Integrating LUCC and forest aging to project and attribute subtropical forest NEP in Zhejiang Province under four SSP-RCP scenarios

Net ecosystem productivity (NEP) serves as a key indicator of the ecosystem carbon balance. However, the combined effects of various drivers, particularly land use/cover change (LUCC) and forest aging, on NEP remain uncertain, leading to uncertainties in regional and global future NEP simulations. This study integrated Future Land Uses Simulation (FLUS), System Dynamic (SD), and optimized Integrated Terrestrial Ecosystem Carbonbudget (InTEC) models to account for future LUCC and its induced changes in forest age structure into future forest NEP simulations. Taking the Zhejiang Province as the study area, we applied four SSP-RCP scenarios (i.e., SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) to simulate its subtropical forest NEP from 1980 to 2100. Our simulations indicate that the forests existing in 2020 will function as a carbon sink from 2020 to 2060 but will transition to a carbon source from 2060 to 2100, primarily due to the gradual aging of existing forests and the combined influences of climate and CO2 changes. Nonetheless, after considering LUCC such as afforestation, the overall cumulative NEP will continue to increase after 2060. By 2100, cumulative forest carbon sinks from 2020 will reach 631.74 Tg C under SSP1-2.6, 681.75 Tg C under SSP2-4.5, 586.41 Tg C under SSP3-7.0, and 601.28 Tg C under SSP5-8.5. Among these contributions, aging forests existing in 2020 with climate and CO2 changes account for 27.04 % to 63.30 % of cumulative NEP. Climate change exerts a negative impact ranging from -47.39 % to -14.39 %, while CO2 fertilization has a positive contribution of 6.31 % to 73.79 %. Regarding LUCC, afforestation/reforestation contributes significantly, accounting for 43.66 % to 53.65 %, whereas deforestation has a negative impact of -22.77 % to -10.49 %. Additionally, continuous regeneration further supports NEP growth, contributing 12.85 % to 34.77 %. Finally, Partial Least Squares Structural Equation Modeling (PLS-SEM) was used to elucidate the interactions between these factors. The analysis revealed that future LUCC has significant positive impacts on forest NEP whereas forest aging has significant negative impacts. These findings are crucial for understanding the future carbon cycle of subtropical forests and informing adaptation strategies in response to global climate change.