Assessment of the impact of 2022 extreme climate conditions on European forests

The frequency and intensity of extreme climate events is increasing globally. In 2022, Europe experienced extreme heatwaves and prolonged droughts followed by clear indications of a devastating effect across many forest sites. Instrumental measurements show that 2022 contained some of the most extreme heat and dryness conditions ever recorded across Europe, particularly during the forest growing season. Such extreme conditions that follow a number of consecutive extreme years (e.g., 2019, 2018, 2015) with short intervals in between are becoming the new norm and the response of forests in terms of canopy condition, reduced productivity, and potential feedback to the climate is not clear. It is yet not clear how the negative and positive impacts of the most recent extreme temperature and dryness conditions differ from the impact of previous extreme conditions. This study aims to: 1) compare the severity of extreme conditions in 2022 (in terms of heat and dryness) with the preceding extreme years (i.e., 2003, 2015, 2018, 2019) and 2) to quantify forest canopy response (in terms of vegetation browning) and variables responsible for the feedback from forests to the climate (transpiration and solar induced fluorescence as a proxy for photosynthetic activity) across the main classes of forest types in Europe.

For this assessment we use spatially explicit daily modelled top soil water content (0-7 cm), air temperature, and potential transpiration using the ERA5-Land spatial dataset (0.1° × 0.1°) between 1970 to 2022. Reference period for anomaly assessment is set to the 1970-2000 period and MODIS land cover type product is used to aggregate anomalies across forest types. SIF anomalies are extracted using an OCO-2 satellite based SIF dataset, and ground-based ecophysiological measurements collected across a few sites during heatwave and drought events in 2022 are compiled and used to link leaf-level processes to observed canopy response.

Our initial assessment shows that conditions in July 2018 had the largest negative impact on transpiration of European forests, but in June 2022 we observed a far larger spatial extent of positive temperature anomalies across Europe. We compare the impact of 2022 extreme conditions with that of previous extreme years, and discuss our results in the light of underlying ecophysiological mechanisms that control the response and the feedback to climate across different forest types and climate regions.