To explore and describe the species richness patterns along altitudinal, high mountain gradients, two transects – northern exposure (YG) and southern exposure (TD) at Mt. Jiuding (1200–4200 m) in Western China (31º13’– 31º46’N, 103º29’–104º05’E) were selected. They differ from south to north in climate conditions and vegetation zonation, and each transect was sampled according to a uniform method. Every 200 m along the altitudinal gradient we set a sampling belt of 3000 m × 5 m to record the tree species, and 30 plots of 5 m × 5 m within every vegetation belt were used to investigate shrub and herb species. We compared the composition of plant species and calculated the coefficient of similarity between the two transects. A Generalized Additive Model (GAM) was used to describe the richness patterns. For the whole Mt. Jiuding, the richness at all three levels (species, genus and family) showed a monotonically decreasing pattern. As for the different growth forms, richness of the trees, shrubs and pteridophytes showed hump-shaped patterns; and herbs showed a slow decreasing pattern along the altitudinal gradients. In TD transect, the richness of species, genus and family also showed monotonically decreasing patterns; tree richness decreased with the increase of altitude; the shrub richness showed a humpshaped pattern; but pteridophytes and other herbs showed wave-like patterns. In YG transect, altitudinal gradient of richness at different taxonomic levels all showed hump-shaped patterns; and the species richness patterns for different growth forms peaked at middle attitude except for the graminoids and other herbs. The evolutionary history of the vegetation in Mt. Jiuding was quite consistent, and different richness patterns along altitudinal gradients might be resulted from different contemporary ecological conditions. Human disturbance and different range of altitudinal gradients were also important factors for different richness patterns between the two transects. In our study, species in different growth forms showed different altitudinal patterns, but those species with similar requirements to environmental conditions showed similar richness patterns along altitudinal gradients.
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