Afforestation turns cutaway peatland into a carbon sink

Rates of peat extraction have rapidly declined in Finland, leaving thousands of hectares of formerly cutaway peatlands in need of management. Unlike in some other regions, in Finland there is no obligation to restore former peat extraction sites to wetlands. Afforestation is the most popular after use option of cutaway peatlands by landholders in Finland, however it is unclear how the ecosystem will respond to afforestation and whether the system can become a carbon sink quickly. In this study, we present a 3-year record of eddy covariance measurements from an afforested cutaway peatland site in Finland. We examined the carbon dioxide (CO₂)exchange dynamics of the site as it was afforested and calculated annual totals to determine whether it is a carbon source or sink.

The study site, Naarasneva, is in Southern Ostrobothnia, Finland. Peat extraction ceased in 2020 and there is an average of 1 m depth of peat remaining. The eddy covariance system was installed in August 2021. Wood ash fertilisation was applied in January 2022, followed by the planting of 2-year old Pinus sylvestris (Scots pine) saplings in June 2022. Sentinel-2 derived leaf area index (LAI) observations were used to investigate the revegetation of the site during afforestation.

The timeseries of NEE shows the exchange (uptake and emission) of CO₂ increasing over time. There was a clear effect of fertilisation, with a steady increase in the amount of CO₂ uptake in the months following fertilisation. After fertilisation, the most dominant vegetation species growing were Calamagrostis spp. (reedgrass), Epilobium spp. (willow herb) and Betula pubescens (downy birch). The increase in CO₂ uptake corresponded well with the LAI observations, which also showed a year on year increase. Annual totals of NEE show the site was a net source of 5.30 ± 0.46 t CO₂ ha^-1 yr^-1 (mean ± 95% CI) in 2022, followed by two years where it was a net sink of -1.36 ± 0.42 t CO₂ ha^-1yr^-1 in 2023 and -0.75 ± 0.56 t CO₂ ha^-1 yr^-1 in 2024.

Our results show that afforestation of a cutaway peatland can quickly turn the site into a carbon sink. While it is positive that the carbon sink functionality of former peat extraction sites may be restored quickly, the long-term climate impact of afforestation is unclear due to the continued drainage of the peat and the uncertain fate of the carbon stored in wood.