Abstract: The small-scale spatial pattern of plant species, such as aggregation, random and dispersion distribution, varies with environmental conditions, reflecting the ecological adaptation strategies of plant species. From east to west, it gradually transits from the semi-humid region of sub-frigid zone to the temperate arid region on the northern Tibet Plateau, with great variation of plant species diversity and species composition. From July to August 2012, eight typical grassland plant communities in different sites were selected from east to west in northern Tibet Plateau for field investigation. The species composition and species diversity of communities and smallscale spatial pattern of two populations of Kobresia humilisand Stipa purpurea were analyzed. The results showed that species richness and relative coverage of K. humilis were significantly positively correlated with precipitation. From east to west, the distribution of K. humilis changed from dispersion to random pattern, while that of S. purpurea changed from random to dispersion. There was a spatial segregation relationship between both populations with the lowest precipitation. Our results indicate that significant changes in small-scale spatial pattern of species are an important mechanism driving species coexistence in the communities along the transect on the northern Tibet Plateau.

Key words: Prionace glauca, North Atlantic Ocean, demographic analysis, sensitivity analysis, age at first capture.