Forest responses to global change: reconciling evidence from tree to continental scales in Europe

Information on forest responses to global change drivers remains fragmented across scientific disciplines, spatial scales, and methodological approaches, limiting our ability to understand and predict forest vulnerability and resilience. Reconciling ground-based physiological observations with landscape- to continental-scale evidence from modelling and remote sensing is essential to advance our understanding and inform effective policy for forest management.

Here, we synthesise current knowledge on how European forest condition has changed over recent decades by integrating evidence across spatial scales and methodological frameworks. We focus on three key metrics: tree growth, water-use efficiency (WUE), and tree mortality, which together link physiological processes to ecosystem-level responses and forest health. These metrics were assessed in relation to major global change drivers, including climate change (drought and climate extremes) and atmospheric pollution (rising CO₂ concentrations and changing nitrogen and sulphur deposition).

As part of Working Group 2 of the COST Action CA21138 CLEANFOREST, we conducted a systematic review of trends in European forest growth, WUE, and mortality reported since 1990. We extracted the direction of reported trends (positive, negative, or neutral) from more than 500 peer-reviewed studies, spanning dendrochronology, ecosystem flux measurements, forest inventories, modelling, and remote sensing, alongside detailed information on forest type, climate zone, tree species/genus, and site characteristics. The resulting database comprises over 1,300 observations across Europe.

Our synthesis reveals pronounced spatial, temporal, and ecological asymmetries in the European forest evidence base. Studies are heavily concentrated in Central Europe, with substantial gaps in Mediterranean, Eastern European, and high-latitude regions. Reported trends indicate predominantly negative growth and increased mortality in southern Europe, in contrast to more neutral or positive signals in central and northern regions. Across scales, tree-level observations often suggest physiological compensation (e.g. increasing WUE), whereas landscape-scale assessments more frequently reveal stagnating growth and intensified mortality, highlighting a mismatch between local adjustments and ecosystem-level responses.

By reconciling evidence across scales, this systematic review draws a comprehensive picture of the complex response of European forests to the changing climate, identifies key knowledge gaps, biogeographic biases, and opportunities to better integrate long-term monitoring networks with emerging approaches. Such integration is essential for understanding mechanisms underpinning forest responses to climate extremes and atmospheric deposition, and for improving projections of future forest functioning under global change conditions.