Satellite data reveal differential responses of Swiss forests to unprecedented 2018 drought

Abstract

Extreme events such as the summer drought of 2018 in Central Europe are projected to occur more frequently in the future and may cause major damages including increased tree mortality and negative impacts on forest ecosystem services. Here, we quantify the response of >1 million forest pixels of 10 × 10 m across Switzerland to the 2018 drought in terms of resistance, recovery, and resilience. We used the Normalized Difference Water Index (NDWI) derived from Sentinel‐2 satellite data as a proxy for canopy water content and analyzed its relative change. We calculated NDWI change between the 2017 pre‐drought and 2018 drought years (indicating resistance), 2018 and the 2019 post‐drought (indicating recovery), and between 2017–2019 (indicating resilience). Analyzing the data from this large natural experiment, we found that for 4.3% of the Swiss forest the NDWI declined between 2017 and 2018, indicating areas with low resistance of the forest canopy to drought effects. While roughly 50% of this area recovered, in 2.7% of the forested area NDWI continued to decline from 2018 to 2019, suggesting prolonged negative effects or delayed damage. We found differential forest responses to drought associated with site topographic characteristics and forest stand characteristics, and to a lesser extent with climatic conditions and interactions between these drivers. Low drought resistance and high recovery were most prominent at forest edges, but also on south‐facing slopes and lower elevations. Tree functional type was the most important driver of drought resilience, with most of the damage in stands with high conifer abundance. Our results demonstrate the suitability of satellite‐based quantification of drought‐induced forest damage at high spatial resolution across large areas. Such information is important to predict how local site characteristics may impact forest vulnerability to future extreme events and help in the search for appropriate adaptation strategies.

Soil–plant interactions modulated water availability of Swiss forests during the 2015 and 2018 droughts

“…Furthermore, the duration and the drought intensity were higher in 2018 than in 2015, as indicated by higher cumulative T d over the summer months. All this might be responsible for further visible crown damages after 2018 (Braun et al, 2020; Brun et al, 2020; Sturm et al, 2022) but not after 2015.…”

“…Limited water supply triggers a cascade of physiological responses, such as stomatal closure, reduced water and nutrient uptake, downregulation of photosynthesis, early wilting, crown dieback, and mortality, as observed in 2018 in Germany, Austria, and Switzerland (Obladen et al, 2021; Schuldt et al, 2020; Walthert et al, 2021). So far, several studies have focused on the impacts of recent droughts on carbon assimilation using eddy covariance measurements and remote sensing data (Gharun et al, 2020; Ramonet et al, 2020; Stocker et al, 2019; Thompson et al, 2020; Wang et al, 2020), mapping of early wilting via anomalies in remote sensing‐derived vegetation indices (Brun et al, 2020; Schuldt et al, 2020; Sturm et al, 2022), characterizing these droughts in terms of climatic characteristics (Buras et al, 2020) or in terms of its remote‐sensing derived water mass deficit (Boergens et al, 2020), and evapotranspiration anomalies (Ahmed et al, 2021). Only a few studies have evaluated these droughts with mechanistic models (Mastrotheodoros et al, 2020; Moravec et al, 2021), targeting hydrological impacts rather than physiological drought stress.…”

Editorial: Forest monitoring to assess forest functioning under air pollution and climate change

“…New methods (such as point dendrometers or soil CO 2 measurements) provide particularly high temporal information that allows in some cases (Zweifel et al) close to real-time analysis of the interaction between environmental drivers and their impact on ecosystem functioning. With the increased probability of extreme events, now-casting of the forest stress and health status will become more and more important and needs to be complemented by remote sensing approaches that allow high spatial coverage (Sturm et al, 2022) and the detection of early warning signals (D'Odorico et al, 2021) enabling us to detect stress response in the canopy before any visual signs of defoliation or leaf discoloration can be observed. These are, however, additional, complementary methods that cannot replace the methods used since long to generate long-term time series, but make them even more valuable.…”

Are the conditions of the Italian forests worsening due to extreme climatic events? Evidence from the national monitoring networks ICP Forests - CON. ECO.FOR.

“…In quest'ottica il monitoraggio della salute delle foreste rappresenta uno strumento fondamentale per poter conoscere l'entità e la direzione dei cambiamenti, nonché per valutarne le cause scatenanti e i fattori contribuenti. Il monitoraggio, tuttavia, può acquistare maggiore efficacia lavorando in due direzioni: (i) aumentare la capacità di intercettazione dei disturbi forestali e la loro distribuzione nello spazio e nel tempo, incrementando le osservazioni a terra su reti locali di monitoraggio, grazie anche alla collaborazione con cittadini (Castellaneta et al 2021) e la loro restituzione per mezzo di remote sensing (Sturm et al 2022); (ii) migliorare la qualità dell'informazione con l'introduzione di specifici indicatori di tipo fisiologico per misurare la vitalità e la resilienza degli alberi (Bussotti & Pollastrini 2017a, 2017b.…”