Species richness patterns of Picea schrenkiana var. tianschanica communities along an altitudinal gradient at different longitudes in Xinjiang of Northwest China

The Picea schrenkiana var. tianschanica forest in Zhaosu, Gongliu, Wusu, Urumqi and Hami that covered about 12 longitudes (81 degrees 05'-93 degrees 41.5'E) along the Tianshan Mountains was investigated by using vertical transects to analyze the species richness of the forest along an altitudinal gradient at different longitudes. The results showed that at most longitudinal positions, P. schrenkiana var. tianschanica forest was basically of pure forest, with P. schrenkiana var. tianschanica as the dominant species, very few of other arbor and shrub species, and the species richness of herbs being controlled by the growth of P. schrenkiana var. tianschanica. This forest had the widest distribution, highest canopy density, best growth, and highest stand volume at middle altitudes, resulting in the lowest species richness of herbs. The herb species richness in P. schrenkiana var. tianschanica forest in Tianshan Mountains showed an inverse hump-shaped variation along the altitude, which could be described by the