Asynchronous Coupling Between China's Forest Canopy Structure and Belowground Biomass Carbon in the 21 st Century

ABSTRACT The spatio‐temporal relationship between Forest Canopy Structure (FCS) and Belowground Biomass Carbon (BGBC) is fundamental to determining how optimizing spatial structure can maximize forest carbon‐sink potential in land management. However, the lidar data resolution over the space and time exists limitation, causing the coupling relationship between FCS and BGBC to remain insufficiently understood. To explore the coupling mechanism between FCS and BGBC, we construct the Forest Canopy Structure–Function Index (FCSFI) based on the geographic data to assess the change trend and function of FCS in China during 2002–2019. We found that approximately 61.02% of China's forested areas experienced a steady increase in FCSFI. The FCSFI and BGBC displayed a synergistic upward trend, with an average annual growth rate of 0.47%. Evergreen broadleaf forests exhibited the highest annual growth rate (3.54%), whereas deciduous broadleaf forests showed the lowest (0.08%). The Coupling Coordination Degree (CCD) between BGBC and FCSFI increased from 0.47 to 0.51, with BGBC exhibiting a temporal lag effect. CCD hotspots were primarily concentrated in northeastern and southwestern China. The former showed stable enhancement, while the latter demonstrated increasing variability. This study strengthens the theoretical basis for coordinating FCS optimization with carbon‐sink enhancement and provides spatial‐structure‐oriented nature‐based solutions to improve forest carbon sequestration and forest sustainable development.