Forest ecosystem response to weather shocks: case study of a montane mixed forest in Switzerland

Abstract:

Extreme environmental events has 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 shock 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 exchange (NEE) measured over 16 years (2004 – 2019), in a montane mixed forest in Switzerland (CH-LAE, Lägeren). In addition to identification of the weather shock events, we test the hypothesis that extremes associated with continuously varying environmental conditions can modify physiological functionality of a forest ecosystem.

We calculated weather shock events 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 150 shock events (VPD-shocks) ranging from one to seven days, that occurred 30% in spring and 63% in summer. On average NEE decreased by 40% during the shock days as compared to pre-shock days, resulting in mean resistance (NEE_shock/NEE_pre-shock) of 0.6. Mean recovery (NEE_post-shock/NEE_shock) was close to one, indicating post-shock days having similar functionality as shock days. There was no significant trend in resistance and recovery over the 16 years. However, we observed an increasing trend (p < 0.05) in resilience for spring season shock events, with increased resilience during recent (2014-2019) spring seasons, but decreased resilience in summer shock events. Finally, decreased functionality during these VPD-shock events confirm 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 future response of forests to climate change.