Drought shifts ozone deposition pathways in spruce forest from stomatal to non-stomatal flux.

Dry deposition is the primary pathway for tropospheric ozone (O) removal, with forests playing a critical role. However, environmental stressors such as drought can reduce this removal capacity by limiting stomatal O uptake due to stomata closure. Here we test the hypothesis that combined soil and atmospheric drought reduces the O sink capacity of forest ecosystems by diminishing stomatal O flux. For stomatal O flux estimation, we applied a single-layer resistance model, which estimates stomatal O flux based on evaporative resistance method complemented by aerodynamic and laminar sublayer resistances calculation. The model was complemented by detailed sap flow monitoring within the forest footprint, to calculate stomatal O flux, using long-term eddy covariance measurements of total water vapour and O fluxes over four growing seasons (2017-2020), including an unprecedented drought period. The results revealed that non-stomatal O flux compensated for the reduction in stomatal flux in a temperate Norway spruce forest at the Bílý Kříž experimental site in the mountainous region of the Czech Republic, Central Europe. Ozone consumption through interactions with volatile organic compounds, quantified by the MEGAN (Model of Emissions of Gases and Aerosols from Nature) model, contributed only marginally to the non-stomatal flux. These findings suggest that surface reactions, where O interacts with plant surfaces, cuticular layers, and soil particles, likely constitute a dominant non-stomatal O sink during drought. To our knowledge, this is the first report of severe drought influencing O fluxes in temperate mountainous regions, which were previously considered less affected by drought stress.