Objective Long-term monoculture management of Chinese fir (Cunninghamia lanceolata) plantations has led to declines in soil fertility and stand productivity. Mixed-species conversion is a key strategy for improving stand quality, yet the quantitative mechanisms by which mixing patterns influence canopy light interception capacity and spatial heterogeneity remain unclear. This study employed the three-dimensional (3D) radiative transfer model LESS (LargE-Scale remote sensing data and image Simulation framework), which accurately simulates light radiation transfer within forest canopies, to elucidate the 3D light distribution patterns in mixed forests and provide a scientific basis for optimizing light environments in Chinese fir plantations.

Method Using the LESS model, we constructed 3D scenes of Chinese fir-Schima superba mixed forests under four mixing patterns (individual-tree mixing, row-strip mixing, block mixing, and irregular mixing) and five mixing ratios (Chinese fir proportions ranging from 50% to 90%), with pure Chinese fir stands as the control. We quantitatively simulated and analyzed overall canopy light interception characteristics, as well as horizontal and vertical spatial light distribution patterns, to investigate the 3D light distribution dynamics in these mixed stands.

Result (1) Both mixing ratio and mixing pattern jointly influenced canopy light interception. The mean of overall FPAR (Fraction of absorbed Photosynthetically Active Radiation) decreased from 0.748 to 0.624 (a 16.6% reduction) as the Chinese fir proportion increased from 50% to 90%. Among the patterns, individual-tree mixing exhibited relatively higher overall light interception capacity. (2) Light competition between Chinese fir and Schima superba was markedly asymmetric. Chinese fir showed significant light acquisition advantages in row-strip and block mixing patterns, whereas Schima superba maintained higher FPAR levels more effectively under individual-tree and irregular mixing patterns. (3) Light distribution exhibited pronounced spatial heterogeneity. Horizontally, light environment heterogeneity peaked at 50% Chinese fir proportion and gradually became more uniform as the proportion increased. Vertically, FPAR maxima primarily occurred in the main canopy layer at 12−14 m; among mixed scenarios, the most uniform vertical light distribution was observed at 80% Chinese fir proportion.

Conclusion This study quantitatively deciphered the 3D light distribution patterns in Chinese fir-Schima superba mixed forests, demonstrating that mixing ratio and pattern jointly regulate canopy light interception and spatial heterogeneity. These findings provide theoretical foundations and technical support for optimizing spatial configurations in light-environment-oriented mixed-species conversion and enabling precision enhancement of forest quality in Chinese fir plantations.