Abstract: Known as the “nurse effect”,shrubberies in desert ecosystem of ten benefit understory plants through microhabitat amelioration,and they are the typical facilitation structuring plant communities in desert.In this study,the germination method was used to investigate the soil seed bank at three microhabitats relative to the individuals of the dominant shrub species Haloxylon ammodendron,i.e.,under shrubbery,shrubbery periphery and inter-shrubbery open area,across an area between two dunes in the southern marginal zone of the Gurbantunggut Desert.Field seedlings close to the soil seed bank sampled at the microhabitats were also investigated in spring.Our aims were to explore the nurse effect of H.ammodendron on the understory plants at early stage of the life history.The results showed that the soil seed bank and aboveground seedling bank were dominated by annuals and ephemerals.Density of the soil seed bank decreased gradually away from the shrubbery core〔(689±67)seed·m^-2〕to the periphery〔(578±58)deed·m^-2〕 and to the inter-shrubbery open area〔(350±48)seed·m^-2〕,showing an enrichment effect of shrub seeds.However,this effect was not significant at the dune crest.Germination rate of soil seed bank was the highest at the shrubbery periphery(61%),but the difference between the shrubbery core and inter-shrubbery open area was not significant,which indicated that there was a fine-scale zonation of the shrubbery effect on germination from shrubbery core to inter-shrubbery open area.Correspondingly,the nurse effect of seedlings from H.ammodendron was the highest at shrubbery periphery.Through aggregating seeds and promoting seedling settling,H.ammodendron has a significant nurse effect on the understory plants in the Gurbantunggut Desert at the early stage of life history.Our findings provide the insights into the occurrence mechanism of interspecific positive interactions in arid ecosystems.

Key words: Haloxylon ammodendron, nurse effect, soil seed bank, aboveground seedling bank, microhabitat, spatial pattern, Gurbantunggut Desert