Abstract: Competition is a major factor structuring plant communities. Plants contend with limiting resources and space under abiotic and biotic stress. The competition effect of plant populations is of significance in population dynamics of desert vegetation. Our objective is to explore the population dynamics of Haloxylon ammodendron and relative intensity of competition with the population growth in the Linze oasis fringe area in the middle of Hexi Corridor. The goal is to provide helpful information related to understanding the mechanisms and interactions influencing adaptation and succession of artificial sand-fixation desert vegetation. H. ammodendron is a commonly-used sand-fixing species in the desert area of northwestern China; it has been abundantly planted in areas where annual precipitation is about 120 mm in the Hexi Corridor since the 1970s. H. ammodendron in five stages of development (5-10-year,10-20-year,20-30-year,30-40-year,>40-year) were analyzed. Five 0.3-hm² (50 m×60 m) permanent plots were established in each of five developmental stages, H. ammodendron was classified as seedlings, juvenile and mature trees, all individuals were measured and stem-mapped, to gain insights into population dynamics over a course of succession driven by resource competition. One-way ANOVA was used to describe the variation among seedlings, juvenile and mature trees, and the variation among different developmental stages. We then used the tool create thiessen polygons in GIS to create voronoi of H. ammodendron in five developmental stages, On the basis of voronoi, defining the competition unit. Finally, we used a competition index which is called V_Hegyi index put forward by Tang Mengping to evaluate the relative intensity of competition with the population growth of H. ammodendron. The results show: (1) The population density of H. ammodendron revealed a reduction in 10-20-year, and a small increase with a large number of seedlings appeared in 30-40-year, but for the low survival rate of seedlings, the density declined at a later stage. (2) In the same developmental stage, there were significant differences in the relative intensity of competition of seedlings, juvenile and mature trees. The bigger the diameter at basal, the smaller the V_Hegyi index is. The V_Hegyi index of seedlings is the biggest, juvenile trees followed, the V_Hegyi index of mature trees is the smallest. (3)The relative intensity of competition of H. ammodendron in 30-40-year stage is the greatest, the order of the other four stages was 5-10-year, >40-year, 20-30-year and 10-20-year stage. The results showed that the variation of relative intensity of competition of H. ammodendron was influenced mainly by variations of the source of water, varies with population growth. In conclusion, population competition should be considered during breaking wind and fixing sands in the arid desert area where annual precipitation is less than 120 mm.

Key words: competition, Haloxylon ammodendron plantation, spatial analysis, V_Hegyi Index