Ankit Shekhar

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Extreme environmental events have become a major interest of ecologists. Commonly, extreme climatic events are identified based on “changes in the mean conditions” over a discrete period with respect to the longer-term climatology. In this study we aim to: 1) define a different type of extreme event, i.e. weather extreme events: an event with extreme deviation from an expected value (calculated based on past weather conditions) and 2) quantify ecosystem resistance, recovery, and resilience in response to these shock events based on changes in net ecosystem productivity (NEP) measured over 16 years (2004 – 2019), in a montane mixed forest in Switzerland (CH-LAE, Lägeren). In addition to the identification of the physiological extreme events, we test the hypothesis that extremes associated with continuously varying environmental conditions can modify the physiological functionality of a forest ecosystem. We calculated weather extreme;based on half-hourly measurements of atmospheric water demand (i.e. vapor pressure deficit, VPD) measured alongside eddy covariance flux measurements. Between 2004 and 2019, we found 185 such physiological extreme events (VPD-extreme), ranging from one to seven days, that occurred about 27 % in spring and 68 % in summer. On average NEP decreased by 25% during these VPD-extreme days compared to the normal-VPD day before, resulting in mean resistance (NEPextreme/NEPpre-extreme) of 0.75. Mean recovery (NEPpost-extreme/NEEextreme) was about 0.85, indicating about a 15% decrease in NEP on days after the extreme events compared to before. There was no significant trend in resistance, recovery, and resilience over the 16 years.  Finally, decreased functionality during these VPD-extreme days events confirms our hypothesis. Our approach of looking at the forest response to extreme events is independent of “changes of mean conditions from long-term climatology” and focuses on the ability of the ecosystem to maintain functionality within the realm of “continuous environmental variability”. Identification of physiologically-relevant climatic extremes and testing the legacy effect from those events is a crucial requirement for understanding the future response of forests to climate change.
This study estimates the influence of anthropogenic emission reductions on nitrogen dioxide (NO_2) and ozone (O_3) concentration changes during the COVID-19 pandemic period using in-situ surface and Sentinel-5p (TROPOMI) satellite column measurements and GEOS-Chem model simulations. We show that, as a result of reductions in anthropogenic emission in eight German metropolitan cities, meteorology corrected mean in-situ (\& column) NO_2(2020,corr) concentrations decreased by 23 ± 4.7 % (& 16.4 ± 7.2 %) between March 21 and June 30, 2020, whereas meteorology corrected mean in-situ O_3(2020,corr) concentration increased by 4 ± 8.8 % between March 21 and May 31, 2020, and decreased by 3 ± 8.7 % in June 2020, compared to 2019 (uncertainty represents the 1 σ of mean changes of eight metropolitan cities). The impacts of meteorology on in-situ and TROPOMI NO_2 concentration changes during the lockdown compared to 2019 are relatively small (+0.4 % and -0.6 %, respectively), while those on in-situ O_3 concentration changes are more significant (+3.6 % and -13.5 % for March 21 to May 31, 2020 and June 2020, respectively). A NO_X saturated ozone production regime in German metropolitan cities in March to May explains the increased O_3(2020,corr) concentration in response to the decreased NO_2(2020,corr) concentration. This implies that future reductions in NO_X emissions are likely to increase ozone pollution in these cities if appropriate mitigation measures are not implemented. TROPOMI NO_2(2020,corr) concentrations decreased nationwide during the stricter lockdown period, except for North-West Germany, which can be attributed to enhanced NO_X emissions from agricultural soils.

Florian Dietrich

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