Abstract: Forest gaps are common in forest ecosystems, which are important sites for species regeneration. Forest gaps can change the spatial structure and plant community composition of forests, and is of great significance for species regeneration and forest succession. In this study, we analyzed the species diversity and distribution patterns of shrub and herb layers in 24 pairs of differentsized gaps and understory of a Picea crassifolia forest on the eastern slope of the Helan Mountains. The aims of this study were to elucidate the effects of gap size on species diversity of vascular plants and to reveal differences in the interaction network of shrub-regeneration seedling-herb co-occurring species within the forest gaps and understory. The results showed that dominant plant species in the understory of Picea crassifolia forest were Berberis dubia, Juniperus procumbens, Dasiphora fruticosa, and Lonicera microphylla. The seedlings of Picea crassifolia and Juniperus rigida dominated the regeneration layer. Carex was the dominant species in the herb layer. The number of species in the forest gaps was higher than that of understory, with the highest number of family, genus, and species of shrub and herb layers in the large gaps. The richness index of shrub layer increased with increasing forest gap size. The Shannon diversity index was highest at the medium-sized gaps (100-200 m²), and the Simpson index and Pielou index showed a decreasing trend with increasing forest gap size. The herb layer richness index, Shannon index, Simpson index, and Pielou index increased with increasing gap size, and were significantly higher than those of forest understory. Beta diversity tended to decrease and then increase with increasing gap size. The decomposed richness difference components contributed the most to the shrub and herb layers, and showed an increasing trend with increasing gap size. The network structure of forest gaps was larger, more complex, and more concentrated than that of the understory. A close symbiosis between herb-herb species was mostly formed in different-sized gaps and understory. The emergence of medium gap increased the interaction and symbiosis between regenerated seedlings, shrub and herb species. Forest gaps play a positive role in enhancing species diversity of vascular plants and species symbiosis in Picea crassifolia forest in the Helan Mountains. Forest gap with a size of 100-200 m² can improve species diversity and promote the regeneration of understory.

Key words: forest gap, alpha diversity, beta diversity, symbiosis index, interaction network