| China was one of the most severely affected countries suffering from desertification. Haloxylon ammodendron (C. A. Mey.) Bunge (Chenopodiaceae) was an xerophytic desert shrub. As dominate desert plant, H. ammodendron played an important role in the maintenance of the structure and function of the whole ecosystem. During the last five decades, overall land reclamation and cultivation, over-grazing and over-cutting and digging had resulted in the destruction of H. ammodendron vegetation and development of mobile sand dunes. However, the spatial pattern and population genetic structure of H. ammodendron populations were largely unknown. In this paper, the spatial pattern and the genetic structure were investigated in the surface plants and seed banks of the nine natural populations of H. ammodendron. We aimed to detect the characteristics of the spatial distribution and dynamics, to evaluate the level of genetic diversity, to examine the distribution of genetic variation with and among populations and on space, to predict the genetic potential of the present populations, and to provide some suggestions for the practices of ecological conservation and restoration.1. The spatial distribution and population dynamics of Haloxylon ammodendron, were studied in four communities in the southeast of Gurban Tonggut desert. Clumped distributions appeared in most seedlings, saplings. Adults were also clumped in Plot A, B and D, while adults in Plot C showed random distribution. At the population level, a clump distribution was found irrespective of successional status. The class distribution of H. ammodendron was nearly reverse J-shape in Plot A and C, suggesting that the populations regenerated continuously. Despite declined class distribution in Plot D, the limit on regeneration was believed to be temporary. The highly disturbed population B still had the potential ability of regeneration.2. Spatial autocorrelation statistics were used to examine the spatial structure of genetic variation. No significant pattern was detected, which suggested extensive pollen flow. Alternatively, the failure to detect genetic structure within population may be due to intraspecific competition during recruitment and development process, resulting in "thinning effect" within populations.3. Genetic variation within and between nine populations of H, ammodendronfrom two regions of China was investigated using ISSR markers. Eight primers used in this study amplified 219 reproducible bands with 184 (84%) polymorphic. Analysis of molecular variance (AMOVA) revealed high genetic variation within populations (97.63%) and low genetic differentiation between regions (0.62%) and among populations (1.75%). We suggested that the present genetic structure could have arisen by adaptation to highly heterogeneous and stressful environment and high levels of gene flow.4. Genetic diversities of both adults and soil seeds were investigated in five populations, while the comparison of spatial genetic structure was performed in three of the five populations of H. ammodendron. We found similar genetic diversity in both populations, but higher differentiation (Gst = 0.220) in surface plant populations (Gst = 0.103). The soil seeds were positively correlated in the first distance class (0-10 m), while no clear spatial pattern was revealed in surface plant populations. These results suggested that seed banks had the potential to maintain genetic diversity in populations and the effect of slowing differentiation of populations.5. Our studies show that the levels of genetic diversity of the present populations of H. ammodendron are still high, and the gene flows between regions, among and within populations are not obstructed. Therefore, we suggest that the ecological conservation of the species can be carried out in the process of ecological restoration. On selecting optimum populations or seeds in the practices of rehabilitation and afforestation, the genetic structure and its spatial pattern should be fully emphasized. Population size was also an important restoration c...
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