Biomass and carbon stock decline in floodplain forests of the central amazon: evidence from long-term monitoring in the Amanã Reserve

Tropical forests contain approximately 60% of the biosphere’s carbon stocks, making their biomass a crucial buffer against anthropogenic carbon emissions. This study assessed biomass and carbon stocks in two vegetation types within the Amanã Sustainable Development Reserve, Central Amazon. Four permanent 1-ha plots were established, with two plots in each environment: Igapó and Paleovárzea. In 2014, all trees and palms with diameter ≥ 10 cm were measured, including diameter, height, species identification, and wood density. Aboveground biomass was estimated using allometric equations based on these parameters. The plots were re-inventoried in 2023 to assess biomass stocks and forest dynamics over time. Aboveground biomass (215 Mg ha −1 ) and carbon stocks (101 Mg C ha −1 ) in Paleovárzea forests did not differ significantly from those in Igapó forests (177 Mg ha −1 and 101 Mg C ha −1 , respectively). However, both forest types experienced substantial biomass losses, with mortality rates (7.62 and 6.09 Mg ha −1 yr −1 ) exceeding stem net primary productivity (3.65 and 1.37 Mg ha −1 yr −1 ) in Paleovárzea and Igapó, respectively. Consequently, aboveground biomass declined by an average of 36 Mg ha −1 over nine years. The observed imbalance between biomass losses and gains may be associated with increasing climate variability and the greater occurrence of extreme drought events in the Amazon, which can increase tree mortality and constrain biomass recovery. These findings indicate that, despite similar biomass and carbon stocks between forest types, both experienced a decline in biomass over time, highlighting the vulnerability of floodplain forests to increasing climatic stress.