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 (O3) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and clear sky) modulate O3 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. O3 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 O3 depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions. The data show unequivocally that more O3 is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O3 flux. Though there are other mechanisms and pathways involved in the transport of O3 to the plant-soil system, the highest O3 deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O3 ecosystem fluxes were observed under clear sky conditions despite the highest O3 concentrations at this time. These findings suggest that forests growing at sites where conditions are predominantly cloudy are expected to deposit higher extent of O3 than less-cloudy forests being thus more threatened by phytotoxic O3.