A Terrestrial Ecosystem Carbon Sink Assessment Model Considering Forest Age Dynamics (CEVSA-AgeD)

The large variation in net ecosystem productivity (NEP) with forest age was dominated by the dynamics of net primary productivity (NPP)-which in turn was determined by the different response slopes of gross primary productivity (GPP) and autotrophic respiration (Ra) with forest age. However, only few models can comprehensively represent the impacts of forest age and global changes including land-use change, climate change, nitrogen deposition, and atmospheric CO2 from the perspective of ecological processes. Based on a process-based model (CEVSA-ES) that included these global changes, we developed an ecosystem carbon sink assessment model considering forest age dynamics (CEVSA-AgeD) using satellite-based relationships between GPP (or Ra) and forest age to constrain photosynthesis and autotrophic respiration processes. Subsequently, we used a model data-fusion framework combined with carbon flux observations to calibrate the model. The calibrated CEVSA-AgeD model performed well in simulating seasonal (R2 values for GPP, ecosystem respiration, and NEP were 0.86, 0.79, and 0.66, respectively) and annual carbon flux changes (R2 of GPP, ecosystem respiration, and NEP were 0.83, 0.77, and 0.67, respectively). The magnitude of average NEP in China estimated using this model was 0.35 +/- 0.005 TgC/yr from 2001 to 2021, which was close to previous estimates, and the dynamics of forests age increased NEP by 87-92 TgC/yr. These results indicate that the CEVSA-AgeD model performed well in simulating carbon fluxes at the site and regional scales and that it was necessary to incorporate the effect of forest age dynamics on carbon cycling processes into process-based models.