International climate change agreements aim to stabilize atmospheric CO2 concentrations. However, current carbon accounting methods in the land sector have limitations, including not differentiating between forest ecosystems of varying quality and failing to account for historical carbon stock losses. This study addresses a key issue in accounting: the reference level for calculating past and predicting future forest carbon stock changes. Using an ecologically-based reference level, derived from a forest ecosystem's carbon-carrying capacity (CCC), is crucial for consistent assessment of past carbon loss, prediction of potential gains from management changes, and determination of foregone mitigation benefits. The study focuses on European primary forests as a case study, examining their CCC and the potential contribution to climate change mitigation by protecting them and restoring secondary forests.

Existing literature highlights inconsistencies in forest carbon accounting, particularly regarding the reference levels used. Studies have shown that current methods often underestimate carbon stocks, especially in primary forests with high biomass densities. The importance of considering all carbon stocks (above- and below-ground biomass, deadwood, soil carbon) and flows for accurate assessment has been emphasized. There's a need for a more ecologically-based approach that considers the natural potential of forest ecosystems to store carbon, rather than relying solely on historical management practices.

The study compiled tree inventory data from 7982 sites across 27 European countries, encompassing boreal, temperate, and subtropical Global Ecological Zones. Data sources included research studies, literature, and forest inventories (NFIs). Above- and below-ground biomass, dead biomass, and (where available) soil carbon data were aggregated by country and forest type. Allometric equations were used to convert tree dimensions to biomass. Global modelled biomass data (GlobBiomass and GeoCarbon) were used for comparison and to estimate soil carbon where field measurements were lacking. The CCC of primary forests was estimated, and compared with existing modelled data. The potential carbon stock gain from forest restoration was calculated by comparing current carbon stocks in secondary forests with the CCC estimated from primary forest data. The mitigation potential was assessed by evaluating avoided emissions from primary forest protection and additional removals through secondary forest restoration.

The study found that biomass carbon stock density in European primary forests was significantly higher than estimates from global modelled maps (1.6 times higher on average for primary forests and 2.3 times higher for all forests). Large trees (DBH ≥ 60 cm) accounted for 50% of the total biomass carbon stock. The carbon stock foregone by harvesting ranged from 12% to 52%, depending on the harvest diameter threshold. The estimated annual carbon gain potential from protecting and restoring forests to reach their CCC was 309 MtCO2e, exceeding the current forest sink in the EU and comparable to the Green Deal 2030 target for net removals. Variations in carbon stocks were observed across different Global Ecological Zones (GEZs) and forest types, with higher stocks generally found in temperate and subtropical zones, and in mountain systems.

The significant discrepancies between measured carbon stocks in primary forests and those estimated by global models highlight the need for improved carbon accounting methodologies. The substantial mitigation potential identified through primary forest protection and restoration underscores the critical role these ecosystems play in achieving climate targets. The study's findings suggest that current policies undervalue the mitigation benefits of primary forests and underestimate the carbon sequestration potential of secondary forest restoration. The results provide strong evidence supporting the implementation of conservation measures and active restoration strategies to leverage the full potential of forests for climate change mitigation.

This research demonstrates that carbon stocks in European primary forests have been significantly underestimated, emphasizing their crucial role in climate mitigation. Protecting existing primary forests and restoring degraded forests to reach their carbon carrying capacity offer a significant pathway to meet the European Green Deal's 2030 target. This requires robust conservation policies, effective governance, and active restoration efforts, highlighting the substantial potential of nature-based solutions for climate action.

The spatial distribution of the sampled primary forest sites was not entirely representative of all remaining primary forests or forest types across Europe. Some forest types, such as subtropical dry and mountain systems, were under-represented in the dataset. While efforts were made to use multiple data sources, uncertainties remain regarding the accuracy of allometric equations and global model estimates, particularly at high biomass densities. The estimated potential carbon stock gain for secondary forests is based on the assumption of a linear carbon accumulation rate towards the CCC which might not entirely reflect reality.