Differences in soil water and nutrients under catchment afforestation and natural restoration shape herbaceous communities on the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) is one of the areas with the most serious soil erosion worldwide. Vegetation restoration is the main way to control soil erosion and combat land degradation in this region. Using the methods of afforestation and natural restoration, there are large differences in the diversity pattern and community structure of the herbaceous plants, which affects the ecological security and restoration effectiveness. However, current studies are often less focused on this aspect, or only woody plants are considered. In this study, two adjacent catchments with similar geological backgrounds but different restoration methods on the CLP were selected; one catchment was subject to afforestation, and the other was subject to natural grassland revegetation. To clarify the environmental causes of the different performances of the herbaceous communities under these two restoration methods, the functional traits and species diversity patterns of the herbaceous plants, soil physicochemical properties and topographic factors in the two catchments were investigated. The results showed that there were significant differences in the functional traits of the herbaceous communities between these two different restoration methods. Natural restoration produced a significantly higher diversity of herbaceous plants than did afforestation. Differences in soil water, nitrate nitrogen and rapidly available phosphorus contents between the two catchments were the key reasons for the functional traits and diversity differences between the herbaceous communities. Moreover, we found that hillslopes and gullies showed different patterns in water-related functional traits, especially in the naturally restored catchment, which was probably caused by the soil water differences in these two landforms. The results of this study can help us better understand the diversity patterns of herbaceous communities under the measures of natural and artificial vegetation restoration on the CLP.