Background The plant diversity and soil physico-chemical properties are important indicators reflecting the quality of forest management. The purpose of this study is to explore the plant diversity, soil physico-chemical properties and their relationships to evaluate the quality of plantation in the loess area of western Shanxi province. It could provide a theoretical reference for the construction of artificial mixed forest and reconstruction of vegetation in the loess area of western Shanxi province.

Methods Three stands were studied, including Pinus tabulaeformis pure forest, Robinia pseudoacacia pure forest and the mixed forest of Pinus tabulaeformis and Robinia pseudoacacia. Standard sample land survey method was carried out. Plant diversity, soil physico-chemical properties and their correlations were analyzed. Besides, the quality of different stands was evaluated by the method of principal component analysis (PCA).

Results 1) The species quantities of herb layer were higher than that of shrub layer. The highest important value was Rosa xanthina (50.32) in shrub layer, and Artemisia sacrorum (27.41) and Patrinia scabiosaefolia (23.63) in herb layer. The diversity values of plant were ranked as follows: mixed forest of P. tabulaeformis and R. pseudoacacia > R. pseudoacacia pure forest > P. tabulaeformis pure forest. 2) The total soil porosity, soil capillary porosity and total phosphorus in the mixed forest of P. tabulaeformis and R. pseudoacacia were significantly higher than that of P. tabulaeformis pure forest. The organic carbon and the total nitrogen in the mixed forest of P. tabulaeformis and R. pseudoacacia and R. pseudoacacia pure forest were significantly higher than P. tabulaeformis pure forest. 3) There was an significant correlation between Shannon-Wiener index and soil bulk density and total porosity in the shrub layer, while in the herb layer Shannon-Wiener index was positively correlated with soil total porosity and total phosphorus (P < 0.05). There was a highly significant correlation between Shannon-Wiener index and capillary porosity in the shrub layer, while in the herb layer Shannon-Wiener index was positively correlated with capillary porosity and total nitrogen (P < 0.01). 4) The comprehensive evaluation values of the quality of artificial forests in different forests were ranked as follows: mixed forest of P. tabulaeformis and R. pseudoacacia > R. pseudoacacia pure forest > P. tabulaeformis pure forest.

Conclusions Mixed broadleaf-conifer forest of P. tabulaeformis and R. pseudoacacia should be firstly considered in the construction of soil and water conservation forests in the loess area of western Shanxi province, which has the effect of improving soil physico-chemical properties and maintaining plant diversity, followed by broad-leaved forest of R. pseudoacacia. For the existing P. tabulaeformis pure forest, practical forest management measures should be taken to improve the soil physico-chemical properties and promote the increase of the plant diversity.