The spatial structure of stands is one of the important factors affecting the species diversity of saplings. Studying the influence of stand spatial structure on the species diversity of undergrowth saplings can provide a theoretical basis for improving the stability of ecosystem structure and reasonable forest management.

In this study, 49 typical 20 m×30 m plots of oak natural secondary forest were set up and investigated in Qingyanghu forest farm in Ningxiang County, Wugaishan forest farm in Chenzhou City, Lutou forest farm in Pingjiang County, Longhushan forest farm in Yiyang City and Badagongshan nature reserve in Sangzhi County. The stand spatial structure was described by the mingling index, crown index, opening degree index and aggregation index, and the species diversity of saplings was described by the Margalef index, Auclair & Goff index and Pielou's index. Pearson's correlation analysis, step-wise multiple linear regression models and canonical correlation analysis (CCA) were used to explore the influence of spatial structure of tree layers on the species diversity of saplings under the forest.

1) Pearson's correlation analysis showed that the influence of mingling degree on the Margalef's evenness index was extremely significant (P < 0.01). The aggregation index had a significant effect(P < 0.05), and mingling degree and the crown index had an extremely significant effect (P < 0.01) on the Pielou's richness index. The crown index had a significant influence (P < 0.05), and the mingling degree had an extremely significant influence (P < 0.01) on the Auclair & Goff's diversity index; 2) Step-wise multiple linear regression models showed that the dominant factors affecting the three species diversity indices were the mingling degree index and crown index; 3) The results of canonical correlation analysis showed that the number of canonical correlation variables in the first group was 0.647 1 (P < 0.01), indicating that the overall correlation between stand spatial structure and species diversity of saplings was extremely significant. The results of canonical load analysis indicated that mingling degree and the crown index were the key factors for the species diversity of saplings. 4) According to the analysis results of the three statistical methods above, mingling degree and the crown index were the main driving factors affecting the species diversity of saplings in the natural secondary oak forests in Hunan.

The mingling index and crown index are the key stand spatial structure factors affecting the species diversity of saplings in the natural secondary oak forest in Hunan. Therefore, the mingling index and crown index of stand structure can be changed by regulating the mingling degree and canopy structure of tree species, so as to promote their natural regeneration and achieve the purpose of protecting and enriching the species diversity of saplings.