Record reduction in European forest canopy greenness during the 2022 drought

Abstract

<p>According to climate projections, extreme summer drought conditions as those striking Central and Southern Europe in 2022 will become more frequent under climate change. Consequently, studying forests’ response to such extreme conditions may reveal important insights on how forests will cope with anticipated climate conditions. Of particular interest are questions related to forest-type specific drought sensitivities (e.g. broadleaved vs. coniferous, Mediterranean vs. temperate) and the existence of legacy effects from previous droughts (e.g. the extreme 2018 drought, see Buras <em>et al</em> 2020).</p> <p>While many approaches exist to address these questions at local scale, satellite borne remote sensing offers the opportunity to tackle these topics at large scale. Here, the MODIS mission provides a valuable source of information due to a relatively long observational period since the year 2000 at sufficient spatial resolution (250 m x 250 m) and a high sampling frequency (daily images which are used to compute 16-day maximum value composites). In context of monitoring forests' response to environmental conditions, MODIS NDVI renders a frequently considered data source since it reflects canopy greenness and consequently mirrors – among others – early leaf coloration and senescence as direct responses of trees to extreme drought. Yet, MODIS NDVI time-series need to pass a multi-step processing chain to mask poor-quality pixels, remove remaining outliers, gap-fill, and finally apply a pixel-specific standardization to achieve relative measures of canopy greenness. The recently launched European Forest Condition (EFCM, Buras <em>et al</em> 2021) provides correspondingly processed data, which can be used to monitor forest canopy condition in Europe through space and time.</p> <p>Here, we present first insights on the impact of the 2022 drought on European forest ecosystems based on the EFCM. Preliminary results indicate the drought 2022 to supersede previous droughts with regards to the spatial extent of severely affected pixels, thus breaking the former record from the 2018 drought (Buras <em>et al</em> 2020) just four years later. Our analyses suggest that legacy effects from previous years have contributed to this development. Moreover, we found different drought sensitivities of different forest types. In combination, these factors draw a complex picture of forests climate-change resilience, which we here seek to disentangle. Corresponding knowledge will likely provide valuable empirical information to improve model-based projections of tree-species performance under anticipated climate change.</p> <p>Buras A, Rammig A and Zang C S 2020 Quantifying impacts of the 2018 drought on European ecosystems in comparison to 2003 <em>Biogeosciences</em> <strong>17</strong> 1655–72</p> <p>Buras A, Rammig A and Zang C S 2021 The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline <em>Frontiers in Plant Science</em> <strong>12</strong> 2355</p>

Coherency and time lag analyses between MODIS vegetation indices and climate across forest and grasslands in European temperate zone

“…Here, a very big, negative VPD anomaly occurred in August and resulted in decreased condition of the vegetation, especially of pastures. Another severe droughts occurred in Europe in 2018 (Buras and Rammig and Zang, 2020;Schuldt et al, 2020;Boergens et al, 2020), 2019 (Boergens et al, 2020;Hari et al, 2020) and 2022 (Buras and Meyer and Rammig, 2023;Wang et al, 2023). All of them were observed also in our study area.…”

Coherency and time lag analyses between MODIS vegetation indices and climate across forests and grasslands in the European temperate zone

“…Here, a very big negative VPD anomaly occurred in August and resulted in a decreased condition of the vegetation, especially of pastures. Other severe droughts occurred in Europe in 2018 (Buras et al, 2020;Schuldt et al, 2020;Boergens et al, 2020), 2019 (Boergens et al, 2020;Hari et al, 2020) and 2022 (Buras et al, 2023;Wang et al, 2023). All of them were observed also in our study area.…”

Impacts and damages of the European multi-year drought and heat event 2018–2022 on forests, a review

“…Reduced greenness was also observed in the spring of 2019 when compared to the greenness before the drought in spring 2018 (Brun et al, 2020). During the hottest summer on record in Europe in 2022, large parts of temperate forest regions were negatively affected, and forest greenness decreased stronger than any other summer since 2002 by breaking the former record drought in 2018 (Hermann et al 2023, Buras et al 2023; the five aforementioned studies are based on satellite-derived Normalized Difference Vegetation Index (NDVI).…”