Introduction
Two invasive herbaceous species, A. artemisiifolia and A. trifida , have recently become troublesome weeds in several regions of the world, especially in central and eastern Europe as well as in China (Chrenová, Mičieta, & Ščevková, 2010; Kasprzyk et al., 2011; Qin et al., 2014; Hamaoui-Laguel et al., 2015). The existence of these two ragweed plants has greatly changed the biodiversity, structure, and function of the invaded ecosystems, seriously threatening agricultural production and human health (Katz & Carey, 2014; Qin et al., 2014; Hamaoui-Laguel et al., 2015; Page & Nurse, 2015). Many reports have addressed the invasion process and distribution of A. artemisiifolia and A. trifida (Chauvel et al., 2006; Aikio, Duncan, & Hulme, 2010; Pinke et al., 2011; Joly et al., 2011; Cunze et al., 2013;Richter et al., 2013;Storkey et al., 2014;Bae et al., 2015; Chapman et al., 2016;Leiblein-Wild et al., 2016;Skálová et al., 2017).Ambrosia artemisiifolia and A. trifida belong to the same genus and originated in North America (Bazzaz, 1979; Essl et al., 2015). They spread to other continents as early as 1836 (Essl et al., 2015) or 1829 (Verloove, 2016) without considering its cultivation in botanical gardens. According to the global geographical distribution of the two species (CABI Invasive Species Compendium, https://www.cabi.org/isc/search/index?q=Ambrosia , accessed February 21, 2020), the distribution of A. artemisiifolia is more extensive than that of A. trifida. In addition, A. artemisiifolia and A. trifida occur in 80 and 40 countries, respectively (Montagnani et al., 2017).Why does A. artemisiifoliainvade a larger area than A. trifida worldwide? In order to provide information necessary for the early warning of invasion by the two species, a more in-depth study is necessary.
Differences in distribution between species are normally caused by differences in genetic adaptation to environmental conditions.Ambrosia artemisiifolia and A. trifida often invade roadsides, farmland ecotones, wastelands (Essl, Dullinger, & Kleinbauer, 2009; Pinke et al., 2013; Milakovic, Fiedler,& Karrer, 2014), residential habitats(Ziska et al., 2003), and other disturbed areas (Bassett & Crompton, 1982; Fumanal et al., 2008; Milakovic et al., 2014; Essl et al., 2015). Ambrosia artemisiifolia is rarely found in grasslands (Bullock et al., 2012); however, A. trifidaoccurs in grasslands (Regnier et al., 2016). In terms of specific regions, there is partial overlap between the two species’ niches, but these two plants tend to invade different types of microhabitats. The main reason for the differences in habitat that they invade is not clear. Generally, invasive plant species have strong performance-related traits, including those related to physiology, leaf-area allocation, shoot allocation, growth rate, size, and fitness than do non-invasive plant species (van Kleunen, Weber, & Fischer, 2010; van Kleunen et al., 2015). Ambrosia artemisiifolia and A. trifida both have relative strong interspecific competitive ability (Montagnani et al., 2017). The effect of interspecific competitive ability on the distribution differences of two species is unclear.
Water availability affects plant seed germination, growth, and reproduction, factors that are the basis of species distribution and competition, especially in arid and semi-arid areas. Leiblein-Wild and Lösch (2011) found that A. artemisiifolia grew well under moist soil conditions and that it can survive in dry soils. Ambrosia trifida needs more water than A. artemisiifolia (Abul-Fatih & Bazzaz, 1979; Bassett & Crompton, 1982). It is not clear how the water use capacity affects the distribution difference of the two species. Moreover, the link between the differences in distribution and water demand of these two species during seed germination, plant growth, and reproduction period remain unclear.
Temperature has a significant effect on the distribution and growth of the two species (Pinke et al., 2011; Storkey et al., 2014; Qin et al., 2014). Ambrosia artemisiifolia and A. trifida have become widespread in temperate regions (Bassett & Crompton, 1975; Essl et al., 2015; Montagnani et al., 2017). Seeds of the two species require prolonged chilling to break dormancy (Davis, 1930; Bazzaz, 1979; Shrestha et al., 1999; Essl et al., 2015). Following seedling emergence, the rate of vegetative growth depends on temperature, but development occurs over a wide thermal range (Deen, Hunt, & Swanton, 1998).
The Yili Valley, Xinjiang, China covers an area of 56,400 km2 and contains a rich variety of habitats, including grasslands, farmlands, mountains, and residential areas (Jia et al., 2011). Our previous study found that A. artemisiifolia andA. trifida simultaneously invaded the same area of the Yili Valley in 2010, and we also found that the dominant habitat distributions of two species were different (Dong et al., 2017). Therefore, the Yili Valley provides a large, relatively closed field experiment site in which to study the beginning of an invasion by the two species along with their subsequent diffusion. This study can therefore help to explain the distribution differences and causes for successful invasion of A. artemisiifolia and A. trifida , providing insight into the reasons for the resulting distribution of these two species worldwide.
Distribution and abundance of A. artemisiifolia and A. trifida were surveyed and measured from 2010 to 2017 in the Yili Valley, Xinjiang, China. The soil physical and chemical properties, soil temperature and humidity, and the main companion species were determined in farmland ecotone, residential area, roadside and grassland in 2017. Also, biological characteristics, such as density and coverage, plant height, number of seeds per plant, 100-seed weight, and seed size of these two species and companion species (density and coverage, plant height) in four habitats were measured in 2017. Moreover, the differences in water demand between the two species were studied through seed germination and garden experiments from October 2017 to October 2018. The following questions were explored: What were the differences in the distribution of these two species in the Yili Valley? What caused the differences in the distribution of these two species?