Objective The aim of this study was to study the effects of gap size and light factor on understory plant diversity in the Larix principis-rupprechtii natural secondary forest.

Method Taking the natural secondary Larix principis-rupprechtii forest in Guandi Mountain of northern China as the research object, 12 gaps were selected according to the area grade, and three 5 m × 5 m quadrats were set in each gap to investigate the shrubs in the quadrats. Each 5 m × 5 m quadrat was set up with a 1 m × 1 m herb investigation quadrat, and another 12 forest control checks (CK) with the same area were set up to investigate vegetation in the same way. Four diversity indicators, Simpson dominance index, Shannon-Wiener index, Pielou evenness index, and Margalef richness index were used to analyze the differences in species composition and species diversity in different area grades of the Larix principis-rupprechtii natural secondary forest. We investigated the light factor indicators of different areas of forest gaps, and analyzed the relationship between light factors and species diversity in different areas of Larix principis-rupprechtii natural secondary forest.

Result In the 12 forest gaps investigated, a total of 8 species of shrubs and 32 species of herbaceous plants were recorded. The species diversity index of the herb layer in the gap was larger than that in the forest, while the species diversity index of the shrub layer was only higher than the forest gap of grade Ⅲ and grade Ⅳ. In the shrub layer, with the increase of the gap area, the H value of the Shannon-Wiener index showed a trend of first increasing and then decreasing. The H value was the smallest, and both were greater than CK, but there was a significant correlation between the H′ index of the herb layer and the gap area (P < 0.05). In the herb layer, the Shannon-Wiener index H, Simpson index H′, Pielou evenness index J_sw, and Margalef abundance index D were the largest in the small gap, and gradually decreased with the increase of gap area, but it was higher than CK. The D value of the herb layer was higher than that of the shrub layer, and the changing trend of the D value of herb layer with the increase of the gap area was more obvious than that of the shrub layer. The direct radiation, scattered radiation and total radiation in the gap changed significantly with the increase of the gap area (P < 0.05). With the increase of gap area, the amount of scattered radiation and total radiation also increased, and the amount of direct radiation was reduced first and then increased as the area of the forest gap increased, and the direct radiation was the smallest when the forest gap area was 70−100 m².

Conclusion When the forest gap area is 100−150 m², it is beneficial to improve the species diversity level of the shrub layer, and it is beneficial to improve the species diversity level of the herb layer when it is 0−50 m², and the species richness of the understory community is the highest in 50−100 m². Appropriate light factors in forest gaps are beneficial to increase the diversity of understory vegetation species, however, too high light factor is not conducive to the formation of understory vegetation diversity. Therefore, in the process of protection of forest species diversity of Larix principis-rupprechtii natural secondary forests, small and medium-sized gaps should be the focuses.