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?