Relationship between seed germination and invasion of Ambrosia artemisiifolia and A. trifida at different positions

Ambrosia artemisiifoliaLinn. andA. trifidaLinn. are exotic invasive plant species in China. They are often distributed in populations at high density, and they yield abundant fruits. It is important to study the characteristics of germination(dormancy) and diffusion(distribution) of seeds of both species produced on different parts of the mother plant to enhance the understanding of the mechanisms of their invasion. In this study, we usedA. artemisiifolia and A.trifidaplants collected from Xinyuan County, Yili, Xinjiang. Plants were proportionally divided into nine positions from top to bottom according to plant height and branch length. Morphological characteristics, quantity and germination characteristics of the seeds produced, and diffusion of seeds originating from the nine positions were compared. The relationship between population density regulation and invasion was also studied. The results showed that: 1) There was no significant difference in length, width and 100-seed weight of seeds produced at different positions between the two species,but the lengths and widths of the seeds ofA. trifidawere 2-3 times greater than those ofA. artemisiifolia. The 100-seed weight ofA. trifidawas seven times greater than that ofA. artemisiifolia. In combination with the different distributions of the two species in Xinyuan, it was considered that seed size is a factor affecting the regional differences in distribution of the two species. 2) The number of seeds produced in positions Up Up(UU), Middle Up(MU) and Up Middle(UM) ofA.artemisiifoliaandA. trifidawas about 50% of the total number of seeds produced by the plant. Seed yield in positions MM and BU was about 23% of the total, while production in the lower positions of UB, MB, BM and BB was about 27%. This meant that 73% of the seeds produced in the year had the potential for long distance diffusion. 3) The germination rates of seeds produced at different plant positions ofA. artemisiifoliaandA. trifidashowed a trend of upper positions > middle positions > lower positions; while the initial germination time was lower positions > middle positions > upper positions; and germination duration followed the trend of upper positions > middle positions > lower positions. This pattern of seed germination avoided the risk of simultaneous germination by large numbers of seeds in the same growing season, which might lead to high mortality at high seeding density. On the basis of the above analysis, it was concluded that