Despite the rates of carbon stored continuing to increase, the ability of European forests to absorb carbon has fallen significantly.
European trees are slowing down the amount of carbon dioxide that they are absorbing from the atmosphere by 170 million tonnes a year between 2010 and 2020, a new study has found.
The research, led by the University of Birmingham and the University of Münster, has provided further insights into growing pressures faced by European forests.
The paper, published in Forest Ecology and Management, reveals that while forests continue to sequester a growing amount of carbon dioxide from the atmosphere, their uptake capacity decreased by almost a third between 2010 and 2020.
During that time, the amount of new carbon dioxide taken out of the air and into trees fell from 466 to 295 million tonnes of CO₂ equivalents per year.
Professor Sami Ullah, from the University of Birmingham and a co-author on the study, said: “Our study highlights the importance of collaboration across Europe to better understand the evolving pressures that our forests are facing. Strengthening links between existing forest research manipulation and monitoring facilities will enable more accurate predictions of how forests respond to global change, helping to inform future forest protection policies.”
Strengthening links between existing forest research manipulation and monitoring facilities will enable more accurate predictions of how forests respond to global change, helping to inform future forest protection policies.
European forests play a central role in meeting the EU’s climate targets, covering around 40% of its land area. However, the declining carbon sink has left them trailing behind climate goals.
Forests often show relatively predictable responses to global change factors. Hotter temperatures increase both photosynthesis and ecosystem respiration, elevated atmospheric CO₂ promotes photosynthesis and carbon sequestration and moderate nitrogen deposition can boost productivity in nitrogen-limited systems.
Hotter temperatures alone can extend the growing season and enhance carbon capture via photosynthesis. However, when combined with drought, trees increasingly close their stomata, which are microscopic pores on their leaves through which they absorb CO₂; reducing their overall carbon uptake. Similarly, the positive impact of rising CO₂ concentrations on productivity can be offset by nutrient limitations, drought and heat stresses.
To investigate which key factors are driving this decline, the international research team conducted a comprehensive review study – with their findings suggesting that while individual drivers can have predictable effects, their combined impacts are far more complex, often amplifying or even reversing expected outcomes.
Professor Mana Gharun, from the University of Münster and the lead author of the study, said: “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. Our study’s findings show that multiple drivers can amplify, dampen or even reverse one another's effects.”
The study also sheds light on extreme events that have not yet been the focus of much research such as winter warming, excessive precipitation and late frosts. These events are expected to become increasingly frequent in the future. Current model projections indicate that forests are increasingly vulnerable to extreme environmental stressors like this – for example, more than a third of European forest areas are at an increased risk of frost damage.
To improve the accuracy of predictions for the future, long-term climate strategies must move beyond single factor approaches and look at the combined effects of climate, pollution, biodiversity, and management.
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Professor of Biogeochemistry
Dr Sami Ullah's research is mainly focused on the biogeochemistry of nitrogen and its linkages to carbon and phosphorus cycling under global change in soils under crops
