Causes of differences in the distribution of the invasive plants Ambrosia artemisiifolia and Ambrosia trifida in Yili Valley, China

<p class="1"><i>Ambrosia artemisiifolia</i> and <i>Ambrosia trifida</i> are two species of very harmful and invasive plants of the same genus. However, it remains unclear why <i>A. artemisiifolia </i>is more widely distributed than <i>A. trifida</i> worldwide.</p> <p class="1">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.</p> <p class="1">The area occupied by <i>A. artemisiifolia </i>in the Yili Valley was more extensive than that of <i>A. trifida</i>, while the abundance of <i>A. artemisiifolia </i>in grassland was less than that of <i>A. trifida</i> 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, <i>A. trifida </i>had greater interspecific competitive ability than other plant species in grassland. The seed size and seed weight of <i>A. trifida</i> were five times or eight times those of <i>A.artemisiifolia</i>. When comparing the changes under simulated annual precipitation of 840 mm versus 280 mm, the seed yield per m² of <i>A. trifida</i> decreased from 50,185 to 19, while that of <i>A. artemisiifolia </i>decreased from 15,579 to 530.</p> <p>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 <i>A. artemisiifolia </i>has a smaller seed size and stronger drought tolerance, which allows <i>A. artemisiifolia </i>to spread farther than <i>A. trifida</i>. The reason for wider distribution of <i>A. trifida</i> in grassland is that <i>A. trifida</i> has stronger interspecific competitive ability than <i>A. artemisiifolia </i>under sufficient water.</p>