Effect of the 2022 summer drought across forest types in Europe

Abstract

Abstract. Forests in Europe experienced record-breaking dry conditions during the 2022 summer. The direction in which various forest types respond to climate extremes during their growing season is contingent upon an array of internal and external factors. These factors include the extent and severity of the extreme conditions and the tree ecophysiological characteristics adapted to environmental cues, which exhibit significant regional variations. In this study we aimed to: 1) quantify the extent and severity of the extreme soil and atmospheric dryness in 2022 in comparison to two most extreme years in the past (i.e., 2003, 2018), 2) quantify response of different forest types to atmospheric and soil drought in terms of canopy browning and photosynthesis, and 3) relate the functional characteristics of the forests to the emerging responses observed at the canopy level. For this purpose, we used the ERA5-Land spatial meteorological dataset between 1970 to 2022 to identify conditions with extreme soil and atmospheric dryness. We used the near-infrared reflectance of vegetation (NIRv) derived from the MOderate Resolution Imaging Spectroradiometer (MODIS), and the OCO-2 solar induced fluorescence (SIF) as an observational proxy for photosynthesis based on the SIF data product, to quantify the response of forests at the canopy level. In summer 2022, particularly southern regions of Europe experienced the most pronounced atmospheric and soil dryness. As a result, the extremely dry conditions led to an average 30 % more widespread decline in SIF across forests compared to drought in 2018, and 60 % more widespread decline compared to drought in 2003. Although the atmospheric and soil drought were more extensive and severe (indicated by a larger observed max z-score) in 2018 compared to 2022, the negative impact on forests, indicated by declined SIF, was significantly larger in 2022. Across different forest types, the deciduous broad-leaved forests were most negatively affected by the extreme conditions in 2022, but Evergreen Needle-Leaf Forests (ENF) distributed in northern regions of Europe showed enhanced canopy greening and SIF signals as a benefit of warming. Higher degree of canopy damage in 2022 in spite of less extreme conditions compared to the previous extreme year points to a legacy effect on forest canopies, and a declined forest resilience in response to more frequent drought events.