Background The shoulder line is the transition zone between hill slope and gully slope in the hilly-gullied Loess Plateau region. Along the shoulder line, the disturbance from human activities is relatively fewer. The plant communities that distribute along the shoulder line play an important role in protecting gully head from headward erosion and providing habitat for native plant species. However, how the plant communities characteristics change with the different micro-topographies along the shoulder line is not well known.

Methods We selected small watersheds with different orientations in hilly-gullied Loess Plateau region as study objects, and divided the micro-topography along shoulder line into two different micro-topographies, gully head area and inter shallow gully area, for vegetation investigation. The species composition, coverage, height and number of individual species were investigated. And species diversity characteristics of plant communities in different micro-topographies along the shoulder line were analyzed.

Results The abundant species were retained along the shoulder line in the study region, and 90 species, belonging to 73 genera and 34 families were recorded. Most of these species were perennial herbs and grasses, and a few species were shrub and trees. Among them, the species with high distribution frequency and high importance value were potential anti-erosion species in this region, and were important seed resources for spontaneous restoration of abandoned slope land and understory layer in artificial forest. The community coverage changed from about 40% in the inter shallow gully area to 60% in the gully head area. The community height showed that the gully head area was higher than inter shallow gully area, and the maximum average height of vegetation was 29.43 cm in the gully head area. There was no significant difference in the number of species in different micro-topographies, but the importance value of individual plant species varied with micro-topographies, indicating that micro-topography affected community structure. The species diversity along the shoulder line was affected by the slope aspect, micro-topography and their interaction. Average height, average coverage and species diversity index of community in shady slope were higher than those in sunny slope. Compared with the inter shallow gully land, the plant community in the gully head had higher species richness, lower evenness index, and no significant difference in species diversity.

Conclusions As a transitional zone between hill slope and gully slope, the shoulder line preserves abundant species and plays an important role in preventing headward erosion and providing seed for slope plant spontaneous restoration. Moreover, in a small range, the fluctuation of topography changes the distribution of water and nutrient resources, and then affects the structure and diversity of plant communities. The influence of micro-topography should be fully considered in the process of artificial vegetation of restoration and reconstruction in this region.