Trouble in the air: How European forests keep their cool under atmospheric drought

The 21st century has witnessed a steady rise in the frequency and intensity of heatwaves, creating unprecedented challenges for ecosystems. Europe, in particular, has experienced its hottest heatwaves in the past two decades, increasingly coupled with elevated vapor pressure deficit (VPD)—the atmospheric demand for water—resulting in a new form of “atmospheric droughts.” These droughts disrupt plant water relations, diminish carbon sink capacity, and trigger large-scale tree mortality. Our overarching goal is to unravel how European forests respond to rising VPD and temperature, their extremes, and compounding soil drought effects. COOL integrates innovative methodologies, including state-of-the-art laboratory experiments, the first-ever VPD manipulation in a natural forest, a pan-European monitoring network, and advancements in mechanistic modeling. We will (1) systematically assess what drives trees’ thermal and VPD tolerance, an area previously unexplored but crucial for predicting forest resilience; (2) disentangle, for the first time, the impacts of temperature, VPD, and soil moisture in natural settings—an achievement made possible by novel methodological developments; (3) investigate the role of long-term acclimation to temperature and VPD under varying soil moisture scenarios, leveraging unprecedented experimental infrastructure; and (4) quantify the consequences for forest growth, water use, and tree mortality. By combining cutting-edge approaches to collect high-resolution hydraulic and thermal data, COOL overcomes a critical bottleneck in forest research. The project will deliver groundbreaking insights into forest resilience under future climates. These results will advance climate-smart forest management, inform policy frameworks, equip stakeholders with actionable tools, and foster education in global change ecology. COOL will establish new paradigms for predicting and mitigating the long-term consequences of atmospheric droughts on forest ecosystems.