The Impacts of Different Levels of Drought on O3 Deposition in Forest Mountains in Norway
Climate change projections of Central Europe suggest an increase in drought frequency, duration, severity, and heatwaves occurrence (Seneviratne et al. 2006). Low precipitation, soil water deficit, and high air temperatures, associated with drought, usually leads to a decrease in stomatal conductance and limits tropospheric ozone (O3) uptake by stomatal absorption even at higher levels of O3 concentrations (Emberson et al. 2007).
O3, a secondary photochemical pollutant, and greenhouse gas is mainly formed in the vicinity of urban areas, by reactions of precursors such as nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs) (Monks et al. 2009). As predicted, the average concentration of O3 in the atmosphere will go beyond the 40 ppb threshold by 2050, which is used as a measure of oxidative stress to natural ecosystems (Langer et al. 2005). O3 concentrations are mainly regulated by net chemical production, the influx from the stratosphere and loss to surfaces via dry deposition. Dry deposition of O3 is estimated to account for 25% of the total O3 removal from the troposphere on a global scale (Lelieveld and Dentener 2000). Dry deposition of O3 to terrestrial ecosystems is predominantly through plant stomata and range from 30-90% of the total O3 uptake by the ecosystem (Cieslik 2004). The mechanism involved in the opening/closure of stomata is controlled by water availability in the soil, air temperature, vapour pressure deficit (VPD), photosynthetically active radiation (PAR), and wind velocity (Emberson et al. 2000a).
O3 episodes and occurrences of drought usually happen together, when PAR is at its peak during the summer season. The number and severity of drought cases are on the increase in many parts of the world. This increase, together with the projected background O3 concentration is expected to continue (Bates et al. 2008). Drought has been mentioned as a major limiting factor in regulating gas exchange in the Mediterranean oak ecosystem (Vargas et al. 2013). Drought is also known to have the potential of reducing dry deposition of O3 (Pio et al. 2000). With the prediction of severe and more frequent droughts over Central Europe in the future, it is crucial to have a better understanding of variability in the deposition of O3 under drought stress conditions in Norway spruce forest ecosystem, which is the most-widely spread tree species in the Czech Republic. Understanding the impacts of drought on O3 dry deposition is key in air quality management in regions affected by drought conditions. It is, therefore, important to investigate the effect of different levels of drought on O3 deposition in Norway spruce forest, especially when drought is changing openness of stomatal aperture responsible for most of the O3 accepted into the ecosystem (Mills et al. 2011). The main aim of this study was to investigate the impacts of different levels of drought on O3 deposition in a mountainous Norway spruce forest at Bily Kriz experimental research site in the Beskydy Mts. from June to August 2008.
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