Spatial distribution of the herbaceous layer and its relationship to soil physical–chemical properties in the southern margin of the Gurbantonggut Desert, northwestern China

Herbaceous plants, particularly ephemerals, exhibit higher variability than any other plants in desert ecosystems. The spatial distribution of herbaceous plants is restricted by environmental factors and presents obvious regularity. To elucidate the restriction and distribution regularity of herbaceous plants, we investigated the characteristics of the herbaceous layer and the soil physical–chemical properties at the bottom, middle, and top of sand dunes in the southern margin of the Gurbantonggut Desert, northwestern China. Redundancy analysis (RDA) was performed to analyze the relationship between herbaceous layer characteristics and soil physical–chemical properties. Results showed that: (1) the distribution of the herbaceous layer at different positions on the sand dunes exhibited clear selectivity. Soil in the middle and lower parts of the sand dunes demonstrated higher water, organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents, as well as higher electrical conductivity (EC), than that at the top of the sand dunes. Accordingly, the herbaceous layer in the middle and lower parts of the sand dunes presented higher density, coverage, and above-ground biomass and contained fewer species than that at the top of the sand dunes. (2) RDA showed that TN and TP were the two key factors that significantly influenced herbaceous layer characteristics (p < 0.01) and explained 10.1% and 41.9% of the variance, respectively. SOC and EC also significantly influenced herbaceous layer characteristics (p < 0.05) but only explained 4.2% and 3.5% of the variance, respectively. Soil water (SW) and pH did not significant influence herbaceous layer characteristics (p > 0.05). Moreover, herbaceous layer coverage and aboveground biomass showed positive correlations with TP and TN, whereas species number was negatively correlated with soil physical–chemical properties. Our results suggested that the spatial distribution of the herbaceous layer was sensitive to changes in soil physical–chemical properties. In particular, TN and TP significantly influenced the coverage, species diversity, and above-ground biomass of the herbaceous layer.