Species composition and influencing factors of understory woody species in Robinia pseudoacacia plantations on the Loess Plateau

Black locust (Robinia pseudoacacia L.) plantations have contributed significantly to soil and water conservation and ecological reconstruction on China's Loess Plateau. Understanding the impact of stand and environment on species composition of understory woody plants will improve the stability of existing black locust plantations. Ten stands were selected in second-generation black locust plantations in tableland and gully areas of the Loess Plateau. The number of understory tree species in the tablelands was significantly lower than in the gully stands. Regenerated black locust (19.76%) and Rubus corchorifolius L.f. (64.85%) were the most abundant understory tree and shrub species, respectively, in the tableland stands; Broussonetia papyrifera (L.) L'Her. ex Vent. (6.77%) and Acanthopanax senticosus (Rupr. Maxim.) Harms. (37.22%) were most abundant in the gully stands. Species richness (S), Shannon diversity (H), and evenness index (J) of the understory plants were significantly lower in the tableland stands than in the gully stands. More diverse understory species and community structures occurred in the gully stands. Differences in species diversity among landform positions may be attributed to differences in soil moisture. In addition, 77.57% of the variation in understory species composition was explained, among which shrub and herb coverage, stand age, leaf area index, slope and total soil phosphorus in the 10-20 cm layer were the main factors. Soil organic carbon and total potassium significantly impacted S, H and J. Considering the environmental conditions and the biological characteristics of the plants investigated, R. corchorifolius should be given priority in the development of tableland stands, while B. papyrifera and Celtis sinensis Pers. should form mixed forests with black locust in gully stands. This management could promote biodiversity and stability of the existing black locust plantations but also optimize regional landscape patterns.