Abstract
During the range expansion of invasive plants, competitors shared
different co-evolutionary history with invasive plants, as well as
population differentiation, would have different effects on the response
of invaders to global change factors such as increased nitrogen
deposition. Disregard the community responses and potential adaptations
of invaders during the range expansion might bring misleading answers.
To address these challenges, we conducted a greenhouse experiment to
explore the synergistic effects between population differentiation
during range expansion and competitors on the invasion ofGalinsoga quadriradiata in response to increased nitrogen
deposition. Competitors (new or
old that shared short or long co-evolutionary history with the invader,
respectively) were set to compete with the invasive central and edge
populations under different nitrogen addition treatments.Galinsoga quadriradiata from the central population (i.e.,
with longer residence time since invasion) showed significantly higher
total mass, reproduction, interspecific competitiveness when compared to
the individuals from the edge population, and the magnitude of response
to nitrogen addition treatments was larger in the central population
when planted in isolation (single-culture). Nitrogen addition promoted
growth and reproductive performance of G. quadriradiata in
single-culture, in the presence of competitors this effect was weakened.
The old competitors acted more effectively than new competitors in
inhibiting the invader performance, mainly for the central population.
Our results indicate that population differentiation on growth and
competitiveness occurred during the range expansion of G.
quadriradiata , with the central population displaying higher
invasiveness than the edge population. The co-evolutionary history
between invasive species and its competitors has been suggested to be
probably not in favor of invasive plants, especially for central
populations. Our results highlight the synergistic and non-additive role
of population differentiation and shared co-evolution history between
invasive species and its competitors in the range expansion of invaders
in the context of global change factors.
Keywords: central or edge population, co-evolutionary history,
interspecific competition, invasive plant, nitrogen deposition,
population differentiation, range expansion
INTRODUCTION
Human activities have dramatically changed the nitrogen cycle in natural
ecosystems since the industrial revolution (Liu et al., 2020; Yu et al.,
2019; Zhang et al., 2021). This is particularly true for China, as the
largest developing country, it is expected to experience a rapid
increase in nitrogen deposition: the average flux of total nitrogen
deposition for China has been estimated as 20.4 ± 2.6
kg
ha-1 yr-1 as a result of the
industrialization process (Xu et al., 2019; Yu et al., 2019). The
increase of nitrogen deposition leads to anthropogenic disturbances that
increase available soil nitrogen, and different plants have different
responses to this process. Studies have shown that the increase of
available soil nitrogen is beneficial to fast-growing plants that can
quickly transform nitrogen into new biomass, but not to slow-growing
species (Feng et al., 2009; Liu et al., 2017a; Nordin et al., 2005).
Many studies have shown that increased nitrogen deposition will promote
the colonization, diffusion and other invasive processes of exotic
plants, because one of the major characteristics of invasive plants is
rapid growth and preemption of resources (Davis et al., 2000; Radford,
2013; Yu et al., 2020; Zheng et al., 2020). Therefore, it is considered
that the impact of invasive plants may be further strengthened in the
context of future climate change. However, it is still unclear how the
increase of nitrogen deposition affects the invasion process of
widespread alien plants due to the complexity of interactions during
their range expansion.
During their range expansion, invasive plants encounter environmental
heterogeneity that might lead to differentiation across populations
(Dematteis et al., 2020; Helsen et al., 2020; Zhang et al., 2022). This
differentiation may reflect upon species performance and to diverse
responses to the increase of nitrogen deposition. Drastic differences
across populations usually encountered between a central population
where the species initially established in the introduced range and an
edge population where the invasive plant has established recently. While
the central population shares a longer co-evolutionary history with
co-occurring competitors (old competitors), the edge population recently
co-occurs with its competitors at the expansion front (new competitors)
(Dematteis et al., 2020; Halbritter et al., 2015; Tabassum & Leishman,
2020). The differentiation on growth and competitiveness among invasive
populations since time of establishment can significantly affect the
competition dynamic with their competitors and determine invaders
further expansion (Farooq et al., 2017; Kilkenny & Galloway, 2016). For
example, compared with the edge population, the central population of an
invasive plant usually needs to deal with the strong competition caused
by the denser number of individuals due to the longer time since
establishment, so the central population often tends to be more
competitive (Phillips et al., 2010; Shine et al., 2011). This
differentiation may be driven by evolutionary processes (such as spatial
sorting, natural selection, gene flow, mutation, and genetic drift /
gene surfing) as well as ecological processes (such as demography,
dispersal, expansion speed and shape, and expansion variability), and
could be modified by landscape features, trait genetics and biotic
interactions (Miller et al., 2020).
The new and old competitors interact differently with the invasive plant
along the expansion range (Callaway & Aschehoug, 2000; Sun et al.,
2013; Sun & He, 2018), and the outcome of their interactions usually
brings idiosyncratic and mixed results. In general, new competitors tend
to inhibit the growth of invasive species more significantly than old
competitors (Alexander et al., 2015; Sun & He, 2018). Unlike new
competitors, the co-evolution between an older competitor and the
invader seems lead to a gradual development from competition to
coexistence (Huang et al., 2018; Lankau, 2013; Sheppard & Schurr,
2019). Other studies show that invasive species might have an initial
advantage over their competitors, but over time, this advantage
decreases and competitors may limit the performance of invaders (Oduor,
2013; Saul & Jeschke, 2015; Sheppard & Schurr, 2019). Although the
study of pairwise interactions between invasive plants and different
competitors is relatively common across the invasion biology literature,
there is a lack of understanding on whether old and new competitors have
different effects on the response of invasive plants.
Different competitors might have contrasting effects on the response of
invasive plants to elevated nitrogen. Many studies have shown that
invasive plants use nitrogen more efficiently than native species, with
responding more quickly and greatly to the increase of nitrogen
deposition (Parepa et al., 2019; Yu et al., 2020). This differential
response could lead to a predicted enhanced expansion rate of invasive
plants (Liu et al., 2017b; Liu et al., 2019). However, there is no
guarantee that invasive plants will succeed in the future, as their
success will also depend on the outcome of the competition with their
competitors. Some studies have shown that, compared to native species,
invasive plants usually maintain a larger magnitude of response to
increased nitrogen deposition and increased relative competitive
advantage (Eller & Oliveira, 2017; Qin et al., 2018; Valliere et al.,
2017). Other studies suggest that the presence of competitors
significantly reduces the response magnitude of invasive species to
increased nitrogen deposition, and that some native species even gain a
higher competitive advantage over invaders as nitrogen availability
increases (Eskelinen & Harrison, 2014; Luo et al., 2014; Wang et al.,
2017). Thus, the competition outcome between invasive plants and their
competitors under elevated nitrogen deposition is still context
dependent and needs further clarification.
Galinsoga
quadriradiata is an annual invasive forb of Asteraceae, which is native
to tropical America (Du et al., 2014; He et al., 2020; Liu et al.,
2016b). This invasive plant has strong adaptability and has been widely
distributed in most areas of subtropical, temperate and warm temperate
zones around the world, mainly growing in farmland, abandoned land,
roadsides, grasslands and other habitats with frequent human disturbance
(Kabuce & Priede, 2010; Liu et al., 2016b; Liu et al., 2021b; Zhang et
al., 2022). It was first introduced into East China (Jiangxi Province)
around the beginning of the last century (earlier than 1943) (Liu et
al., 2018; Liu et al., 2016b; Yang et al., 2018). It then gradually
spread and established in the Southwest, Central, and North China (Yang
et al., 2018). Galinsoga quadriradiata arrived at the southern
slope of Qinling Mountains, where
our study takes place, in Shaanxi Province about 10 years ago (Liu et
al., 2016b; Liu et al., 2021b). This invasive plant impacts natural
ecosystems and reduce crop production by about 50% when under high
invasion (Kabuce & Priede, 2010; Liu et al., 2021a). Whereas its
impacts are well understood, the mechanisms of invasion and the
predicted expansion under increasing nitrogen deposition, an ongoing
scenario in China, are still far from been fully uncovered. In this
study, we conducted a common garden experiment to detect the differences
in response between central and edge populations ofG. quadriradiata to their
competitors with different co-evolutionary histories in the context of
increased soil nitrogen deposition. We hypothesized that: (1) the
central population of G. quadriradiata would display higher
competitive ability and growth when compared to the edge population; (2)
nitrogen addition will increase the growth and reproductive performance
of both populations, but the central population will have a stronger
response to increases in nitrogen deposition than the edge population;
(3) the competition outcome in the context of increased nitrogen will
depend on the co-evolutionary history between G. quadriradiataand its competitors with the new competitors showing a stronger
inhibition on the invader’s response under high nitrogen, compared to
the old competitors.
MATERIALS AND METHODS
Greenhouse experimental design
We chose Glycine max , Zea mays , and Glebionis
coronaria , as old competitors which have longer co-evolutionary history
with G. quadriradiata ; while Medicago sativa ,Achnatherum splendens , Sorghum bicolor , Sonchus
wightianu , and Artemisia capillaris , as the new competitors.
These competitors belong to Poaceae, Fabaceae and Asteraceae
respectively. They are common species in the distribution range ofG. quadriradiata , and they are also the main competitors. Since
the invasive plant commonly colonize in crop lands, we chose three
common crop species as competitors. The new competitor species are
commonly distributed in north and west China, while the old competitors
are found in central and east China. We chose the species based on field
surveys prior to the beginning of this experiment (Liu et al., 2021a;
Liu et al., 2016b; Yang et al., 2018). The seeds of the central
population ofG.quadriradiata and the old competitors were collected from four
sampling sites in Lushan, Jiangxi Province in the Fall of 2017, while
the seeds of the edge population were collected from three sampling
sites (Fig. 1, Table S1) at different elevations in Qinling Mountains,
Shaanxi Province in the Fall of 2015. All collected seeds were stored in
a 4 °C refrigerator until set to germinate. The seeds of the new
competitors were purchased in the market prior to the beginning of the
experiment in 2018.
In early May 2018, the seeds of all species were sowed in nursery pots
filled with sterile substrate and they were let to germinate in an
artificial climate chamber. Four weeks later, the seedlings (about 5 cm
high) were transplanted into plastic pots (diameter 16 cm, height 14
cm), and moved into the greenhouse. The pots were filled with sand and
soil in a 1:1 proportion by volume. To compare the inter-specific
competition intensity, the seedlings were planted in isolation
(single-culture) or together with a native competitor (mixed-culture).
In the single-culture treatment, one seedling of the central population
or edge population of G. quadriradiata was transplanted into a
pot. In the mixed-culture treatment, one seedling of G.
quadriradiata and one seedling of a new or old competitor were planted
in the same pot. Seedlings from each population of G.
quadriradiata were planted with all the other competitors,
respectively. After transplanting, all potted plants were subjected to
nitrogen treatments in a full factorial design. Each pot was watered
with 5 ml deionized water (ambient nitrogen) or 3 g
L-1 NH4NO3 solution
(low nitrogen addition) or 6 g L-1NH4NO3 solution (high nitrogen addition)
monthly. The low nitrogen treatment
was set to simulate the current3
nitrogen deposition rate in China
(Liu et al., 2013; Zhao et al., 2017) while the high nitrogen addition
refers to double the current nitrogen deposition rate. For each
combination of treatment, there were 10 replications for the
single-culture and 8 replications for the mixed-culture. All pots were
randomly arranged in an 80 m2 greenhouse to avoid the
influence of herbivores, and watered every two days to keep the soil
wet. The experiment had in total 888 pots and 1656 individuals planted.
From July to August 2018, the three uppermost fully expanded young
leaves of each of three randomly selected individuals from each
treatment were chosen for photosynthetic measurements. Photosynthesis
was measured under sunny conditions at a sequence of light levels (1200,
1000, 800, 600, 400, 200, 100, 60, 20, 0
μmol
m-2s-1 PPFD) using a Li-6800
Portable Photosynthesis System with a Red/Blue LED Light Source (Li-Cor,
Lincoln, NE, USA). Light curves and
maximum
net photosynthetic rate (P max, μmol
CO2 m-2 leaf area
s-1) were calculated by a mechanistic model for the
photosynthesis–light response based on the photosynthetic electron
transport of photosystem II in C3 and C4species (Ye et al., 2013).
In early September 2018, all parts of the plants were harvested. The
fresh leaf area was measured immediately after harvest by leaf analysis
system WinFOLIA Pro (WinFOLIATM, Régent Instruments
Inc., Québec, QC, Canada). The number of capitula per plant was counted
before the harvesting as a metric of species performance. The dry weight
of the root, leaf, capitula, and stem of each individual was separated
and weighted after the samples had been dried to a constant weight in an
oven at 60 °C. Elemental analysis for leaf phosphorus concentration
(LPC) and leaf nitrogen concentration (LNC) was performed on a
continuous flow analyzer (SEAL Auto Analyzer III, Germany). Specific
leaf area (SLA) was calculated as the ratio of the leaf area to the leaf
dry weight. For each treatment, 0.5 g leaf powder was burned in a muffle
furnace at 600 °C for 6 hours to measure an average ash content. The ash
content was calculated as ash mass divided by the sample mass. The leaf
caloric value was measured by a calorimeter (IKA-C6000, IKA, Germany)
with 0.5 g leaf powder from each sample. The SLA, ash content, leaf
caloric value, LNC obtained were used to calculate the
leaf construction cost per unit of
mass (CCmass) following Liu et al. (Liu et al.,
2016a; Shen et al., 2011). The CCmass was calculated
using the ash content, leaf caloric value, SLA, and LNC. Root mass ratio
(RMR) was calculated as the ratio of root dry mass and total dry mass.
The relative competition index (RCI), which represents the intensity of
interspecific competition betweenG. quadriradiata and competitors, was calculated according to the
following equation (Vilà & Weiner, 2004):
RCI = (M 1 –M 2) / M 1 (1)
Where, M 1 is the total dry mass of G.
quadriradiata under single-culture conditions.M 2 is the total dry mass of G.
quadriradiata in the mixed-culture conditions. The RCI
values were generally positive,
and the higher RCI value the higher intensity of interspecific
competition.
Data analyses
A generalized linear mixed model (GLMM) was performed to evaluate the
effects of the nitrogen treatments (NTC), invasive plant population
types (PopType), culture types (Culture), and competitor types
(CompType, nested in Culture) on the total mass, the number of capitula
per plant, RMR, LPC, LNC, or P max. The effects of
NTC, PopType, and CompType on the
RCI or CCmass were also evaluated by GLMM.
All analyses were conducted using SAS 9.3 (SAS Institute Inc., Cary, NC,
USA). The map with the sampling locations was made in ArcGIS version
10.2.2 (Environmental Systems Research Institute, Inc.), and other
figures were made by SigmaPlot version 11.0 (Systat Software, Inc., San
Jose, CA, USA).
RESULTS
Growth performance of G. quadriradiata
The result of the GLMM revealed that NTC (nitrogen addition), PopType
(central vs. edge population), Culture (single vs mixed), and CompType
(old vs. new competitors; nested in Culture) all had significant effects
on plant total mass, and the interactions between CompType and PopType
were also significant (Table 1). In general, the total mass ofG.
quadriradiata increased with the nitrogen addition. The central
population of G. quadriradiata obtained significantly higher
total mass than the edge population. The total mass ofG. quadriradiata in the
mixed-culture was significantly lower than that in the single-culture
(Fig. 2). And the old competitors inhibited more the total mass of the
central population when compared to the new competitors (Figs 2, S1).