Discussion
Ambrosia artemisiifolia invaded more quickly than A. trifida in the Yili Valley. In roadside, farmland ecotone, and residential area habitats with relatively poor water availability and weak interspecific competition, A. artemisiifolia was much more abundant than A. trifida . In grassland with relatively rich water availability and strong interspecific competition, A. trifida was much more abundant than A. artemisiifolia (Fig. 2). In the study area, more types and larger areas of suitable habitat are available toA. artemisiifolia than to A. trifida , which is consistent with the distribution of these two species worldwide (Chauvel et al., 2006; Bullock et al., 2012; Follak et al., 2013;Regnier et al., 2016; Montagnani et al., 2017).
Greater population density, higher plant height, and greater coverage are conducive to successful plant invasion (Chapman et al., 2014; Chapman et al., 2016). Although the density, height, and coverage ofA. trifida were higher than those of A. artemisiifolia and companion species in roadside, farmland ecotone, and residential area in the present study (Fig. 4), the distribution points of A. trifidawere all located in low-lying and waterlogged areas (Fig 2).Ambrosia artemisiifolia is highly competitive in continuously disturbed habitats such as roadsides and farmland ecotones (Kazinczi et al., 2008; Novak et al., 2009; Bullock et al., 2012;Gentili et al., 2015; Gentili et al., 2017) as the disturbances decrease competition.Ambrosia trifida is widely distributed in grassland as the density, height, and coverage of A. trifida are higher than those of A. artemisiifolia and companion species (Fig. 4). The life history strategy of A. trifida is mostly based on rapid growth that allows the plants to quickly reach a greater height and biomass than other plants (Abul-Fatih & Bazzaz, 1979). Stronger interspecific competitive ability of A. trifida may explain larger distribution of the species in grassland.
The primary means of dispersal of A. artemisiifolia and A. trifida seeds is barochory (Basset & Crompton, 1975; Montagnaniet al., 2017).The medium-distance and long-distance dispersal of A. artemisiifolia and A. trifida is driven by human activities and obstruction in many ways (Bullock et al., 2012). Seed size is an important factor affecting seed diffusion and species distribution (Washitani & Nishiyama, 1992). Ambrosia artemisiifolia has lighter and smaller seeds (Fig. 5), so A. artemisiifolia seeds are easier to spread in habitats with more human activity such as residential area and roadside (Essl et al., 2009; Bullock et al., 2012; Skálová et al., 2017). Easier spread of seeds of A. artemisiifolia may explain larger distribution of the species in the Yili Valley. In addition, the long-term seed bank of A. artemisiifolia (Webster, Cardina, & White, 2003; Fumanal et al., 2008) may be mentioned as a factor stabilizing populations, especially in very dry years when seed production is low.
Ambrosia artemisiifolia can grow well and produce more seeds thanA. trifida with a limited water supply when the latter produces almost no seeds (Table 4). This shows that A. artemisiifolia has a stronger ability than A. trifida to tolerate drought. The net photosynthetic rate of A. artemisiifolia decreases during periods of reduced soil water content (Bazzaz, 1973), but the plants recover rapidly from short-term droughts (Bazzaz, 1973; Bazzaz, 1974). In unusually dry years or on dry sites, A. artemisiifolia plants have stunted growth but remain able to produce seeds, albeit in small quantities (Raynal & Bazzaz, 1975; Leiblein-Wild & Lösch, 2011). Stronger drought tolerance of A. artemisiifolia may explain larger distribution of the species in roadside, farmland ecotone, and residential area habitats with relatively poor water availability. Low rainfall is a limiting factor for the growth of A. trifida(Basset & Crompton, 1982). Therefore, A. trifida can invasion success only when adequate water is available.
Ambrosia artemisiifolia and A. trifida were mainly distributed in farmland ecotone, roadside, residential area, grassland valley, and other accumulated water in the Yili Valley (Dong et al., 2013), and there was no obvious law for the difference in soil temperature of the four habitats in different periods (Fig. 3). Therefore, we believe that the existing distribution pattern of the two species is not mainly affected by temperature in the Yili Valley.
Since the causes of species distribution include factors other than interspecific competition, seed size, and water demand, other issues need to be discussed in future work if researchers wish to better explain the reasons for the differences between these two species. Additional factors to investigate include: 1) how temperature and water work collectively to affect the germination, growth, and reproduction of these two species; and 2) quantitative analysis of the influence of the difference in seed size on the difference in distribution of the two species.