Ambrosia artemisiifolia and Ambrosia trifida are two species of very harmful and invasive plants of the same genus. However, it remains unclear why A. artemisiifolia is more widely distributed than A. trifida worldwide. Distribution and abundance of these two species were surveyed and measured from 2010 to 2017 in the Yili Valley, Xinjiang, China. Soil temperature and humidity, main companion species, the biological characteristics in farmland ecotone, residential area, roadside and grassland, and water demand of the two species were determined and studied from 2017 to 2018. The area occupied by A. artemisiifolia in the Yili Valley was more extensive than that of A. trifida, while the abundance of A. artemisiifolia in grassland was less than that of A. trifida at eight years after invasion. The interspecific competitive ability of two species was stronger than those of companion species in farmland ecotone, residential, and roadside. In addition, A. trifida had greater interspecific competitive ability than other plant species in grassland. The seed size and seed weight of A. trifida were five times or eight times those of A. artemisiifolia. When comparing the changes under simulated annual precipitation of 840 mm versus 280 mm, the seed yield per m² of A. trifida decreased from 50,185 to 19, while that of A. artemisiifolia decreased from 15,579 to 530.

Germination heterochrony refers to germination of seeds dispersed in a single growing season, which is different from delayed germination. We studied two year’s demographic characteristics, characteristics of fruit heteromorphism, the relationship between fruit heteromorphism and germination heterochrony, effects of moisture and temperature on germination characteristics, as well as seed longevity of four annual Salsola L. species to analyze the adaptive significance and causes of germination heterochrony. We found that the number of individuals of all populations changed drastically in one year. Approximately 41.6–100% of seedlings germinated in spring died. The number of fruit types varied with interspecies and intraspecies. Despite the wide range of germination temperature of different fruit types (0–35 °C), the germination percentage at 0–15 °C was the highest. When the soil moisture content was 20%, the germination percentage was the highest, reaching 50% within the shortest time. The contrary was the case with the decreasing of soil moisture. The seed longevity of the four species was one year. Fruit heteromorphism had no direct relationship to germination heterochrony. Germination heterochrony was caused by precipitation characteristics and short seed longevity of annual Salsola L., which was an effective survival strategy for plant to adapt to the changing environments in arid area.

Giant ragweed (Ambrosia trifida, L. henceforth referred to as GR), an annual non‐native invasive weed, may cause health problems and can reduce agricultural productivity. Chemical control of GR in grasslands may have irreversible side effects on herbs and livestock. In an attempt to propose a solution to the harmful effects of GR on grasslands, this study explores the fate of its soil seed bank (SSB) and considers the physical control of its SSB reduction.By studying GR distributed in grasslands of the Yili Valley, Xinjiang, China, we measured the spatial and temporal changes in seed density, seed germination, dormancy, and death. We analyzed seed germination, dormancy, and death following different storage periods. The study analyzed population characteristics over time, including seed fate, and examined physical control methods for reducing the SSB density.The SSB of GR occurs in the upper 0–15 cm of soil in grasslands. Seed density in the SSB decreased by 68.1% to 82.01% from the reproductive growth period to the senescence period. More than 98.7% of the seeds were rotten, eaten, germinated, dispersed, or died within one year after being produced. The seed germination rate of the SSB decreased with the number of years after invasion. When stored for 0.5 or 3.5 years, seed germination rates fell by 40%, during which time seed death rate increased by almost 40%. When GR was completely eradicated for two consecutive years, the SSB and population densities decreased by >99%.The vast majority of GR seeds germinated or died within one year; the germination rate decreased significantly if the seeds were stored dry at room temperature for a long time. Newly produced seeds are the main source of seeds in the SSB. Therefore, thoroughly eradicating GR plants for several years before the seeds can mature provides an effective control method in grasslands.

The farm-shelter forest network is a complex grid protection system, with a windbreak that is distinctly different from that of the single shelterbelt. We selected the farm-shelter forest network of a jujube field in the Tarim Basin of northwest China and used a combination of field measurements and wind tunnel tests to determine the optimal spacing interval between principal shelterbelts. The wind speed reductive curve of the farm-shelter forest network showed a gradual wind speed tendency to stability. Therefore, a model was established based on the energy transfer balance between the upper and the lower airflows for a steady wind speed. The prediction error of the model was found to be < 1%. The model results indicated that increasing the spacing interval between principal shelterbelts from 10 to 20 H, where H is the shelterbelt height, maintained more than 70% of the windbreak effect of the farm-shelter forest network. If the spacing interval between principal shelterbelts were to be increased from 10 to 20 H, the jujube planting area would be increased by 0.54%. Therefore, a thorough consideration of the windbreak effect of each shelterbelt, the synergistic effects of shelterbelts, the windbreak effects of tall crops, and the effects of temperature and humidity in farm-shelter forest networks indicates that increasing the spacing interval will not only maintain the windbreak effect, but it will also reduce the side effects of shelterbelts, increase the planting area, favor mechanized operation, and improve planting efficiency.

Background: Ambrosia artemisiifoliaandAmbrosia trifidaare globally distributed harmful and invasive weeds. High density clusters play an important role in their invasion. For these two species, the early settled populations are distributed at low densities, but they can rapidly achieve high population densities in a short period of time. However, their response to intraspecific competition to improve the fitness for rapid growth and maintenance of high population densities remains unclear. Therefore, to determine how these species form and maintain high population densities, individual biomass allocations patterns between different population densities (low and high), and plasticity during seedling, vegetative, breeding and mature stages were compared. In 2019, we harvested seeds at different population densities and compared them, and in 2020, we compared the number of regenerated plants across the two population densities.Results: Most biomass was invested in the stems of both species.Ambrosia trifidahad the highest stem biomass distribution, of up to 78%, and the phenotypic plasticity of the stem was the highest. Path analysis demonstrated that at low-density, total biomass was the biggest contributor to seed production, but stem and leaf biomass was the biggest contributors to high-density populations. The number of seeds produced per plant was high in low-density populations, while the seed number per unit area was huge in high-density populations. In the second year, the number of low-density populations increased significantly.A. artemisiifoliaandA. trifidaaccounted for 75.6% and 68.4% of the mature populations, respectively.Conclusions: High input to the stem is an important means to regulate the growth of the two species to cope with different densities. These two species can ensure reproductive success and produce appropriate seed numbers. Therefore, they can maintain a stable population over time and quickly form cluster advantages. In the management, early detection of both species and prevention of successful reproduction by chemical and mechanical means are necessary to stop cluster formation and spread.