Europe's forests under multiple pressure
Forests are indispensable for climate protection. They play a central role in the global carbon budget: forests in the European Union cover around 40 per cent of its land area and absorb approximately 280 million tonnes of carbon dioxide (CO₂) each year. However, this capacity is declining. While the carbon stock in forests continues to grow, their uptake capacity decreased by almost a third between 2010 and 2020, falling from 466 to 295 million tonnes of CO₂ equivalents per year. Consequently, EU forests are at risk of failing to fulfil their role as a key carbon sink for European climate targets. An international research team led by Prof Mana Gharun of the Institute of Landscape Ecology at the University of Münster conducted a comprehensive review study to investigate what exactly lies behind this decline. The central finding of the study is that when multiple drivers act simultaneously, ecosystem responses frequently become unpredictable.
When considered individually, forests often show relatively predictable responses to global change factors: warming increases both photosynthesis and ecosystem respiration; elevated atmospheric CO₂ promotes photosynthesis; and moderate nitrogen deposition can boost productivity in nitrogen-limited systems. In reality however these factors never happen in isolation.
“Forest dynamics are shaped by the interplay of many global drivers. Assessments that consider only a single factor are insufficient for predicting how ecosystems will respond,” explains Mana Gharun. Multiple drivers can amplify, dampen or even reverse one another's effects. Warming alone, for instance, can extend the growing season and enhance photosynthesis. However, when combined with drought, trees increasingly close their stomata, which are microscopic pores on their leaves through which they absorb CO₂. This reduces their carbon uptake. Similarly, the positive impact of rising CO₂ concentrations on productivity can be offset by nutrient limitations or drought stress.
The study also sheds light on extreme events that have not yet been the focus of much research. These include winter warming, excessive precipitation and late frosts, all of which are expected to become more frequent in the future. During the exceptionally warm winter of 2020, for example, elevated air and soil temperatures reduced net carbon uptake in numerous forests across Europe. Furthermore, model projections indicate that more than a third of European forest areas are at an increased risk of frost damage.
The resilience of net carbon uptake has declined across large parts of temperate Europe over the past two decades. “This suggests that forests are becoming increasingly vulnerable to environmental stressors,” says Mana Gharun. Climate extremes can impair carbon uptake for several seasons, or even years. The long-term effects can be as severe as the immediate impacts of the drought itself.
The authors emphasise that the best opportunity to understand the complex effects of multiple global change drivers on forest functioning and to make more realistic predictions for the future lies in long-term observations under field conditions combined with data science and statistical approaches.
Background to the review study
The study, a perspective literature review, was conducted as part of the European COST Action CLEANFOREST, which brought together 34 researchers from 30 institutes across 17 countries. The team summarises the current state of knowledge on the impact of global change factors, including elevated CO₂, droughts, heatwaves, nitrogen deposition, and previously understudied extreme events, on carbon fluxes (CO₂ and CH₄) and forest resilience. The study also identifies key knowledge gaps and discusses priorities for future research and monitoring.
Original publication
Gharun, M. et al. (2026): Perspectives: Effect of global change drivers on carbon fluxes and resilience of European forests. Forest Ecology and Management 616, 123844. DOI: 10.1016/j.foreco.2026.123844