Abstract: Taking the Pinus tabulaeformis plantations with different stand densities in the Loess Plateau of Western Shanxi Province, China as the objects, and by the methods of field survey and laboratory experiment, this paper studied the effects of stand density on the understory plant species diversity, litter waterholding capacity, soil physical properties, and soil waterholding capacity, aimed to provide scientific basis for the determination of reasonable stand density of P. tabulaeformis plantation. With the increasing P. tabulaeformis stand density, the understory species richness index and diversity index of shrub layer decreased, while the species evenness index had no obvious trend. In herb layer, the species richness had the same variation trend as that in shrub layer, while the diversity index and evenness index had different trend, i.e., decreased after an initial increase, and reached the maximum when the P. tabulaeformis stand density was 1675 trees·hm^-2. The species diversity index and evenness index of the total community were also the highest at the stand density 1675 trees·hm^-2. Both the evenness index and the diversity index of the shrub layer were higher than those of the herb layer, no matter the stand density was. The litter natural water content in P. tabulaeformis plantations with different stand densities had no significant difference, but the litter amount, maximal water-holding capacity, maximal water-holding rate, effective interception amount, and effective interception rate had significant differences among some plantations with different stand densities. When the stand density was 1300 trees·hm^-2, the soil bulk density was the minimum, while the total porosity, capillary porosity, and their corresponding water-holding capacity all reached the maximum. At the stand density 1675 trees·hm^-2, the litter amount and water-holding capacity were up to the maximum values, the surface (0-20 cm) soil bulk density was lower (1.07 g·cm^-3), and the non-capillary porosity and non-capillary water-holding capacity were the largest (17.45% and 87.25 mm, respectively). Overall, the P. tabulaeformis plantation of about 30 years old had the greatest understory plant species diversity and the best eco-hydrological effect when the stand density was 1675 trees·hm^-2.

Key words: mercury, Moina macrocopa,  Scenedesmus obliquus density, life table demography.