The contribution of biodiversity to supporting ecosystem multifunctionality (EMF) is well established in natural ecosystems. However, the effects of multidimensional understory diversity, such as taxonomic diversity (TD), functional diversity (FD), and phylogenetic diversity (PD), on EMF remain largely unknown in plantations. Here, we investigated the TD, FD, PD, and EMF in plantations (Pinus tabuliformis, Robinia pseudoacacia, Platycladus orientalis) and natural forests (Quercus wutaishanica) on the Loess Plateau and examined the effects of stand structure, topography, and understory multidimensional understory diversity on EMF. The results showed that on the Loess Plateau, plantations had lower TD and PD than natural forests. However, the differences in FD between plantations and natural forests were nonsignificant. Natural forests were associated with higher EMF than plantations, except R. pseudoacacia, which performed better in N cycling function and water conservation. In general, EMF was positively correlated to TD, PD, and community-weighted means (CWM, represents the first principal component axis composed of eight leaf traits: leaf area, leaf dry matter content, leaf thickness, specific leaf area, leaf water content, leaf nitrogen content, leaf phosphorus content, and leaf carbon content). In contrast, EMF was negatively correlated to FD. In particular, CWM, FD, and PD mainly affected the N and P cycling, water conservation, and productivity of plantations. The structural equation models indicated that forest stand structure had a direct influence on EMF, while both forest stand structure and topography had an indirect influence on EMF through their effects on multidimensional understory plant diversity.
