Extraction of the spatial structure of Chinese fir plantations stands based on unmanned aerial vehicle and its effect on AGB

As the most dominant fast-growing timber species in China, exploring the response of aboveground biomass to stand spatial structure parameters in Chinese fir (Cunninghamia lanceolate) is of great significance for improving forest quality and carbon sequestration capacity. In this study, we used unmanned aerial vehicle (UAV) point cloud data to extract forest parameters and constructed weighted Voronoi diagrams to determine the spatial structure units of the forest to realize rapid acquisition of the spatial structure parameters of the forest stand. Combined with the biomass model constructed using 20 harvested logs, we used stepwise regression to explore the key factors of the spatial structure of Chinese fir that affect biomass accumulation at different age stages. The results showed that: (1) the angle competition index should be kept in the range of (0,0.25] for the biomass of the Chinese fir to be maximized at the young, near-mature, mature, and overmature forest stages; (2) the openness ratio affects the accumulation of the Chinese fir biomass, and its optimal range changes continuously during the growth process from (0.25,0.5]- (0.75,1]-(0.5,0.75] and subsequently has been at (0.5,0.75]; (3) in the mature and overripe forest stages, the forest layer index further affects biomass accumulation, and its optimal range varies from (0.25,0.5] to (0.5,0.75]; (4) in the five Chinese fir growth stages, biomass accumulation was more favorable when the number of nearest trees change was 7, 6, 6, 6, and 5; (5) adopting reasonable measures to regulate the spatial structure of forest stands at different age stages can effectively enhance the biomass of Chinese fir. At the young, middle-aged, and mature forest stages, ensuring the optimal number of neighboring trees through reasonable dense planting and nursery thinning is essential; at the near-mature and mature forest stages, enhancing the use of vertical space of the forest trees by trimming the branches and trunks is crucial.