Introduction: Haloxylon sp. is one of the plant species adapted to sandy and nutrient-poor soils, and it had shown promising results in the stabilization programs of loose sands. Most of the country's hand-planted stands are suffering from leaf wilting and yellowing, along with serious issues  such as heavy infestation of pests and diseases. Lack of genetic diversity among the cultivated seedlings as well as dense  planting and close spacing are among the causes of these problems. Establishing  seed gardens is one of the ways for addressing these problems and improving forest species through the production of suitable and genetically diverse seeds. A requirement for establishing a seed garden is the initial evaluation of different populations and the selection of superior genotypes.
Materials and methods: This research was carried out as a continuation of a national project during which different populations of Haloxylon were evaluated over two consecutive generations, from  2018 to 2022 at the Ba'sat Sand Stabilization Station in Qom city. In this way, in the first generation, 27 populations were evaluated during five years, and then in the second generation, 18 selected top populations were also evaluated during five years, and finally, nine superior and genetically diverse populations were selected to establish the seed garden in the third generation. First, in each superior population, a number of individual trees were selected and seeded by preserving their pedigree information. The seeds of the selected rootstocks were planted in plastic pots and at the end of March 2018, five to seven healthy rootstocks from nine top selected populations were transplanted into the main field. To prevent inbreeding, maximum possible spacing was maintained between  seedlings  of the same maternal genotype. Over the five year period,  longitudinal growth characteristics of the seedlings, main trunk diameter growth and seedling freshness were measured, and from the third to the fifth year,  crown diameter was measured. Composite variance analysis of traits was conducted separately for each generation and comparison of trait means  using Duncan's multi-range test  in each generation and for the five-year averages and the fifth-year data. Also,  genotypic and phenotypic variance of traits was calculated for each generation  based on the results of the composite analysis.
Results: The results of composite variance analysis of the examined traits across the first to third generations of Siah tag showed that for traits of longitudinal and diameter growth of shrubs as well as canopy diameter, the effects of genotype and year were significant at the 1% or 5% probability level in all three generations. Additionally, the genotype effect in the second generation and the year effect in the first and third generations were significant for the freshness score. The  genotype × year interaction effect was significant for longitudinal growth and canopy diameter  in the second generation, as well as for main trunk diameter growth in the third generation, indicating that the temporal trend of these traits varied across  years. The comparison of average traits among different genotypes in the first to third generations also showed that populations 8, 9, 5 and 3 outperformed others in most traits across years and generations. The average  shrub height and trunk diameter increased over three generations, so that the average of these traits in the third generation was usually higher than the previous generations. In the first generation, the heritability of longitudinal and diameter growth of seedlings was 0.34 and 0.29, respectively. In the second generation, the general heritability of traits was lower than the first generation, and in the third generation, the general heritability of traits varied from 0.27 for crown diameter to 0.43 for the length of shrubs.Conclusion: Due to the presence of considerable genetic diversity among the genotypes in the seed garden, the resulting seeds have the potential to introduce the necessary variability in the field of cultivation of this species. Also, analysis of the change process of the traits across the used genotypes and the trend in genotypic variance and heritability over three generations showed  a favorable degree of trait stability in terms of occurrence of traits among the genotypes, this stability  significantly contributes to the stability of seed production as well as stability. The condition of the seedlings produced from these seeds in the natural fields will improve, which is one of the main and major goals of establishing forest tree seed gardens. Also, the evaluation of the change process of genotypic variance and their heritability in the genotypes forming the seed garden showed the improvement of the average of important traits in them. The results of this research clearly showed the importance and effectiveness of seed garden establishment in forest and especially desert species.