Tree-species-specific response of canopy greenness to the extreme droughts of 2018 and 2022

In recent decades, forests have faced increasingly severe droughts due to rising temperatures and longer dry spells. These conditions intensify atmospheric and soil drought, putting forest ecosystems under considerable stress. The extreme drought years in Germany, particularly in 2018 and 2022, have had a profound impact on forests and thus offer the potential to gain important insights into the responses of different tree species to hotter droughts. Given the pivotal role of forests in mitigating climate change and the long rotation periods of managed forests, understanding species-specific drought responses is crucial for developing effective strategies to adapt to evolving climate scenarios.

Currently, our understanding of species-specific drought responses relies heavily on dendroecological analysis and plot-based ecophysiological monitoring networks. While these methods provide insights into tree growth and physiology, their spatial constraints limit widespread replication. To address these limitations and quantify drought responses of specific tree species at a larger scale, our study integrated tree-species maps from the Thünen Institute with remotely sensed canopy greenness and environmental variables, including soil moisture (PAWC), atmospheric vapor pressure deficit (VPD), and climatic water balance (SPEI). Specifically, we focused on four dominant species: two with more anisohydric characteristics (beech and oak, which keep their stomata largely open under drought) and two with more isohydric strategies (pine and spruce, which close their stomata already under less extreme drought). Using statistical methods such as linear regression and machine learning within a gradient-boosting framework, we aimed to explore the factors influencing changes in canopy greenness for different species from 2018 to 2022.

We found that nearly all trees of these species had lower PAWC in 2022 than in 2018, while only one-third of beech, oak, and pine trees and more than 70% of spruce trees had higher VPD in 2022. More isohydric species showed a greater decline in canopy greenness over this period compared to more anisohydric species, despite similar soil moisture conditions. Our models suggest that more isohydric species were primarily affected by extremely low soil moisture, whereas more anisohydric species were primarily affected by atmospheric moisture deficit. Our statistical analysis showed that oak is the only species with significantly higher canopy greenness in 2022 compared to 2018. Linear regression models showed very low importance of PAWC for oak canopy greenness but much higher importance of VPD. However, we hypothesize that all species are still susceptible to carry-over effects from previous drought years or secondary factors related to biotic pathogens.

Our study provides critical insights into the diverse responses of different tree species to changing environmental conditions over large spatial scales. It elucidates the complex interactions between soil moisture, climate variables, and canopy greenness. These findings contribute significantly to our understanding of the resilience of forest ecosystems to climate variability and provide invaluable guidance for informed forest management and conservation strategies.