Abstract: Understory plant diversity and soil physicochemical properties are key indices reflecting the stability and growth capacity of soil and water conservation forest. We examined the understory plant species diversity and soil physicochemical properties of Robinia pseudoacacia plantations with stand densities of 900, 1075, 1450, 1850, 2325 trsse·hm^-2 in Caijiachuang watershed of western Shanxi loess region, aiming to provide the basis for the highquality development and sustainable management of R. pseudoacacia plantations in the loess region of western Shanxi Province. With the increases of R. pseudoacacia stand density, the average tree height and diameter at breast height (DBH) decreased gradually. The Margalef, Simpson and Shannon indices of shrub layer and herb layer increased firstly and then decreased, with a peak at 1850 plants·hm^-2. Periploca sepium was the dominant species (importance value: 40.01) in shrub layer, and Artemisia gmelinii was the dominant species (importance value: 41.03) in herb layer. The bulk density of 0-60 cm soil layer decreased first and then increased with increasing R. pseudoacacia stand density, with the lowest value at 1850 trees·hm^-2 (1.19 g·cm^-3). Porosity, organic matter content, and total nitrogen content increased first and then decreased with increasing stand density, with the highest values at 1850 trees·hm^-2. The dominance and diversity indices of shrub layer and herb layer were negatively correlated with soil bulk density (P<0.05), while positively correlated with capillary porosity and organic matter content (P<0.01). The evenness index was positively correlated with total porosity and capillary porosity (P<0.05). According to principal component analysis, the comprehensive score of R. pseudoacacia plantations with different stand densities was ranked as follows: 1850 trees·hm^-2 (0.772), 1075 trees·hm^-2 (0.354), 1450 trees·hm^-2 (0.001), 2325 trees·hm^-2 (-0.012), 900 trees·hm^-2 (-1.115). Our results suggested that too high or too low stand densities would not be suitable for the sustainable development of R. pseudoacacia plantations. A stand density of 1850 trees·hm^-2 would be more conducive to the growth of understory vegetation and the accumulation of soil nutrients.

Key words: understory vegetation diversity, stand density, vegetation restoration, Robinia pseudoacacia, the loess region of western Shanxi Province.