Altitudinal patterns of stand structure and herb layer diversity of Picea schrenkiana forests in the central Tianshan Mountains, Northwest China

Altitude is a useful indicator to examine patterns of forest structure and species diversity in relation to environmental factors. In this study, the altitude patterns of forest stand structure and species diversity were analyzed across 20 plots in the Tianchi Nature Reserve, Northwest China. The results showed that mean stem height (H-m), maximum stem height (H-max) and mean stem diameter at breast height (D-m) of Picea schrenkiana trees all decreased significantly with increasing altitude. Potential tree height (H-star) decreased while stem taper increased significantly as altitude increased, suggesting remarkable altitudinal changes in biomass allocation between the diameter and height growth of Picea schrenkiana. Understory herbaceous richness increased significantly with increasing altitude, or with decreasing total basal area (TBA), Hm and stand volume (Volume). High light availability for understory herbs might account for the higher species richness at high altitude. Sorensen Index decreased significantly with the increase in altitude intervals, while the Cody Index demonstrated a converse pattern, suggesting greater differences in species composition with larger distances.