Abstract:Aims Our objective was to reveal the altitudinal pattern of species diversity in woody plant communities of Mountain Helan, which is located on the ecotone between temperate steppe and desert regions in arid northwestern China. Methods We surveyed forests and shrublands with the quadrat method and used Generalized additive model (GAM) to model relationships between species richness of tree, shrub and herbaceous layers with altitude. Important findings Altitude accounted for 30%-40% of the variances of species richness patterns, including tree, shrub, herb and total species richness. Both for forest and shrubland communities, 60%-80% of the species were herbaceous and patterns of total species richness with altitude were almost determined by herbaceous species. In forest communities, species richness of the tree layer peaked at middle altitude, because of favorable precipitation and temperature. Species richness of shrub and herbaceous layers in forest communities decreased with altitude, showing a significant influence of forest canopy cover. For shrub communities, species richness of shrub and herb layers also peaked at middle altitude. Aridity at low altitude and cold at high altitude limited species’ survival, and only at middle elevations could coexist large numbers of plant species.