Ecosystems carbon budgets of differently aged downy birch stands growing on well-drained peatlands

Estimation of the carbon (C) storages and fluxes in different forest ecosystems is essential for understanding their C sequestration ability. The net ecosystem production (NEP) and the net primary production (NPP) in five downy birch (Betula pubescens) stands, aged between 12 and 78 years, growing on fertile well-drained Histosols, were studied. Drainage of swamp forests is a large-scale manipulation, which causes significant shifts at the ecosystems level, altering C and nutrient cycling a great deal. Young and middle-aged downy birch stands (12-30-year-old) acted as C sink ecosystems, accumulating 1.4-3.0 t C ha−1 yr−1. In the 38-year-old stand NEP was roughly zero; annual C budget was almost in balance. The over-matured downy birch stand (78-year-old) acted as a C source emitting 0.95 t C ha−1 yr−1. Annual woody biomass increment of the stand was the main factor which affected the forest to act as a C accumulating system. Although the highest heterotrophic respiration (Rh) values were measured in the middle-aged stands, mean soil C emission did not differ significantly between the studied stands. Annual total soil respiration (Rs) and Rh ranged from 7.4 to 8.8 t C ha−1 and 4.7 to 6.2 t C ha−1, respectively. Soil temperature appeared to be the dominant driver of the soil CO2 effluxes. Temperature sensitivity (Q10 value) of respiration rates (3.0-5.5), as well as the Rh/Rs (0.6-0.7) varied irrespective of stand age. Both the annual aboveground litter (1.5-1.9 t C ha−1 yr−1) and fine root litter (0.9-1.5 t C ha−1 yr−1) input fluxes were quite similar for the studied stands. However, the annual organic C input into the soil via above- and belowground litter was smaller than the annual Rh efflux, indicating that continuous mineralization of the peat layer reduces the soil organic C pool. The main share of the C stock in the drained swamp downy birch stands was soil C; the storage of C accumulated in the woody biomass of the trees accounted for only 5-20% of the total C storage of the ecosystem.