Forest Carbon Exchange Dynamics and Integrated Carbon Budget of a Subtropical Island Ecosystem

Abstract

Island ecosystems are on the frontline of climate change, yet their terrestrial carbon exchange remains poorly quantified because direct ecosystem‐scale observations are rare. Based on year‐round (2024) eddy covariance measurements of net ecosystem exchange (NEE) in the forest ecosystem of a representative island (Dachen Island) in the East China Sea, we analyzed the seasonal dynamics of carbon fluxes and identified their main drivers. The annual forest carbon sink was further constrained using multiple complementary approaches and integrated into an island‐scale carbon budget assessment. The island forest showed pronounced seasonal variation in NEE, with higher net carbon uptake during the wet season than in the dry season. Within both seasons, shortwave downward radiation was the dominant driver of temporal variation in NEE. The island forest functioned as a carbon sink, with estimated net carbon uptake rates ranging from 55.6 to 114.4 gC m −2  yr −1 across different approaches, while the age‐biomass curve approach yielded a modeled upper bound of 262.6 ± 57.5 gC m −2  yr −1 for potential biomass accumulation. However, the total carbon sinks from forests and fishery‐related blue carbon (2564.1–4060.0 t CO 2  yr −1 ) were generally insufficient to offset total anthropogenic emissions (4363.8–5450.8 t CO 2  yr −1 ) on Dachen Island. Our findings improve direct observational evidence that island forests can function as meaningful carbon sinks, but also show that they are unlikely to compensate for local emissions without substantial emission reductions. This study offers a framework for assessing carbon budgets on small islands, thereby informing integrated carbon dioxide removal strategies toward carbon neutrality.