Record reduction in European forest canopy greenness during the 2022 drought

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 et al 2020).

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 et al 2021) provides correspondingly processed data, which can be used to monitor forest canopy condition in Europe through space and time.

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 et al 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.

Buras A, Rammig A and Zang C S 2020 Quantifying impacts of the 2018 drought on European ecosystems in comparison to 2003 Biogeosciences 17 1655–72

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 Frontiers in Plant Science 12 2355