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 were 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. The differences in the
distribution of the two species are mainly due to differences in
interspecific competitive ability, seed size, and water dependence. The
two species have stronger interspecific competitive ability than that of
companion species, but A. artemisiifolia has a smaller seed size and
stronger drought tolerance, which allows A. artemisiifolia to spread
farther than A. trifida. The reason for wider distribution of A. trifida
in grassland is that A. trifida has stronger interspecific competitive
ability than A. artemisiifolia under sufficient water.