Background The canopy is the basis of the material and energy exchange of forest ecosystem with external environment. It is through the regulation and redistribution of the spatial allocation of natural resources such as rainfall and sunlight, and the formation of microclimate under the forest, which profoundly affects the material accumulation and growth of the plant. At present, researches on the diversity of vegetation and its succession are mainly focused on gaps, gaps and other canopy in subtropical and temperate regions, and there are few studies on the influence of canopy structure on understory vegetation diversity in alpine region.

Methods This study was carried out in Baoku Forest Farm of Datong county in the Beichuan River basin in Xining city of alpine Qinghai province. The five forest stands of Betula platyphylla, Picea crasslia, Hippophae rhamnoides, Larix principis-rupprechtii and Picea crassifolia + Larix principis-rupprechtii mixed forest were selected. Six plots in each stand were set up, and the 5 quadrants in each plot, one in the 4 corners and the center of each plot, were set up, thus there were 30 arbor quadrants, 150 shrubs and herb quadrants. The canopy structure (gap fraction, leaf area index, and mean leaf angle) and understory light (direct PPFD under canopy, diffuse PPFD under canopy, the total PPFD under the crown) were obtained by fisheye cameras to photograph canopies in the quadrants.

Results There were some differences in the species richness, diversity and evenness of the forest shrub layer. The species richness in the herb layers of five forest stands was higher than that of shrub layer, and there was a significant difference between individual stands by single factor analysis of variance. Regarding species diversity, the herb layers of other stands were better than shrub layer, except for B. platyphylla forest. The species uniformity of shrub layer in H. rhamnoides forest stand was the lowest, and the species uniformity of herb layer was the lowest in L. principis-rupprechtii forest stand. The species diversity in the mixed forest of P. crassifolia forest and L. principis-rupprechtii forest was higher than that of either pure P. crassifolia forest or L. principis-rupprechtii forest. Via the typical analysis, it was found that there was a significant correlation between canopy structure and understory light, and the gap score and the total light under canopy weighted the most in the two groups of variable canopy structure and understory light. Moreover, there was a significant negative correlation between the gap score and the species diversity and evenness of the shrub layer, but not so with the species richness of shrub layer. There was no significant negative correlation between the gap score and the diversity index of the herbaceous layer, and the significant positive correlation between the total radiation under the crown and the diversity of the herbaceous layer occurred.

Conclusions The effect of light on the growth and development of herbaceous layer was significantly higher than that of canopy structure. While the canopy structure, especially the gap score affects the formation of shrub layer, because the demand for light by shrub layer is lower that by the herbaceous layer.