Effects of canopy structure and topography on seedling species diversity in an evergreen and deciduous broad-leaved mixed forest

Forest canopy structure and topography are the most obvious characteristics of forest ecosystems and directly affect forest regeneration, seedling survival, and diversity. To reveal the mechanisms of canopy structure, topography, and spatial variables on understory species diversity and functional diversity, we performed semi-spherical photography and extracted indices of canopy structure in 1.2 hm² seedling monitoring plot in the 25 hm² Badagongshan forest plot, then investigated species composition and abundance of seedling communities in each 20 m×20 m quadrat in the seedling monitoring plot. We also sampled leaves of common species close to the reserve and measured leaf functional traits. We applied the spatial simultaneous autoregressive error model (SARs) to test the effects of canopy structure, topography, and spatial factors on seedling species diversity and functional diversity. Results showed that: (1) canopy structure of adult tree communities in the 1.2 hm² plot was relatively complex and the mean maximum canopy height, mean leaf area index (LAI), mean leaf angle (MLA), and Gndcover were 19.94 m, 2.94, 30.88°, and 0.87, respectively; (2) Canopy structure and topographical variables explained 32.6% - 48.4% and 28.5% - 70.2% of variation in the species and functional diversity indices, respectively, but not all topographical variables were entered in the best-fitting model of species diversity indices; (3) Seedling species diversity was higher in habitats with a high altitude and slope and where canopy structure had a lower LAI and MLA, whereas, seedling functional diversity was higher in habitats with a low altitude and slope and where canopy structure had a lower LAI and MLA. These results provide suggestions and guidance for forest regeneration evaluation and vegetation inventory.