechanisms and pathways involved in the transport of O to the plant-soil system, the highest O deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O ecosystem fluxes were observed under clear sky conditions despite the highest O 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 than less-cloudy forests being thus more threatened by phytotoxic O .