Effect of climate extremes on carbon fluxes, acclimation, and resilience of European forests

Forests are pivotal in global carbon and biogeochemical cycles, covering nearly 40% of Europe's land area and sequestering approximately 290 million tons of CO₂ equivalent annually (as of 2020). However, the strength of this carbon sink is declining, having dropped by roughly one-third between 2010 and 2020 (from 430 to 290 million tons CO₂ equivalent per year, according to the national inventories). This decline threatens the role of European forests in achieving EU climate mitigation targets.

The resilience of forest ecosystems and their ability to mitigate climate change depend on how they respond and acclimate to intensifying climate extremes and disturbances. CLEANFOREST COST Action (CA21138, https://cleanforest.eu/) unites almost 400 participants organized in 4 Working Groups (WG) to develop a comprehensive understanding of the combined impacts of climate extremes and atmospheric deposition on European forests. One of the key objectives of CLEANFOREST is to disentangle the interactions between global change drivers and the responses of tree- and soil-related biogeochemical processes. Specifically, within WG3, we examine the three primary carbon fluxes—gross primary productivity (GPP), ecosystem respiration (Reco), and their balance, net ecosystem productivity (NEP)—that determine the net forest carbon sink. While GPP reflects canopy-level photosynthesis, Reco includes heterotrophic respiration from soil decomposition and autotrophic respiration from vegetation and soil.

In this study, we review the effects of multiple global change drivers—such as droughts, heatwaves, nitrogen deposition, elevated atmospheric CO₂, and understudied extremes like winter warming—on carbon fluxes, acclimation, and resilience of European forests. We synthesize findings on the interacting roles of these drivers and propose a conceptual framework that links biotic and abiotic factors with forest conditions. This framework provides insights into how forests' carbon sink capacity responds to these drivers, offering a foundation for strategies to enhance their resilience and climate mitigation potential.