Relationship Between Understory Plant Diversity and Soil Characteristics of Different Plantations in the Mu Us Sandy Land

To elucidate the effects of different plantation types on understory vegetation and soil properties in the Mu Us Sandy Land, this study investigated five plantation types: Pinus sylvestris var. mongolica Litv. pure forest, P. sylvestris var. mongolica-Salix psammophila C. Wang and Ch. Y. Yang mixed forest, Salix matsudana Koidz. pure forest, Amorpha fruticosa L. pure forest, and A. fruticosa-S. psammophila mixed forest, using bare sandy land as a control. Through quadrat survey and stratified soil sampling, we assessed the understory plant diversity, soil characteristics, and their interrelationships. The results showed the following: (1) A total of 11 understory plant species, belonging to 11 genera and five families, were recorded, predominantly shrubs from Asteraceae and Fabaceae, and perennial herbs. The S. matsudana pure forest exhibited the highest understory plant diversity, with Shannon and Margalef indices (1.72 and 1.87, respectively) significantly higher than other stands. (2) Soil nutrients varied significantly with forest type. The mixed forest of A. fruticosa and S. psammophila notably improved the contents of soil total nitrogen and organic matter, whereas the soil total phosphorus content was generally low in all stands. (3) Spearman correlation analysis revealed the differential responses of diversity indices to soil factors: the Simpson dominance index was significantly negatively correlated with 10~20 cm soil water content (SWC) (r = −0.9, p < 0.05), and significantly positively correlated with 20~40 cm total phosphorus (TP) (r = 0.9, p < 0.05), with no significant correlations with other soil factors (TN, TK, OM, pH); the Shannon diversity index was highly significantly negatively correlated with 0~10 cm SWC (r = −0.97, p < 0.01), and no significant correlations with other soil factors (TN, TK, TP, OM, pH); the Pielou evenness index was significantly positively correlated with 0~10 cm pH (r = 0.9, p < 0.05) and positively correlated with 20~40 cm pH (r = 0.8) (not statistically significant); by contrast, the Margalef richness index showed no significant correlations with all measured soil physicochemical factors. This study demonstrates that S. matsudana pure forest is more conducive to the development of understory plant diversity, and that soil factors exert a stronger regulatory effect on community structure (evenness and dominance) than on species richness in this arid sandy ecosystem.