Based on the association classification of northern tropical karst seasonal rainforest in Nonggang plot, species diversity for all associations, trees, sub-trees, and shrub layers along an altitudinal gradient were evaluated using the indices of species richness, diversity, and evenness. The species diversity indices were significantly different among associations (P < 0.01). The species diversity of various associations decreased in the following order:the Chionanthus ramiflorus, Vitex kwangsiensis, and Excentrodendron tonkinense association > the Clausena dunniana-Boniodendron minius-Excentrodendron tonkinense association > the Catunaregam spinosa-Vitex kwangsiensis-Excentrodendron tonkinense association > the Ardisia thyrsiflora-Sterculia monosperma-Saraca dives association > the Clerodendrum wallichii-Sterculia monosperma-Cychidendron guangxiensis association > the Walsura robusta-Cleistanthus sumatranus-Excentrodendron tonkinense association > the Aglaia odorata-Cleistanthus sumatranus-Excentrodendron tonkinense association > the Ficus hispida-Sterculia monosperm-Caryota obtusa association. The Patrick, Shannon, and Pielou indices indicated a "mid-altitude bulge" phenomenon below the middle slope (relative altitude < 76.3 m in the Nonggang plot); the species diversity of the associations located at middle altitudes was higher than those of associations at lower and higher altitudes. However, these indices increased as the relative altitude above the middle slope increased (i.e., when the relative altitude was greater than 76.3 m in the Nonggang plot). Species diversity indices for various layers decreased as follows:tree layer > sub-tree layer > shrub layer. This is similar to the pattern observed in tropical montane rain forests but contrary to that of subtropical karst evergreen broad-leaved forests. Differences in water and temperature reflect spatial habitat heterogeneity at various altitudes and influence the species diversity in northern tropical karst seasonal rainforests. The species diversity of trees was higher in relatively lower altitudes, which are characterized by beneficial water and temperature conditions. However, the observed species diversity of sub-trees and shrubs was higher for relatively higher altitudes characterized by harsh habitat conditions. Hence, sub-trees and shrubs are suitable for vegetation restoration in habitats for karst hills at high altitudes.