Net ecosystem carbon losses are driven by high ecosystem respiration in a temperate drained forested peatland

Approximately 334,000 ha of forest in Ireland are established on peat soils due to historical afforestation and are estimated to contribute around one quarter of national peatland greenhouse gas emissions. Drainage required for tree establishment lowers the water table, accelerating peat oxidation and soil organic matter decomposition. The net carbon (C) balance of these systems depends on the trade-off between C loss through peat decomposition and C accumulation via tree growth and litter inputs. However, whole-ecosystem carbon exchange measurements from Irish forested peatlands remain limited. We quantified net ecosystem carbon exchange (NEE) using the eddy covariance method over a drained peatland planted with Picea abies L. and Betula pendula Roth over a three-year period (2023–2025). The site was a substantial net carbon source, with a mean annual NEE of 3.71 ± 0.56 t C ha⁻¹ yr⁻¹. A temperature sensitivity analysis using semi-empirical models showed that warming relative to the 1993–2022 mean increased annual NEE by 1.27, 0.88, and 1.06 t C ha⁻¹ yr⁻¹ in 2023, 2024, and 2025, respectively, corresponding to increases in mean annual temperature of 1.1, 0.7, and 1.0 °C. Projections of ecosystem carbon fluxes using semi empirical models predict a more than two-fold increase in annual NEE by the end of century, under the SSP5-8.5 scenario (2.70 ± 2.28 t C ha⁻¹ yr⁻¹), although high uncertainty remains due to variability in CO2 fertilisation effects. These findings indicate that tree growth at this site is insufficient to offset the increased ecosystem respiration associated with peat oxidation from drainage and elevated carbon losses due to high temperatures, highlighting the need to evaluate alternative land-use strategies including forest to bog restoration.