Background Robinia pseudoacacia is one of the tree species with the largest afforestation area in the loess region of western Shanxi province. However, as the forest age increases, due to factors such as soil moisture and nutrients, the R. pseudoacacia forest has experienced varying degrees of degradation. To explore the change characteristics of undergrowth vegetation and soil nutrients in the restoration process of artificial R. pseudoacacia forest, the 18, 22, 26, 31 and 36 years artificial R. pseudoacacia forests in the loess region of western Shanxi province were taken as the research objects in this study.

Methods The method of spatial sequence instead of time successional sequence was adopted, and one R. pseudoacacia plot of 20 m×20 m, 5 shrub plots of 5 m×5 m, and 5 shrub plots of 1 m×1 m were selected in forests of different ages with similar site conditions. Then vegetation growth indicators were investigated, soil profiles were excavated, and soil samples were collected to determine soil nutrients indicators, then correlation analysis were conducted, so as the change characteristics of growth of R. pseudoacacia forest, undergrowth vegetation and soil nutrients with stand age were clarified.

Results 1) With the increase of the age of R. pseudoacacia forest, the average DBH and height of the trees increased gradually, and the average crown diameter increased first and then decreased. 2) The soil nutrient content of R. pseudoacacia forest land with different ages tended to gather on soil surface, the soil nutrient content of the forest land decreased with the increase of the depth of the soil layer, and content in the soil layer with 0-10 cm was significantly higher than that in the soil layer below 30 cm (P < 0.05), in which the vertical spatial distribution of organic matter content and total nitrogen content was similar to changes with forest age; the content of soil organic matter, total nitrogen and total phosphorus in the forest lands increased first and then decreased with the increase of R. pseudoacacia stand age, but the increase trend of soil available nitrogen was not obvious, and the content of soil available phosphorus decreased. 3) With the increase of R. pseudoacacia stand age, the number of undergrowth vegetation species increased, and the height and coverage first decreased and then increased. In addition, the growth of undergrowth vegetation was affected by the growth characteristics of R. pseudoacacia and soil nutrients. The restoration of R. pseudoacacia forests had an impact on the undergrowth vegetation and soil nutrient. R. pseudoacacia forests improved soil nutrients, but at a certain stand age, this effect was weakening.

Conclusions The growth of R. pseudoacacia forest has a certain effect on improving soil nutrients. The 26 years old forest performed better overall. This study reveals the changing laws of vegetation and soil nutrients in the R. pseudoacacia forest during the growth process, and the results are conductive to understanding the overall characteristics of the artificial R. pseudoacacia forest in the area, and it may provide a scientific basis for vegetation restoration and the management of the R. pseudoacacia forest, and then improve the stability of the ecosystem in this area.