The effects of changes in water and nitrogen availability on alien plant invasion into a stand of a native grassland species.

TL;DR: Testing whether high water availability, nitrogen (N) enrichment and their interaction promote performance of three invasive alien plants when competing with a native Chinese grassland species indicates that future changes in water availability and N enrichment may affect the invasion success of different alien species differently.

Abstract: Plant invasions are a major component of global change, but they may be affected by other global change components. Here we used a mesocosm-pot experiment to test whether high water availability, nitrogen (N) enrichment and their interaction promote performance of three invasive alien plants (Lepidium virginicum, Lolium perenne and Medicago sativa) when competing with a native Chinese grassland species (Agropyron cristatum). Single plants of the three invasive and the one native species were grown in the center of pots with a matrix of the native A. cristatum under low, intermediate or high water availability and low or high N availability. The invasive species L. virginicum and M. sativa grew larger, and produced a higher biomass relative to competitors than the native species A. cristatum did. Increasing water availability promoted biomass production of all species, but water availability did not change the biomass of the central plants relative to that of the competitors. Nitrogen addition also increased biomass production of all species, and it increased the biomass of the central plants more so than that of the competitors. The positive effect of N addition on the biomass of the central plants relative to that of the competitors increased with increasing water availability. However, compared to central plants of the native species, the positive effect of N addition on the relative biomass of L. virginicum decreased when water availability increased. These interactions indicate that future changes in water availability and N enrichment may affect the invasion success of different alien species differently.

Summary

Introduction

• Owing to the increasing influence of human activities, at least 3.9% of species in the global vascular flora have established naturalized populations in regions where they did not naturally occur (van Kleunen et al. 2015a; Pyšek et al. 2017) .
• Therefore, understanding the competition between invasive alien and native plants and assessing the potential invasion risk of naturalized alien plants are hot topics in ecology.
• With global climate change, precipitation is likely to increase in some regions and decrease in other regions (Naz et al. 2016) .
• In temperate grasslands, the effect of soil N on grassland productivity and composition usually depends on soil-water availability (Harpole et al.

Study location and species

• To test interactions between the effects of water availability and N enrichment on alien plant invasion into a stand of a native grassland species, the authors did a mesocosm-pot experiment at the field station (40°16 N, 115°36 E) in Huailai of China, which belongs to the State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, China.
• During the 30 years , the mean annual temperature and precipitation of Huailai are 9.6 °C and 370 mm, respectively (http://data.cma.cn).
• To test whether the invasive alien species differ in their responses to water availability and N enrichment, the authors chose three invasive alien species as invaders based on the database of invasive alien species in China (http://www.china ias.cn): L. perenne , L. virginicum and M. sativa .
• Both L. perenne and M. sativa are perennial herbs, and were introduced from Europe to China.
• Seeds of L. perenne and L. virginicum were bulk sampled in four different natural populations, and within each populations, seeds were collected from at least ten individuals.

Experimental set-up

• To compare the growth performance of invasive alien and native plants when growing in a stand of the native grassland species under different water and N conditions, the authors did a full factorial experiment.
• The authors grew each of the three invasive alien plant species (L. virginicum, L. perenne and M. sativa) in the center of a matrix of the native species (A. cristatum) under three different water availabilities (low, intermediate and high) and two N conditions (low and high).
• The authors used a white plastic roof to intercept the rainfall when it was raining, and removed it when it was not raining.
• The authors checked the germination in each pot every day until there were no new seedlings germinating from the soil.
• For the low and high nutrient treatments, the authors used N concentrations of 2 and 6 mmol, respectively.

Statistical analysis

• To test the effects of water availability and N enrichment on performance of the three alien and the one native target species in the stand of the native grassland species, the authors fitted linear models using the lm function in R 3.3.2 (R Core Team 2016).
• Aboveground biomass production of the target species, aboveground biomass production of the native competitor, total aboveground biomass per pot and aboveground biomass proportion of the target species (i.e., target biomass/total biomass) were the response variables.
• The authors included water treatment (i.e., low, intermediate and high water availability), N treatment (i.e., low and high availability), target species identity and their two-way and three-way interactions as explanatory variables in the model.
• In the linear model described above, the authors assessed the significance of all variables and their interactions with likelihood-ratio tests (Zuur et al. 2009 ; for details, see Online Resource 3) using the lr test function in R 3.3.2 (R Core Team 2016).
• The outputs (including model estimates) of all linear models are presented in Online Resource 4.

Results

• The invasive alien target species L. perenne and the native target species A. cristatum did not significantly differ in aboveground biomass (Table 1 and Fig. 1a ), proportion biomass (Table 1 and Fig. 1d ), and their effects on biomass of the competitor plants and total biomass per pot (Table 1 ; Figs. 1b, 1c ).
• The two other invasive alien target species, L. virginicum and M. sativa, produced more biomass than the native target species A. cristatum (Table 1 and Fig. 1a ).
• This was not only because these two invasive alien target species produced more biomass than the native target species (Table 1 and Fig. 1a ), but also partly because the alien target species M. sativa had a positive effect on the biomass of the native competitor plants (Table 1 and Fig. 1b ).
• The biomass proportions of L. Table 1 Results of linear models testing the effects of water availability (low, intermediate and high), nitrogen addition (low and high), target species identity and all interactions, thereof on biomass produc-tion and biomass proportion of target species, total biomass per pot and biomass production of the native competitor Agropyron cristatum.

Effects of water availability

• An increase in water availability significantly promoted biomass production of the three invasive alien and the one native target species (Table 1 and Fig. 1a ).
• Compared to the native target species A. cristatum, the invasive alien species L. perenne increased its biomass more strongly in response to an increase in water availability, whereas the invasive alien species L. virginicum and M. sativa showed similar biomass increases (Table 1 and Fig. 1a ).
• An increase in water availability also significantly enhanced the biomass production of the native competitors (Table 1 and Fig. 1b ) and thus the total biomass production per pot (Table 1 and Fig. 1c ).
• In pots with the invasive alien species L. virginicum, the increase in total biomass production was stronger than in the pots with the native target species A. cristatum (Table 1 and Fig. 1c ).
• The proportional biomass of the target species, however, was not affected by the watering treatments (Table 1 and Fig. 1d ).

Effects of N addition

• As the target biomass increased more strongly than the competitor biomass, the proportion biomass of the target species was also increased by N addition (Table 1 , Fig. 1d ).
• Effects of N addition for pots with the invasive alien target species L. perenne and M. sativa were similar to those with the native target species A. cristatum (Table 1 and Fig. 1c ).
• The positive effects of N addition on target plant biomass (Table 1 and Fig. 1a ), biomass proportion of the target plant (Table 1 and Fig. 1d ) and total biomass (Table 1 and Fig. 1c ) were stronger when the target plant was L. virginicum than when it was the native A. cristatum.

Interactive effects between water availability and N addition

• The biomass production of the native competitors increased more strongly in response to N addition when the water availability also increased (Table 1 and Fig. 1b ).
• Nevertheless, the biomass proportion of the target species increased significantly more in response to N addition when water availability also increased (Table 1 and Fig. 1d ).
• The invasive alien species L. perenne and M. sativa showed similar responses of biomass proportion as the native A. cristata.
• The invasive alien species L. virginicum in contrast to native A. cristata showed the strongest response to nutrient addition under low water availability (Table 1 and Fig. 1d ).

Discussion

• Here the authors tested how water availability and changes in nutrient availability might affect performance of three invasive alien species into a stand of the native grass A. cristatum.
• Averaged across the four target species and the three water availabilities, N addition also increased biomass production of the plants, but more so for the target plants than for the competitors, as the biomass proportion of the target species increased.
• Invasive alien plant species often show higher values than native plants for traits reflecting physiology, size and fitness (van Kleunen et al. 2010) , and hence invasive plants often appear to be more competitive than native species (Kuebbing and Nunez 2016).
• The effects of soil N and water availability on ecosystem processes are generally interdependent (Harpole et al.
• The positive effect of N addition on the biomass proportion of the target species increased with increasing water availability.the authors.

Conclusions

• Like many previous studies, the authors found strong evidence that soil-water availability and soil N are both major limiting factors for the productivity of temperate grasslands.
• As their study only simulated plant invasion of three invasive alien plant species into a stand of one native grassland species, and the experimental duration was relatively short, the authors cannot generalize from their results.
• More multi-species studies and long-term field studies are needed to test for more general patterns.
• Nevertheless, the authors found that performance of some invasive alien species in stands of native species can be affected by interactions between multiple global change components.

Figures

Fig. 1 a Mean values of biomass of invasive alien (Lepidium virginicum, Lolium perenne, Medicago sativa) and native (Agropyron cristatum) target species, b biomass proportion of target species, c total biomass per pot d and biomass of the native competitor A. cristatum under different water (W) and N availabilities. Error bars represent SEs of the means. Individual points indicate the values for the replicates per species under each treatment combination. Data plotted in the figure are transformed

Table 1 Results of linear models testing the effects of water availability (low, intermediate and high), nitrogen addition (low and high), target species identity and all interactions, thereof on biomass production and biomass proportion of target species, total biomass per pot and biomass production of the native competitor Agropyron cristatum

Full text

The eects ofchanges inwater andnitrogen availability onalien plant
invasion intoastand ofanative grassland species
YanjieLiu
1,2
· MinLiu
1,3
· XingliangXu
1
· YuqiangTian
4
· ZhenZhang
5
· MarkvanKleunen
2,6
Abstract
Plant invasions are a major component of global change, but they may be affected by other global change components. Here
we used a mesocosm-pot experiment to test whether high water availability, nitrogen (N) enrichment and their interaction
promote performance of three invasive alien plants (Lepidium virginicum, Lolium perenne and Medicago sativa) when
competing with a native Chinese grassland species (Agropyron cristatum). Single plants of the three invasive and the one
native species were grown in the center of pots with a matrix of the native A. cristatum under low, intermediate or high
water availability and low or high N availability. The invasive species L. virginicum and M. sativa grew larger, and produced
a higher biomass relative to competitors than the native species A. cristatum did. Increasing water availability promoted
biomass production of all species, but water availability did not change the biomass of the central plants relative to that of
the competitors. Nitrogen addition also increased biomass production of all species, and it increased the biomass of the
central plants more so than that of the competitors. The positive effect of N addition on the biomass of the central plants
relative to that of the competitors increased with increasing water availability. However, compared to central plants of the
native species, the positive effect of N addition on the relative biomass of L. virginicum decreased when water availability
increased. These interactions indicate that future changes in water availability and N enrichment may affect the invasion
success of different alien species differently.
Keywords Exotic· Global change· Non-native· Plant invasion· Plant–plant interaction
Introduction
Owing to the increasing influence of human activities, at
least 3.9% of species in the global vascular flora have estab-
lished naturalized populations in regions where they did
not naturally occur (van Kleunen etal.
2015a; Pyšek etal.
2017). It is very likely that the number of these naturalized
Communicated by Wayne Dawson.
Yanjie Liu and Min Liu contributed equally to this work.
* Yanjie Liu
yanjie.liu@uni-konstanz.de
* Xingliang Xu
xuxingl@hotmail.com
1
Key Laboratory ofEcosystem Network Observation
andModeling, Institute ofGeographic Sciences andNatural
Resources Research, Chinese Academy ofSciences, 11A
Datun Road, Chaoyang District, Beijing100101, China
2
Ecology, Department ofBiology, University ofKonstanz,
Universitätsstraße 10, 78457Konstanz, Germany
3
University ofChinese Academy ofSciences, Beijing100049,
China
4
State Key Laboratory ofEarth Surface Processes
andResource Ecology, Center forHuman-Environment
System Sustainability (CHESS), Beijing Normal University,
Haidian District, Beijing100875, China
5
School ofResources andEnvironment, Anhui Agricultural
University, NO. 130 Changjiang West Road, Hefei230036,
China
6
Zhejiang Provincial Key Laboratory ofPlant Evolutionary
Ecology andConservation, Taizhou University,
Taizhou318000, China
Konstanzer Online-Publikations-System (KOPS)
URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-ygvz7yjisif6
Erschienen in: Oecologia ; 188 (2018), 2. - S. 441-450
https://dx.doi.org/10.1007/s00442-018-4216-1

442
plant species will continue to increase in the future (See-
bens etal.
2015, 2017). Some naturalized plant species can
successfully spread and occupy large areas in high abun-
dances in the introduced ranges (i.e., become invasive sensu
Richardson etal.
2000). The human-caused introduction,
naturalization and subsequent spread of alien plant species
into new regions has become a major component of global
environmental change, and one of the defining character-
istics of the Anthropocene (Lewis and Maslin
2015). The
invasive plant species frequently have higher values than
native plants for traits reflecting physiology, size and fit-
ness (van Kleunen etal.
2010). Consequently, it is generally
thought that invasive alien plant species outperform and dis-
place native plants, and thus threaten native diversity, dis-
rupt ecosystem functions and services (Vitousek etal.
1996;
Vilà etal.
2011). Therefore, understanding the competition
between invasive alien and native plants and assessing the
potential invasion risk of naturalized alien plants are hot
topics in ecology.
As a major component of global environmental change,
plant invasions are also likely to interact with other global
change components (Bradley etal.
2010a). For example,
a recent meta-analysis suggests that different components
of global environmental change can promote the growth
of alien plants mainly via an increase in growth rate and
size (Jia etal.
2016). Furthermore, our recent meta-analysis
comparing the growth performance responses to global
environmental changes between 74 invasive alien and 117
native plants has shown that invasive alien species benefitted
significantly more from increased atmospheric CO
2
levels
and temperatures than native species, and also tended to ben-
efit more from nutrient addition (Liu etal.
2017). However,
both meta-analyses point out that most studies only tested
how plant invasions interact with individual components of
global environmental change, whereas the interacting effects
of multiple global change components on invasions remain
unclear (Jia etal.
2016; Liu etal. 2017).
Global environmental change has a strong effect on
plant growth and community structure in grasslands
(Shaw etal.
2002; Zavaleta etal. 2003; Keller etal. 2014).
Temperate grasslands are widely distributed across the
Eurasian continent and known as “the Eurasian steppes”
(Bredenkamp etal.
2002). Such grasslands constitute
the main habitat in northern China. In this region, low
water availability is the major factor limiting grassland
productivity. With global climate change, precipitation is
likely to increase in some regions and decrease in other
regions (Naz etal.
2016). Moreover, there are some uncer-
tainties about future precipitation levels, and it is likely
that the frequencies of extremely dry and wet years will
increase (IPCC
2013). The changes in water availability
could affect competition between alien and native plants
(Liu etal.
2017; Pearson etal. 2017). For example, wet
condition may favor (Bradley etal.
2010b), and drought
condition may inhibit invasive plant species (Liu etal.
2017) more than native plant species. However, it has not
yet been explored how water availability changes interact
with alien plant invasion in temperate grasslands.
As a major component of global environmental change,
nitrogen (N) deposition is likely to further increase soil N
availability in many terrestrial ecosystems worldwide (Hol-
land etal.
2005; Phoenix etal. 2006; Liu etal. 2013). It
might affect the productivity of temperate grasslands by
changing plant–plant interactions, because N is another
important factor limiting the productivity of grasslands
(LeBauer and Treseder
2008). Invasive plant species may
outperform native plant species under conditions of low
nutrient availability (Funk and Vitousek
2007). However,
successful alien plant species are often associated with a
particular suite of traits that enable them to respond more
positively to increased N availability. (Dawson etal.
2012;
Keser etal.
2014, 2015; Liu and van Kleunen 2017). Con-
sequently, it is frequently suggested that invasive plants are
more successful and outperform the native plants in areas
with high N deposition (Scherer-Lorenzen etal.
2000, 2007;
González etal.
2010). Therefore, it is important to test how
N deposition interacts with alien plant invasion in the tem-
perate grasslands of China.
Various components of global environmental change may
occur simultaneously, and these changes may additively or
interactively impact plant performance (Dukes etal.
2005;
Bloor etal.
2010). In temperate grasslands, the effect of
soil N on grassland productivity and composition usually
depends on soil–water availability (Harpole etal.
2007; Bai
etal.
2008; Lu and Han 2010; Li etal. 2011). As water
can enhance N delivery to the root surface, the effects of N
addition on grassland productivity might become stronger
with increasing water availability (Bai etal.
2008; Lu and
Han
2010). As it remains poorly understood how these two
environmental factors interact in their effects on alien plant
invasion (Jia etal.
2016; Liu etal. 2017), we performed
a mesocosm-pot experiment to test whether N enrichment
could interact with water availability to promote further
invasion of invasive alien plant species into a stand of the
native grassland species Agropyron cristatum. We grew sin-
gle plants of three invasive alien plant species (Lepidium
virginicum, Lolium perenne and Medicago sativa) and one
native species (A. cristatum) in a matrix of the native spe-
cies under three water conditions (low, intermediate and
high) and two N conditions (low and high). We compared
the responses in biomass production of the studied species
and how these changes relative to the biomass of the com-
petitor to address how invasion of the three alien species
into a stand of the native grassland species is affected by
(1) changes in water availability, (2) N addition and (3) the
interaction between water availability and N addition.

443
Materials andmethods
Study location andspecies
To test interactions between the effects of water availabil-
ity and N enrichment on alien plant invasion into a stand of
a native grassland species, we did a mesocosm-pot experi-
ment at the field station (40°16ƍN, 115°36ƍE) in Huailai of
China, which belongs to the State Key Laboratory of Earth
Surface Processes and Resource Ecology, Beijing Normal
University, China. During the 30years (1971–2000), the
mean annual temperature and precipitation of Huailai are
9.6°C and 370mm, respectively (
http://data.cma.cn).
The average temperature and precipitation in the warmest
month (August) are about 23.1°C and 77.3mm, respec-
tively (
http://data.cma.cn). To create a stand of a temper-
ate native grassland species, we chose the perennial grass
species A. cristatum as a representative native species. It
commonly occurs in Inner Mongolia grasslands, and is
dominant in many places (Li etal.
2016). To test whether
the invasive alien species differ in their responses to water
availability and N enrichment, we chose three invasive
alien species as invaders based on the database of inva-
sive alien species in China (
http://www.china ias.cn): L.
perenne (Poaceae), L. virginicum (Brassicaceae) and M.
sativa (Leguminosae). Both L. perenne and M. sativa
are perennial herbs, and were introduced from Europe to
China. L. virginicum is an annual herb, and was introduced
from North America to China. All three invasive alien spe-
cies occur frequently in Inner Mongolia grasslands. Seeds
of L. perenne and L. virginicum were bulk sampled in four
different natural populations, and within each populations,
seeds were collected from at least ten individuals. Seeds
of M. sativa and A. cristatum were acquired from Ulanqab
Grassland Station (Inner Mongolia, China).
Experimental set-up
To compare the growth performance of invasive alien and
native plants when growing in a stand of the native grassland
species under different water and N conditions, we did a full
factorial experiment. In this experiment, we grew each of the
three invasive alien plant species (L. virginicum, L. perenne
and M. sativa) in the center of a matrix of the native species
(A. cristatum) under three different water availabilities (low,
intermediate and high) and two N conditions (low and high).
To compare whether the different water conditions and N
conditions also affect the native species itself, we also grew
the native species in the center of the same matrix of con-
specific native species as a control. The experiment started
on 13 July and ended on 6 September 2016.
We first filled 2 L pots with a 1:1 mixture of sand and
vermiculite. To create a stand of the studied native grass-
land species, we sowed 15–20 seeds of A. cristatum in a
circle around the center of each pot (diameter = 15cm).
We also sowed 3–5 seeds of the native species or one of
the three invasive alien plant species in the center of each
pot to simulate plant invasions. After this, we randomly
assigned the pots to positions in a common garden. We
used a white plastic roof to intercept the rainfall when it
was raining, and removed it when it was not raining. To
ensure that the seeds would germinate well, we watered
the soil to saturation every day at nightfall. Ten days after
sowing, we applied the N treatment for the first time (see
below for details). We checked the germination in each pot
every day until there were no new seedlings germinating
from the soil. On the 4th of August (i.e., 20days after sow-
ing), we thinned the seedlings to have one individual of
the native or alien species in the center of the pot, and we
thinned the native competitors to five individuals, so that
they were positioned at equal distances in a circle around
the central plant.
Twenty days after sowing, we applied the N treatment
for the second time and started the water treatments. We
applied the low and high soil N conditions using a modified
Hoagland nutrient solution (see Online Resource 1). The
two nutrient solutions differed in the concentration of N,
but contained the same concentrations of the other nutrients.
To each pot, we supplied 100mL of nutrient solution once
every 10days. For the low and high nutrient treatments, we
used N concentrations of 2 and 6mmol, respectively. For the
water treatments, we supplied 200ml of water for the high
treatment and 120ml of water for the intermediate treatment
every 2–3days. In the low treatment, we daily checked all
pots and supplied 100ml of water to these pots if plants had
started to wilt (i.e., lost leaf turgor). As the nutrients were
supplied in liquid form, we did not apply the water treat-
ments on the day when the nutrient solutions were supplied.
We replicated each treatment combination five times, result-
ing in 120 pots (4 species [3 invasive species and 1 native
species] × 3 water treatments × 2N treatments × 5 replicates).
Eight weeks after the start of the water treatments, we
separately harvested the aboveground biomass of the plant
in the center of the pots (i.e., the target species) and the five
individuals of A. cristatum around the center (i.e., the native
competitor). As some of the central plants died during the
experiment, we only harvested 110 pots at the end of the
experiment. The harvest was finished in one day. All above-
ground biomass was dried for at least 72h at 80°C, and then
weighed. Based on the final aboveground biomass, we cal-
culated the total biomass per pot (biomass of the target spe-
cies + biomass of the five native competitor plants) and the
biomass proportion of the target species (i.e., the biomass
of the target species divided by the total biomass per pot).

444
Statistical analysis
To test the effects of water availability and N enrichment
on performance of the three alien and the one native target
species in the stand of the native grassland species, we fit-
ted linear models using the lm function in R 3.3.2 (R Core
Team
2016). Aboveground biomass production of the tar-
get species, aboveground biomass production of the native
competitor, total aboveground biomass per pot and above-
ground biomass proportion of the target species (i.e., target
biomass/total biomass) were the response variables. To meet
the assumption of normality, biomass proportion of the tar-
get species, total biomass per pot and biomass production
of the native competitor were square-root transformed, and
the biomass production of the target species was natural-
log transformed. We included water treatment (i.e., low,
intermediate and high water availability), N treatment (i.e.,
low and high availability), target species identity and their
two-way and three-way interactions as explanatory variables
in the model. As we wanted to test the effects of the treat-
ments on each alien target plant species versus the native
target plant species, we coded the target species factor as
three dummy variables T
Lp
(i.e., L. perenne vs A. cristatum),
T
Lv
(i.e., L. virginicum vs A. cristatum) and T
Ms
(i.e., M.
sativa vs A. cristatum) to obtain the different contrasts of
interest (Online Resource 2; Schielzeth
2010). In the linear
model described above, we assessed the significance of all
variables and their interactions with likelihood-ratio tests
(Zuur etal.
2009; for details, see Online Resource 3) using
the lr test function in R 3.3.2 (R Core Team
2016). The
outputs (including model estimates) of all linear models are
presented in Online Resource 4.
Results
The invasive alien target species L. perenne and the native
target species A. cristatum did not significantly differ in
aboveground biomass (Table
1 and Fig.1a), proportion bio-
mass (Table
1 and Fig.1d), and their effects on biomass of
the competitor plants and total biomass per pot (Table
1;
Figs.
1b, 1c). However, the two other invasive alien target
species, L. virginicum and M. sativa, produced more bio-
mass than the native target species A. cristatum (Table
1
and Fig.
1a). In addition, the total biomass per pot was also
higher when the target species were either the invasive alien
species L. virginicum or M. sativa rather than the native A.
cristatum (Table
1 and Fig.1c). This was not only because
these two invasive alien target species produced more bio-
mass than the native target species (Table
1 and Fig.1a),
but also partly because the alien target species M. sativa
had a positive effect on the biomass of the native competitor
plants (Table
1 and Fig.1b). The biomass proportions of L.
Table 1 Results of linear models testing the effects of water availabil-
ity (low, intermediate and high), nitrogen addition (low and high), tar-
get species identity and all interactions, thereof on biomass produc-
tion and biomass proportion of target species, total biomass per pot
and biomass production of the native competitor Agropyron cristatum
The effect of target species was included as three contrasts (T
Lv
, T
Lp
, T
Ms
), each comparing one of the alien target species (Lepidium virginicum,
Lolium perenne, Medicago sativa) to the native target species (A. cristatum)
Parameters Biomass produc-
tion of target spe-
cies (ln)
Biomass produc-
tion of native
competitor (sqrt)
Total biomass per
pot (sqrt)
Biomass proportion
of target species
(sqrt)
df χ
2
Pχ
2
Pχ
2
Pχ
2
P
Nitrogen addition (N) 1 28.303 <0.0001 3.9202 0.0477 15.312 <0.0001 19.42 <0.0001
Water availability (W) 2 65.197 <0.0001 127.42 <0.0001 125.97 <0.0001 0.1038 0.9494
Lepidium virginicum vs Agropyron cristatum (T
Lv
) 1 13.386 0.0002 0.049 0.8248 7.1516 0.0075 17.379 <0.0001
Lolium perenne vs Agropyron cristatum (T
Lp
) 1 0.6831 0.4085 0.1501 0.6984 0.5593 0.4545 0.5667 0.4516
Medicago sativa vs Agropyron cristatum (T
Ms
) 1 14.707 0.0001 6.0879 0.0136 10.26 0.0014 7.3681 0.0066
N:W 2 2.0687 0.3554 8.6614 0.0132 4.8317 0.0893 6.4177 0.0404
N:T
Lv
1 12.298 0.0005 1.2367 0.2661 10.223 0.0014 10.373 0.0013
N:T
Lp
1 0.319 0.5722 1.143 0.2850 0.9219 0.3370 0.8968 0.3436
N:T
Ms
1 0.0019 0.9650 0.2442 0.6212 0.3638 0.5464 0.1095 0.7407
W:T
Lv
2 2.9453 0.2293 6.7752 0.0338 11.138 0.0038 0.8873 0.6417
W:T
Lp
2 6.7807 0.0337 2.0637 0.3563 4.2801 0.1177 4.6043 0.1000
W:T
Ms
2 4.3071 0.1161 3.7848 0.1507 5.2536 0.0723 5.3851 0.0677
N:W:T
Lv
2 3.378 0.1847 2.8366 0.2421 3.7061 0.1568 7.423 0.0244
N:W:T
Lp
2 3.2979 0.1922 0.2861 0.8667 0.9104 0.6343 3.375 0.1850
N:W:T
Ms
2 0.9329 0.6272 1.4075 0.4947 1.4895 0.4748 0.4495 0.7987

445
virginicum and M. sativa were nevertheless still higher than
that of the native target species (Table
1 and Fig.1d).
Eects ofwater availability
An increase in water availability significantly promoted
biomass production of the three invasive alien and the one
native target species (Table
1 and Fig.1a). Compared to
the native target species A. cristatum, the invasive alien
species L. perenne increased its biomass more strongly in
response to an increase in water availability, whereas the
invasive alien species L. virginicum and M. sativa showed
similar biomass increases (Table
1 and Fig.1a). An increase
in water availability also significantly enhanced the biomass
production of the native competitors (Table
1 and Fig.1b)
and thus the total biomass production per pot (Table
1 and
Fig.
1c). In pots with the invasive alien species L. virgini-
cum, the increase in total biomass production was stronger
than in the pots with the native target species A. cristatum
(Table
1 and Fig.1c). Compared to the native target species
A. cristatum, the invasive alien target species L. virginicum
had a negative effect on biomass of the native competitors
under low water availability, but a positive effect under inter-
mediate and high water availability (Table
1 and Fig.1b).
The proportional biomass of the target species, however, was
not affected by the watering treatments (Table
1 and Fig.1d).
Eects ofN addition
Nitrogen addition significantly increased the biomass pro-
duction of the target species (Table
1 and Fig.1a) and the
native competitors (Table
1, Fig.1 b), and as a consequence
the total biomass per pot (Table
1; Fig.1c). As the target
biomass increased more strongly than the competitor bio-
mass, the proportion biomass of the target species was also
increased by N addition (Table
1, Fig.1d). Effects of N
addition for pots with the invasive alien target species L.
perenne and M. sativa were similar to those with the native
Fig. 1 a Mean values of biomass of invasive alien (Lepidium virgini-
cum, Lolium perenne, Medicago sativa) and native (Agropyron cris-
tatum) target species, b biomass proportion of target species, c total
biomass per pot d and biomass of the native competitor A. cristatum
under different water (W) and N availabilities. Error bars represent
SEs of the means. Individual points indicate the values for the rep-
licates per species under each treatment combination. Data plotted in
the figure are transformed

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "The effects of changes in water and nitrogen availability on alien plant invasion into a stand of a native grassland species" ?

Here the authors used a mesocosm-pot experiment to test whether high water availability, nitrogen ( N ) enrichment and their interaction promote performance of three invasive alien plants ( Lepidium virginicum, Lolium perenne and Medicago sativa ) when competing with a native Chinese grassland species ( Agropyron cristatum ). 

Q2. What have the authors stated for future works in "The effects of changes in water and nitrogen availability on alien plant invasion into a stand of a native grassland species" ?

The reason for this is not known and deserves further research. 

Q3. What did the invasive alien species show in the center of each pot?

Increasing water availability promoted biomass production both for the invasive alien and native target species in the center of each pot, as well as of the native competitors around the target plant. 

Q4. What is the main limiting resource for aboveground net primary production of grassland ecosystems?

Nitrogen is another major limiting resource for aboveground net primary production of grassland ecosystems (LeBauer and Treseder 2008; Fay et al. 2015). 

Q5. How was the biomass of the native competitor transformed?

To meet the assumption of normality, biomass proportion of the target species, total biomass per pot and biomass production of the native competitor were square-root transformed, and the biomass production of the target species was naturallog transformed. 

Q6. What is the effect of N addition on the biomass proportion of the target species?

Averaged across the three invasive alien and one native target species, the positive effect of N addition on the biomass proportion of the target species increased with increasing water availability. 

Q7. What is the effect of water on grassland productivity?

As water can enhance N delivery to the root surface, the effects of N addition on grassland productivity might become stronger with increasing water availability (Bai et al. 2008; Lu and Han 2010). 

Q8. How did the authors test the effects of water availability and N enrichment on the native target species?

To test the effects of water availability and N enrichment on performance of the three alien and the one native target species in the stand of the native grassland species, the authors fitted linear models using the lm function in R 3.3.2 (R Core Team 2016). 

Q9. What is the effect of N addition on biomass production of the target species?

Averaged across the four target species and the three water availabilities, N addition also increased biomass production of the plants, but more so for the target plants than for the competitors, as the biomass proportion of the target species increased. 

Q10. What was the effect of nitrogen addition on the biomass of the target species?

As the target biomass increased more strongly than the competitor biomass, the proportion biomass of the target species was also increased by N addition (Table 1, Fig. 1d). 

Q11. What was the effect of watering on the biomass of the native competitors?

Compared to the native target species A. cristatum, the invasive alien target species L. virginicum had a negative effect on biomass of the native competitors under low water availability, but a positive effect under intermediate and high water availability (Table 1 and Fig. 1b). 

Q12. What is the main reason why the authors found that invasive alien plants are likely to be adapted?

Successful alien plants often originate from anthropogenic habitats in their native range (Kalusova et al. 2017), and are thus likely to be adapted to high N levels (Dostal et al. 2013).