Carbon cycling and net ecosystem production at an early stage of secondary succession in an abandoned coppice forest

Secondary mixed forests are one of the dominant forest cover types in human-dominated temperate regions. However, our understanding of how secondary succession affects carbon cycling and carbon sequestration in these ecosystems is limited. We studied carbon cycling and net ecosystem production (NEP) over 4 years (2004-2008) in a cool-temperate deciduous forest at an early stage of secondary succession (18 years after clear-cutting). Net primary production of the 18-year-old forest in this study was 5.2 tC ha^-1year^-1, including below-ground coarse roots; this was partitioned into 2.5 tC ha^-1year^-1 biomass increment, 1.6 tC ha^-1year^-1 foliage litter, and 1.0 tC ha^-1year^-1 other woody detritus. The total amount of annual soil surface CO₂ efflux was 6.8 tC ha^-1year^-1, which included root respiration (1.9 tC ha^-1year^-1) and heterotrophic respiration (RH) from soils (4.9 tC ha^-1year^-1). The 18-year forest at this study site exhibited a great increase in biomass pool as a result of considerable total tree growth and low mortality of tree stems. In contrast, the soil organic matter (SOM) pool decreased markedly (-1.6 tC ha^-1year^-1), although further study of below-ground detritus production and RH of SOM decomposition is needed. This young 18-year forest was a weak carbon sink (0.9 tC ha^-1year^-1) at this stage of secondary succession. The NEP of this 18-year forest is likely to increase gradually because biomass increases with tree growth and with the improvement of the SOM pool through increasing litter and dead wood production with stand development.