Tree-neighborhood scale structure–radiation coupling analysis and modeling using multi-source remote sensing and ensemble learning

Forest structure regulates the spatial heterogeneity of absorbed photosynthetically active radiation (APAR), thereby shaping ecosystem functioning and productivity. However, how neighborhood spatial configuration and competition dynamics control APAR distribution remains poorly understood, particularly in structurally heterogeneous and topographically complex natural forests. To address this, we proposed FASAR (Forest Architecture–Solar Absorption Response), a structure–radiation coupling framework for complex-terrain natural forests. FASAR establishes an integrated workflow for structure–radiation coupling analysis by integrating multi-source LiDAR-based structural parameterization, Adaptive Leaf Synthesis Algorithm (ALSA)-assisted 3D forest reconstruction, 3D radiative transfer simulation, and structure-stratified ensemble modeling. It converts heterogeneous forest architecture into plot-, neighborhood-, and tree-level descriptors and links these descriptors with spatially explicit APAR patterns. Applied to 144 plots (5.76 ha, 3,417 trees) in a natural Qinghai spruce forest in western China, FASAR revealed that large trees absorb more light with vertically stable patterns, while canopy stratification, competition, and size disparity intensify vertical displacement of light and suppress central-tree absorption. Structural dominance and near-natural spatial configurations enhanced APAR interception (ρ = 0.08–0.49, p < 0.001), while moderate aggregation and openness improved light scattering (ρ = 0.06–0.19, p < 0.001). Slope (ρ = –0.37, p < 0.001) and gap density (ρ = –0.18, p < 0.05) showed dual facilitation–inhibition effects on mean APAR. Structure-specific Bagging models achieved moderate performance (R 2 = 0.457–0.546). Variable importance analysis identified crown area (CA) as the dominant driver across all structural types (contribution: 68.95–81.78 %), followed by tree height, spatial density, and other structural heterogeneity indices. This framework provides an operational pathway for quantifying how neighborhood-scale forest architecture regulates individual-tree light absorption under complex-terrain conditions. The structure–radiation mechanisms revealed in this study may also provide physically based constraints for APAR/fAPAR remote sensing retrieval and related research. FASAR has the potential to be extended across forest types and successional stages, with its performance further evaluated in future studies.