Carbon balance of afforested peatland in Scotland

Summary The annual net CO2 exchange over undisturbed deep peatland in Scotland was measured continuously for 22 months using eddy covariance. Annual CO2 exchanges over peatlands that had been drained, ploughed and afforested with conifers 1, 2, 3, 4, 8, 9 and 26 years previously were estimated by extrapolating two to four weekly measurements, using relationships between daytime fluxes and solar radiation and night-time fluxes and air temperature. The contribution of the trees to the overall net CO2 flux was estimated using a carbon accounting model, calibrated to fit conifer volume yield data. The carbon exchange of the peat and ground vegetation was then the difference between the overall carbon flux and the amount accumulated in trees and tree litter. The undisturbed peat accumulated ~0.25 t C ha ‐1 a ‐1 . Newly drained peatland (2‐4 years after ploughing) emitted between 2 and 4 t C ha ‐1 a ‐1 , but when ground vegetation recolonized, the peatland became a sink again, absorbing ~3 t C ha ‐1 a ‐1 4‐8 years after tree planting. Thereafter, the trees dominated the budget and afforested peatlands absorbed up to 5 t C ha ‐1 a ‐1 . Assuming that the trees accumulated carbon at rates commensurate with yield class 10 m3 ha‐1 a‐1, the peat beneath the trees after canopy closure was estimated to be decomposing at only ~1 t C ha‐1 a‐1 or less. This is slower than previously thought and suggested that afforested peatlands in Scotland accumulate more carbon in trees, litter, forest soil and products than is lost from the peat for 90‐190 years.