Plant facilitation (positive interaction) can drive plant-community structuring in degraded and arid ecosystems. Mound-forming shrubs often accelerate plant establishment on the mounds via amelioration of chemical and physical conditions. We hypothesized that such effects of shrub mounds might expand to the inter-mound areas with increasing shrub density. In this study, the density-dependent effect of shrub mounds was examined using Caragana shrubs in a desert steppe, Mongolia.

We designed 13 pairs of high and low shrub-density plots. Species richness (SR) and plant density (PD) were compared between the two habitat types (on mounds and inter-mound areas) with different shrub-densities for annual and perennial herbs.

Although SR of annuals was higher on mounds than in inter-mound areas, SR of perennials and PD of annuals and perennials were higher in inter-mound areas. Within inter-mound areas, SR and PD of annuals increased in high shrub-density plots, but those of perennials decreased.

Thus, the density-dependent facilitation of shrub mounds was shown on annuals. Shrub mounds enhanced the establishment of annuals, particularly in inter-mound areas with high shrub density. Our results suggest that shrub mounds alter plant species-composition via the density-dependent process of environmental alteration.