Soil and shrub differentially determine understorey herbaceous plant richness and abundance in a semi-arid riparian meadow

It had been widely recognized that abiotic and biotic filters determined local community assembly and species diversity hierarchically. However, few studies distinguished the effects of abiotic and biotic filters on species richness and abundance, respectively. We evaluated the difference of abiotic (soil) and biotic (overstorey shrub) filters affecting richness and abundance of understory communities and further disentangle the direct and indirect effects of these filters, focusing on three habitats types defined along an elevational gradient (river bank, floodplain and upper terrace), in a semi-arid riparian vegetation dominated by Caragana aurantiaca Koehne, a dwarf legume shrub, in Yili valley, Sinkiang, China. Understorey herbaceous species richness presented a single effect of soil abiotic filter, which was significantly correlated with soil pH, but not with shrub height and cover, indicating that soil constitutes the main filter determining the species pool available for assembly of understorey vegetation. In contrast to species richness, understorey species abundance presented a joint effect of soil and shrub filters, but the variation explained by the shrub canopies was higher than that of soil variables, indicating overstorey shrub acted as the main filter determining species abundance. In addition, the direct effects of overstorey shrubs through competition and facilitation on understory species abundance are greater than the indirect effects through influencing soil status at local scale. Greater attention to shrub for understory vegetation assembly is needed to assess the role of biotic filter in the semi-arid riparian zone