Short-Term Effects of Thinning on Stand Carbon Density and Sediment Carbon Burial Indicators in Kandelia obovata Sheue & al. Plantation

To explore the patterns of biomass accumulation and sediment carbon burial indicators in mangrove forests under different thinning intensities, a study was conducted on an 8-year-old Kandelia obovata Sheue & al. plantation on Shupaisha Island, Longwan District, Wenzhou City, Zhejiang Province. Three treatments were designed: no thinning (CK), 20% thinning, and 40% thinning. Stand growth and plant carbon density were evaluated for all three treatments at the initial thinning stage and two years later. Sediment carbon density and organic carbon burial rate were assessed only for CK and 20% thinning. Thinning significantly enhanced mangrove growth and plant carbon storage. Compared with unthinned stands, 20% and 40% thinning treatments significantly increased branch diameter and biomass (p < 0.05). The order of mangrove height was 20% thinning > 40% thinning > CK. The plant carbon densities in the 20% and 40% thinned stands were 16.31 Mg C·ha−1 and 15.30 Mg C·ha−1, respectively, far exceeding that of the control (4.80 Mg C·ha−1). In contrast, sediment carbon responses were negative in the short term. After thinning, the sedimentation rate and organic carbon content in mangrove sediments decreased. Sediment carbon density decreased from 88.10 Mg C·ha−1 in unthinned stands to 85.02 Mg C·ha−1 under 20% thinning, accompanied by a reduction in carbon burial rate. Overall, these two-year results indicate increased plant carbon storage under thinning, whereas measured sediment carbon indicators under moderate thinning declined over the same period. Longer-term monitoring is needed to assess whether these short-term responses translate into net ecosystem carbon consequences.