Response of understory plant species diversity to plant mixture on the Loess Plateau: A case study of Hippophae rhamnoides

Afforestation with nature-based solutions is beneficial for restoring biodiversity. On the Loess Plateau of China, numerous monocultures have been converted to mixed plantations following nature-based solutions. However, the effect of plant mixture on understory plant species diversity remains largely unknown. Here, we investigated the understory plant species diversity of different Hippophae rhamnoides mixed plantations resulting from nature-based solutions and assessed the relative importance of associated factors driving their changes. The results showed that the response of understory plant species diversity to plant mixture was nonsignificant, except for Armeniaca sibirica, which significantly reduced the understory plant species diversity. However, mixed plantations had higher variability in plant species diversity and environmental factors among individual plots than monocultures. In mixed plantations, understory plant species diversity was mainly affected by shrub stratum, followed by overstory stratum, soil nutrients, herbaceous stratum, and topography. Specifically, understory plant species diversity was positively associated with stand density and herb biomass but negatively associated with elevation and soil nitrogen. In particular, soil nitrogen also had negative impacts on herb biomass but was beneficial for the growth of trees and shrubs, indicating that trees and shrubs outperformed herbs in exploiting soil nutrients, a consequence of intense interspecific competition. In addition, the impacts of canopy cover and slope aspects on understory plant species diversity were nonsignificant. Overall, in mixed plantations on the Loess Plateau, understory plant species diversity was primarily affected by interspecific competition for preempting soil nitrogen.