Objective In natural secondary forests, large trees play an important role in shaping the species diversity pattern of understory saplies. In order to provide a scientific basis for promoting saplings regeneration, this study explored the mechanism of the effect of large trees on species diversity of understory saplings and revealed the internal mechanism of natural secondary forest regeneration.

Method This study took the natural secondary forests of Quercus in Hunan Province as the research object. 45 fixed plots of 600 square meters were set up in the typical distribution areas of Q. communities in Hunan Province (Lu Tou in Pingjiang, Badagong Mountain in Sangzhi, Wugai Mountain in Chenzhou, and Qingyang Lake in Ningxiang). The DBH, tree height, crown width, biomass and tree number density were selected as the growth indicators of large trees. Margalef index, Pielou evenness index and Audair & Goff diversity index were used to characterize species diversity of saplings. Pearson correlation analysis, multiple stepwise regression model, canonical correlation analysis and path analysis were used. The effects of large trees on species diversity of understory saplings were explored from different diversity dimensions.

Result (1) The results of Pearson correlation analysis showed that the biomass of large trees had extremely significant positive and negative effects on Margalef index and Audair & Goff index of sapling, respectively (P < 0.01). The average crown width of large trees had significant positive effects on Margalef index (P < 0.01), and positively and negatively affected the Pielou index and Audair & Goff index, respectively (P < 0.05). The average DBH of large trees had no significant effect on the diversity related indices of saplings. The number density of large trees had extremely significant positive and negative effects on the Margalef index and Audair & Goff index of young trees respectively (P < 0.01). (2) The results of multiple stepwise regression analysis showed that biomass and average tree height were the main influencing factors of Margalef index of saplish trees, average crown width was the main influencing factor of Pielou index, and biomass and plant density were the dominant factors of Audair & Goff index. (3) The results of canonical correlation analysis showed that there was a significant correlation between the species diversity of large trees and understory saplings, and the biomass and average crown width of large trees were the key factors affecting the comprehensive index of sapling species diversity; (4) Results from the path analysis showed that the average crown width of large trees had a significant positive direct effect on species richness and evenness of understory saplings. Large tree biomass indirectly affects understory sapling species diversity by significantly affecting large canopy width.

Conclusion In the natural secondary forest of Quercus Hunan, the biomass and canopy width of large trees are the main driving factors affecting the richness, evenness and diversity index of sapling species, and have a significant positive regulation effect on sapling species diversity. Through building a suitable growth environment for large trees reasonably and optimizing the stand structure, the species diversity of sapling under the forest can be effectively promoted.