Carbon stock and stock changes across a Sitka spruce chronosequence on surface-water gley soils

We assessed age-related alterations in carbon (C) stocks and sequestration rates of first rotation Sitka spruce (Picea sitchensis (Bong.) Carr) plantations on predominantly surface-water gley soils. Sites were selected to represent a typical Sitka spruce chronosequence following land use transition from grasslands dominated by surface-water gley soils. Based on inventory, eddy covariance, physiological and modelling assessments of net ecosystem productivity (NEP), we show that afforested stands are a C sink at 10 years, and possibly earlier, followed by an increase to a maximum of 9 t C ha−1 year−1 before the first thinning cycle. NEP subsequently declined from 9 t C ha−1 year−1, at closed canopy, to 2 t C ha−1 year−1 in older and thinned stands. Reductions in the C sequestration rate of older stands were coupled with a decrease in gross primary productivity, increases in maintenance/growth respiration and decomposition losses following harvest. We suggest that the high sequestration potential of these forests may be associated with the high net primary productivity of these plantations in Ireland, a high allocation of assimilates and litter into the belowground C pool and accumulation of C in mineral gley soils following afforestation.