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\begin{document}

\title{Is shrub-encroached grassland a stable state in the semi-arid steppe
region?}



\author[1]{Guang Hao}%
\author[1]{Nan Yang}%
\author[1]{Ke Dong}%
\author[1]{YuJuan Xu}%
\author[1]{Xinfeng Ding}%
\author[1]{Xinjian Shi}%
\author[1]{Lei Chen}%
\author[2]{Jinlong Wang}%
\author[1]{Nianxi Zhao}%
\author[1]{Yubao Gao}%
\affil[1]{Nankai University}%
\affil[2]{Tianjin Agricultural University}%


\vspace{-1em}



  
  \date{May 18, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Understanding community restoration state and the corresponding assembly
mechanism is helpful to assess the restoration measures and predict
community dynamics. We collected plots by shrub cover (low, medium and
high) in three Caragana mircophylla shrub-encroached grasslands which
were fenced since 1979, 1983, and 2003 (fencing duration) in the
northern China, to explore the effect of fencing duration and shrub
cover on the community restoration by vegetation investigation and
phylogenetic approach. There were significant differences in community
composition among different fencing duration or shrub cover treatments.
Species richness in the site of fencing since 1979 or in the plots of
high shrub cover was relatively higher than that in any other sites or
plots. By phylogenetic analysis, functional traits were phylogenetically
convergent. Based on the standardized effect sizes of mean pairwise
distance (SESMPD) ranged from -1.96 to 1.96 in six out of nine plots,
which suggested that stochastic processes dominated community assembly.
SESMPD were lower than -1.96 in the rest three plots which indicated
that competitive exclusion drove community assembly. These results
indicated that the increase of fencing duration or shrub cover could
enhance competitive exclusion. The present findings highlighted the
importance of shrub in influencing the community composition and
community assembly, supporting that shrub-encroached grassland is
another stable state in the semi-arid northern China. Therefore, it is
essential to distinct shrub-encroached grassland from degraded
grasslands when formulating relevant conservation and management
measures in the semi-arid regions.%
\end{abstract}%



\sloppy


\textbf{Is shrub-encroached grassland a stable state in the semi-arid
steppe region?}

Guang Hao\textsuperscript{1, 2}, Nan Yang\textsuperscript{1}, Ke
Dong\textsuperscript{1}, Yujuan Xu\textsuperscript{1}, Xinfeng
Ding\textsuperscript{1}, Xinjian Shi\textsuperscript{1}, Lei
Chen\textsuperscript{1}, Jinlong Wang\textsuperscript{3}, Nianxi
Zhao\textsuperscript{1,*}, Yubao Gao\textsuperscript{1}

1 College of Life Science, Nankai University, Tianjin 300071, China

2 College of Environmental Science \& Engineering, Nankai University,
Tianjin 300071, China

3 College of Agronomy \& Resources and Environment, Tianjin Agricultural
University, Tianjin 300071, China

* The corresponding author: Zhao Nianxi,
email:\emph{zhaonianxi@nankai.edu.cn}

\textbf{A short informative containing the major key words:} We
investigated the community restoration state and assessed the community
assembly mechanisms in shrub-encroached grasslands affected by fencing
duration and shrub cover crossed.

\textbf{Running title} : Is shrub-encroached grassland a stable state?

\textbf{Abstract} :

Understanding community restoration state and the corresponding assembly
mechanism is helpful to assess the restoration measures and predict
community dynamics. We collected plots by shrub cover (low, medium and
high) in three \emph{Caragana mircophylla} shrub-encroached grasslands
which were fenced since 1979, 1983, and 2003 (fencing duration) in the
northern China, to explore the effect of fencing duration and shrub
cover on the community restoration by vegetation investigation and
phylogenetic approach. There were significant differences in community
composition among different fencing duration or shrub cover treatments.
Species richness in the site of fencing since 1979 or in the plots of
high shrub cover was relatively higher than that in any other sites or
plots. By phylogenetic analysis, functional traits were phylogenetically
convergent. Based on the standardized effect sizes of mean pairwise
distance (SES\textsubscript{MPD}) ranged from -1.96 to 1.96 in six out
of nine plots, which suggested that stochastic processes dominated
community assembly. SES\textsubscript{MPD} were lower than -1.96 in the
rest three plots which indicated that competitive exclusion drove
community assembly. These results indicated that the increase of fencing
duration or shrub cover could enhance competitive exclusion. The present
findings highlighted the importance of shrub in influencing the
community composition and community assembly, supporting that
shrub-encroached grassland is another stable state in the semi-arid
northern China. Therefore, it is essential to distinct shrub-encroached
grassland from degraded grasslands when formulating relevant
conservation and management measures in the semi-arid regions.

\textbf{Key words} : Fencing duration; Shrub cover; Community assembly;
Phylogenetic patern; Shrub-encroached grassland\textbf{1 INTRODUCTION}

Understanding the mechanisms underlying community assembly and
predicting community dynamics has long been a central problem in ecology
(Kraft et al., 2007; Dong et al., 2019), especially in the context of
restoration communities (Jones et al., 2019). Proposed mechanisms range
from niche-based assembly rules (Diamond 1975) to neutral assembly
(Hubbell, 2001). Early workers relied on taxonomic ranks (e.g.,
species-to-genus ratios; Elton 1946) to infer ecological processes that
structure communities. Recently, the availability of molecular
phylogenies has provided a new method for more accurately measuring
evolutionary relatedness, and has led to a rapidly growing body of
research in this field (Weeb, 2002). A local community can be considered
as a subset of a regional pool. Processes contribute to local community
assembly (Karft et al., 2007; Liu et al., 2018), including environmental
filtering, competitive exclusion and stochastic processes, and outcomes
of these processes may be reflected in patterns of relatedness within a
community (Cornwell et al., 2006).

Recently, assessing phylogenetic conservatism, i.e., assessing
relationships between phylogenetic relatedness and ecological
similarities, is usually done before give a conclusion by phylogenetic
approach, because more and more ecologists found that ecological traits
are not always conserved in evolution but convergent sometimes
(Cavender-Bares et al., 2004; Liu et al., 2018). Phylogentic
conservatism indicated that the closely related species tend to be
similar of ecological and functional traits (conserved traits). Under
this condition, phylogenetic clustering pattern suggests environmental
filtering, phylogenetic overdispersal indicates competitive exclusion,
and phylogenetic random pattern implies neutral processes dominating the
community assembly (Webb et al. 2002; Kembel \& Hubbell, 2006; Liu et
al., 2018). However, when ecological traits are convergent, phylogenetic
clustering suggests competitive exclusion, and phylogenetic
overdispersal indicates environmental filtering driving the community
assembly processes (Karft et al., 2007; Liu et al., 2018).

Due to global changes, large areas of terrestrial communities are
degrading, especially in arid and semi-arid regions (Eldridge et al.,
2011; Chen et al., 2014; Dong et al., 2019). For example, in the
semi-arid Inner Mongolia Steppe in the northern China, large areas of
native climax communities, \emph{Stipa grandis} -- \emph{Leymus
chinensis} communities, degraded or even thicketized because of drought
caused by climate change and over-grazing by cattle or/and sheep, with
the loss of plant diversity and the decline of ecosystem functioning
(Peng et al., 2013; Dong et al., 2019). In response, a number of
restoration practices were carried out to restore the semi-arid
grassland vegetation, and fencing is regarded as an effective management
practice (Wu et al., 2009). Therefore, many researches have been focused
on the restoration processes (or secondary succession) of different
communities, including forest (Muscarella et al., 2015; Liu et al.,
2018; Pinho et al., 2018), wetlands (Lozada-Gobilard et al., 2019),
grasslands (Conradi et al., 2017; Barber et al., 2019; Dong et al.,
2019) and abandoned agricultural fields (Dinnage, 2009).

Secondary succession is community assembly in action with changes in
species richness as well as community composition, and therefore, it
offers a chance to test the relative importance of different assembly
processes along restoration duration and its consequences on the
structure and functioning of plant communities (Pinho et al., 2018). As
for the secondary succession of non-shrub-encroached degraded
grasslands, both arrivals of propagules from outside and germinations
from soil seed bank are very important at the the beginning of
restoration, thereby, both random processes (arrival) and environmental
filtering (conditions for germination) are expected to dominate the
community assembly during the early stage of succession (Meiners et al.,
2015; Marteinsd\selectlanguage{ngerman}óttir et al., 2018). As successions proceed, competitive
exclusion increasingly influences community assembly due to the
improvement of abiotic conditions (Chen and Tang, 2016) and
interspecific competition (Purschke et al., 2013; Dong et al., 2019).
Consequently, the community gradually tends to be relatively stable
(Zhang et al., 2019). However, there is still a lack of research on the
community assmbly mechanisms in secondary succession of shrub-encroached
grassland considering the significant differences between
non-shrub-encroached degraded grassland and shrub-encroached grassland.
Although some reseachers pointed out that shrubs can facilitate the
establishment of surrounding herbaceous communities (Koyama et al.,
2015) by reducing abiotic stress (Maestre et al., 2009; Soliveres and
Eldridge, 2014), many studies comfirmed that shrub encroachment
influenced the cycling of nutrient such as nitrogen, carbon and surface
water and energy flow (Peng et al., 2013), causing grassland patches or
degradation (Xiong et al., 2004; Zhou et al., 2018; Pierce et al.,
2019).

For shrub-encroached grassland, shrub formed different cover at the
landscape scale along the degree of shrub encroachment (Eldridge et al.,
2011; Peng et al., 2013; Soliveres and Eldridge, 2014). On one hand, the
increase of shrub cover across entire landscapes may affect the
heterogeneity of soil nutrients and light availability, provide more
niche and promote plant differentiation (Breshears, 2006; Soliveres and
Eldridge, 2014). On the other hand, microclimatic conditions (Soliveres
et al., 2012; Zhou et al., 2018) affected by shrubs could act as
environmental filters for plant communities. So far, no research has
explored the impact of shrub encroachment on the assembly of plant
community in shrub-encroached grasslands although some studies have
shown that shrub encorachment has changed the plant composition,
community structure and abiotic environment of local habitat (Eldridge
et al., 2011; Peng et al., 2013).

In this study, we chose communities treated by different fencing
duration and shrub cover in the \emph{Caragana
mircophylla}shrub-encroached grassland in the northern China, and used
phylogenetic distances to assess assembly mechanisms along fencing
duration and shrub cover gradient. We aimed to answer the following
questions: 1) did the increase in fencing duration or shrub cover
promote the restoration of herbaceous communities? 2) were key traits of
species in the community phylogenetically conserved? and 3) how did
fencing duration and shrub cover affect phylogenetic structure of
herbaceous community and which process(es) played important roles in
a\selectlanguage{english}ffecting community assembly.

\textbf{2 MATERIALS AND METHODS}

\textbf{2.1 Study region and experiment design}

The present study was conducted in the restoration area of \emph{C.
microphylla} shrub encroachment (43deg33\selectlanguage{english}'-43deg38\selectlanguage{english}' N, 116deg40\selectlanguage{english}'-116deg44\selectlanguage{english}'E) in
the middle of the Inner Mongolia Steppe, China. The climate in this
region is typical semi-arid continental climate with a mean annual
temperature of -0.4\selectlanguage{english}, ranging from -22.3\selectlanguage{english} (minimum monthly temperature,
January) to 18.8\selectlanguage{english} (maximum monthly temperature, July), and mean annual
precipitation \textasciitilde{}348mm. The typical vegetation type of
this region belongs typical steppe, and the perennial rhizome grass
\emph{Leymus chinensis} (Trin.) Tzvel. and the perennial bunchgrass
\emph{Stipa g randis} P. Smirn. dominate these communities (Bai et al.,
2004). The major soil types of this area are calcic chestnuts and calcic
chernozems (Schonbach et al., 2011). Due to the irrational utilization,
most of this area has degraded with the deceasing plant diversity and
vegetation cover. In the seriously degraded area, \emph{C. microphylla}
shrub expanded rapidly and showed a gradient of cover along the
degradation degree (Peng et al., 2013).

Three sites that had been fenced since 1979, 1983 and 2003 were
selected, where \emph{C. microphylla} shrub encroachment had emerged
before fencing. These sites are characterized by an average altitude of
1,182 m, a mild slope less than 5\%, and shared the similar climatic
conditions (because the geographical distance between pairwise sites was
less than 5000 m). Within each site, there was a shrub cover gradient
along the disturbance intensity.

In August 2018, we selected three plots of 100 m x 100 m, representing a
gradient in shrub cover and density from low (\textless{}10\%) to medium
(10\%-30\%) to high (\textgreater{}30\%) within each site. We quantified
shrubs cover and density using the average value method in each plot
within three 20 x 20 m sub-plots (established randomly). Within each
plot, we sampled five 0.5 x 0.5m quadrats placed entirely beneath the
canopy of shrubs and the same number of quadrats in open areas. In each
quadrat, each vascular plant species and its density (cloned plants were
calculated by tiller number) were recorded.

\textbf{2.2 Trait selection}

We selected seven plant traits for each species: specific leaf area
(SLA), leaf dry matter content (LDMC), leaf area (LA), leaf thickness
(LT), leaf carbon content (LCC), leaf nitrogen content (LNC) and
vegetation height (VH). These traits are widely recognized as good
predictors for reflecting different plant species strategies for
acquiring, using and conserving resources, including light, nutrients
and water (Maire et al., 2009; Pinho et al., 2018). All of these were
measured in field, based on standard methods of Perez-Harguindeguy et
al. (2013). More details of the trait information were listed in
Supplementary material (Table S1).

\textbf{2.3 Measurement of species richness, species composition and
community stability}

We calculated species number in a quadrat as species richness. To
distinguish differences in species composition among different fencing
duration or shrub cover treatments, non-metric multidimensional scaling
(NMDS) based on Bray-Curtis distance was run. We used the envfit
function from vegan to test the community composition difference between
treamtments of different fencing duration or shrub cover, with treatment
type as a categorical factor variable. Significance of these differences
was tested using Analysis of Similarity (Anosim) by calculating 999
random permutations. Theses anlyses were performed in the `vegan'
package in R (R Core Development Team).

M-Gordon stability index, a popular international method, is used for
determining community stability (Hou et al., 2019). It uses the number
and frequency of all species in a plant community to establish a
mathematical model, and calculates the reference point of the community.
Then, the stability of the plant community was calculated according to
the reciprocal of the distance between the reference point and the
stable point. More details for calculation methods of M-Gordon stability
index see Supplementary material. The closer the reference point to the
stable point, the higher the stability of plant community (Hou et al.,
2019).

\textbf{2.4 Measurement of phylogenetic structure and trait
phylogenetic} \textbf{conservatism}

Before quantifying phylogenetic structure and phylogenetic conservatism,
we constructed a phylogenetic tree for our study species. All the 19
species investigated in this study were identified based on Angiosperm
Phylogeny Group III system (The Angiosperm Phylogeny Group, 2009) using
the Plant List database (http://www.theplantlist.org/). And then, the
completed phylogenetic tree with branch lengths (Figure. S1) was
directly assembled within the online tool Phylomatic (http://
phylodiversity.net/phylomatic/) based on the ultrametric phylogeny of
Zanne et al. (2014). Next, a phylogenetic tree was randomly solved by
`multi2di' function, and then we ultrametrized the tree using
`compute.brlen' function from `ape' package in R (R Core Development
Team).

To characterize community phylogenetic diversity and structure, we
calculated Faith's phylogenetic diversity (PD) (Faith 1992), mean
nearest taxon distance (MNTD) and mean pairwise distance (MPD) (Webb,
2000). PD is defined as the total branch length among all taxa in a
comnunity. MPD generally reflects phylogenetic structure across the
entire tree, whereas MNTD is sensitive to patterns among closely related
species towards the tips of the phylogeny (Blaimer et al., 2015). In
additon, we calucaluted the standardized effect sizes (SES, differences
of observed values versus null communities) of MPD and MNTD
(SES\textsubscript{MPD} and SES\textsubscript{MNTD}) by weighting
species abundance and their associated quantiles (\emph{p} value) to
assess statistical significance.

To further strengthen the conclusion, we analyzed the SES with two null
models: 1) randomizing the community data matrix (`independent swap')
and 2) shuffling taxon labels randomly across the phylogeny (`taxa
labels'), this algorithm randomizes the relatedness of species to one
another, but it maintains species occurrence frequency and abundance.
SES\textsubscript{MPD} and SES\textsubscript{MNTD} are equivalent to net
relatedness index (NRI) and nearest taxon index (NTI) multiplied by --
1. Phylogenetic clustering is indicated by negative SES values and low
quantiles (\emph{p} \textless{}0.05), corresponding to -1.96, and
overdispersion by positive SES values and high quantiles (\emph{p}
\textgreater{}0.95), corresponding to 1.96. Non-significant positive or
negative values indicate observed values close to the median of the
random dispersion.

To quantify trait phylogenetic conservatism, we employed a widely used
Blomberg's K statistic with 999 iterations (Blomberg et al., 2003).
Blomberg's K statistic is assessed by using the Brownian motion (BM) of
trait evolution, and is considered as one of the most powerful
approahces (Pavoine and Ricotta, 2013). K-value close to 1, greater than
1 and less than 1 indicate that the trait distribution perfectly matches
BM expectation of trait evolution on the phylogeny, is more conserved
and is less conserved than BM expecatation, respectively (Kraft et al.,
2007). K-values between 0.4 and 0.7 are hereby in the following
considered to represent intermediate, whereas values below 0.4 express
low levels of conservatism. We assessed the statistical significance of
K by comparing observed variances of phylogenetic independent contrasts
to a null model generated by shuffling tips randomly across the
phylogeny. Analyses of phylogenetic structure and trait phylogenetic
conservatism were performed in the `picante' package in R (R Core
Development Team).

\textbf{2.5 Statistical analysis}

All data used in this study met the Shapiro--Wilk test of normality and
the Levene's test of homogeneity of variance.

Two-way ANOVA of general linear models were used to assess the effects
of fencing duration, shrub cover and their interaction on species
richness, phylogenetic diversity and phylogenetic structure. A one-way
ANOVA analysis with Tukey's HSD test was conducted to evaluate the
difference among levels of fencing duration or shrub cover when
interaction were not significant. Alternatively, a simple-effect
analysis was used to evaluate the difference of levels of one factor
under a certain condition of another factor when significant interaction
were found. Linear model was applied to test the relationship between
phylogenetic diversity and species richness.

\textbf{3 RESULTS}

\textbf{3.1 Species richness, species composition and community
stability}

Nineteen species from 17 genera of 11 families were recorded in these
plots, and Poaceae, Amaranthaceae and Amaryllidaceae comprise of 57.9\%
of the species recorded (Table S2). Species richness was significantly
higer in the site of fencing since 1979 than that in site of fencing
since 1983 or 2003; and species richness was significantly higher in the
high shrub cover plots than that in the low shrub cover plots (Figure
1). The results of NMDS plots showed strong segregation in species
composition among different fencing duration communities
(\emph{p}\textless{} 0.05) (Figure 2A; Table S3). In addition, there was
a significant segregation (\emph{p} \textless{}0.001) in species
composition between plots of high shrub cover and any of the other two
shrub cover treatments, but there was no siginifcant segregation between
plots of low and medium shrub cover (\emph{p} =0.407) (Figure. 2B; Table
S3). The site of fencing since 2003 had a lower M-Gordon stability index
of the plant community, and the plots of high shrub cover had higher
M-Gordon stability index (Figure. S2; Table 1).

\textbf{3.2 Community phylogenetic diversity and structure}

Fencing duration significantly increased phylogenetic diversity (PD),
and PD in the sites of fencing since 1979 was the highest (Figure 3).
There was no significant difference in PD among different shrub cover
treatments. There was a significantly positive relationship between
phylogenetic diversity and species richness (R\textsuperscript{2} =
0.824, \emph{p} \textless{} 0.001) (Figure. 4).

SES\textsubscript{MPD} (TL) was significantly affected by fencing
duration, shrub cover and their interaction. SES\textsubscript{MPD} (TL)
was significantly lower in the plots of high shrub cover (around -1.96,
phylogenetic clustering) than in the plots of low or medium shrub cover
under fencing since 2003 and 1983 (higher than -1.96 and lower than
1.96, phylogenetic random); and significantly lower in the plots of low
shrub cover under fencing since 1979 (around -1.96, phylogenetic
clustering) than in the plots of low shrub cover under fencing since
1983 or 2003 (around zero, phylogenetic random). There was no
significant difference in SES\textsubscript{MPD} (IS) among different
fencing duration or shrub cover treatments (Figure. 5B).

The values of SES\textsubscript{MNTD} (IS or TL) were between -1.5 and
1, which were significantly affected by shrub cover (Table 2), and
SES\textsubscript{MNTD} in the high shrub cover plots was obviously
lower than that in the low or medium shrub cover plots.

\textbf{3.3 Trait phylogenetic conservatism}

All K values of investigated traits were less than 1, ranging from 0.15
to 0.53 (Table 1), which indicated that all traits exhibited low to
intermediate levels of conservatism. In addition, the \emph{p} values of
five traits (VH, LNC, LCC, LDMC, LT) were less than 0.05.

\textbf{4} \textbf{DISCUSSION}

Community phylogenetic approach reveals the relationship of evolutionary
processes, regional environments, and interactions between species,
which is important to understand the mechanisms of species co-occurrence
and community assembly (Cavender-Bares et al., 2009). The present
findings demonstrated that the restoration processes of shrun-encroached
grassland were affected by fencing duration and shrub cover..

The increase of fencing duration increased the species richness and
phylogenetic diversity, suggesting that longer fencing duration was
conducive to the restoration of community diversity in shrub-encroached
grasslands (Yoshihara et al., 2010; Soliveres \& Eldridge, 2014; Koyama
et al., 2015). Generally, species richness usually increases first and
then decreases in secondary succession of grazing exclusion (Chen et
al., 2014). In this study, species richness did not decline even in the
communities that had been fenced since 1979. The result of NMDS
confirmed the differences in species composition between fencing
durations. Meanwhile, there were only 19 species in species pool in this
study area, far lower than the number 27-58 in the typical grassland and
degraded grassland of this region (Bai et al., 2000). These results
indicated that the community was still in recovering stage, but the
restoration rate was slow. We also found that the communities with
higher fencing duration were more stable. However, the restoration
processes of shrub-encroached grassland were inconsistent with
non-shrub-encroached grassland which could return to the climax state
after about 11-year fencing (Li et al., 1993). Previous studies have
suggested that \emph{C. microphylla} had higher water capturing
capability than grasses, it will lead to irreversible stage once
\emph{C. microphylla} dominated the landscape (Peng et al., 2013),
mainly due to that there was a threshold point for transitions between
states (Xiong et al., 2004; D'Odorico et al., 2012). In addition, the
existence of shrub canopy would ameliorate abiotic environment for the
growth of neighbor spceis, enhancing plant richness and influencing
community composition (Maestre et al., 2009; Koyama et al., 2015). This
also explains why the species richness and community composition of high
shrub cover communities were significantly different from that of low
and medium shrub cover community.

Phylogenetic diversity reflects evolutionary history of coexisting
species and clarifies their capacity to adapt to environmental changes
(Alberti et al. 2017). In addition, it can represent independent aspects
of community structure (Barber et al., 2019; Jones et al., 2019).
Previous studies found that phylogenetic diversity was positively
correlated with species richness but differed in their temporal or
successional patterns (Purschke et al., 2013). In the present study, the
phylogenetic diversity positively correlated with species richness
overall. However, phylogenetic diversity didn't increased obviously with
species richness in different succession gradient of shrub cover. This
result indicated that most plant lineages present in the high shrub
cover communities also occurred in the low and medium shrub cover and
that they are generally less speciose in the last two shrub cover
communities (Blaimer et al., 2015; Bares et al., 2019). This also
reflected that the impact of shrub cover on species diversity was not
consistent with fencing duration.

In this study, we found that the majority of traits were
phylogenetically convergent, with six out of seven phylogenetic signals
were lower than 0.40. Pipenbaher et al. (2013) found traits were
convergent for most of traits when they tested phylogenetic signal in
the North Adriatic Karst grassland. However, most studies in tropical
and temperate forests demonstraed that traits were phylogenetically
conservatism (Muscarella et al., 2015; Dobert et al., 2017). This
difference may occur, because compared with woody plants, herbaceous
plants tend to have shorter life cycles, more rapid reproduction and
more adaptable to the environmental changes (Donoghue, 2008). The
evolutionary relatedness of ecological traits have an important
influence on the interpretation of community assembly (Liu et al.,
2018). Cavender-Bares et al. (2004) emphasized the importance of
detecting phylogenetic conservatism, and suggested that the results of
testing phylogenetic conservatism should be combined with the observed
phylogenetic dispersion pattern in order to make a strong inference on
the ecological and functional similarity of coexisting species.

The phylogenetic pattern varied widely among different fencing duration
and shrub cover communities. The low shrub cover communities under
fencing since 1979 and high shrub cover communities under fencing since
1983 and 1979 showed evidence of phylogenetic clustering (SES
\textless{} -1.96), with co-occurring species significantly more closely
related than would be expected by chance. In contrast, phylogenetic
random dispersion (-1.96 \textless{} SES \textless{} 1.96) dominated in
low shrub cover communities under fencing since 2003 and 1983, medium
shrub cover under fencing since 2003, 1983 and 1979 and high shrub cover
communities under fencing since 2003 (based on MPD). All of communities
showed a random assembly regarding MNTD, but it showed a trend of
phylogenetic clustering with fencing duration or shrub cover increasing.
When ecological traits evolve convergently, the interpretation of the
community assembly becomes complicated, and its predictive ability
declines (Kraft et al. 2007). Convergent traits can result in clustering
patterns even though competitive exclusion is the dominant process of
community assembly (Webb et al. 2002). With regard to the present
results, clustering in low shrub cover communities under fencing since
1979 and high shrub cover communities under fencing since 1983 and 1979
could be linked to competitive exclusion rather than habitat filtering.
The combination of phylogenetic clustering and convergent trait
evolution seen in our study has also been found in other studies
(Pipenbaher et al., 2013; Purschke et al., 2013; Liu et al., 2018).
Research results from non-shrub-encroached grassland indicate that
competitive exclusion would play more important role in driving the
community assembly with the fencing duration due to the improvement of
abiotic conditions (Wellstein et al., 2014; Conradi et al., 2017; Barber
et al., 2019). Our results indicated that the communities assembly
during the fencing restoration in shrub-encroached grassland was
consistent with that of non-shrub-encroached grassland. In addition, we
aslo found shrub cover significantly affected the community assembly
process. On one hand, previous studies have shown that higher shrub
cover could increase soil fertility (Maestre et al., 2009; Pipenbaher et
al., 2013) and ameliorate abiotic environment(Molina-Montenegro et al.,
2016; Thompson et al., 2017). The decrease in abiotic stress and the
increase in species richness may intensify resource competition among
herbaceous species (Chapin et al.,1994; Purschke et al., 2013). On the
other hand, there may be direct competition between shrubs and
herbaceous species, and higher shrubs have a higher competitive
advantage in light and water resources than herbaceous species (Peng et
al., 2013; Pierce et al., 2019), accelerating competitive ability for
resources. Thus, niche overlap in open space should become significant
and competition should consequently intensify.

More recently, the importance of random colonization and stochasticity
in early community has become evident (Ulrich et al. 2016;
Marteinsdottir et al., 2018; Liu et al., 2018). Our results found a
random assembly process (-1.96 \textless{} SES \textless{} 1.96) in six
out of nine communities (Figure 5). Generally, random community assembly
is considered to be strongest in communities that are dominated by
colonization. In arid and semi-arid communities, colonization was often
limited by unfavorable local environmental conditions (Conradi et al.,
2017; Marteinsdottir et al. 2018), seed availability (Knappova et al.,
2017) or lack of facilitating species (Callaway, 1995). At sites like
Inner Mongolia steppe with low soil nutrient status, limited, seed rain
and low seedling survival (Koyama et al., 2015), a successful
colonization requires a sequence of favorable events. This also proved
that these communities have lower stability than those with longer
fencing duration and higher shrub cover (Figure S2; Table 1).

\textbf{5 CONCLUSION}

The present findings showed that the presence of shrubs inhibited the
restoration rate and suggested that shrub-encroached grasslands is
another stable state in this region. If we want to restore the diversity
and composition of grassland community and maintain its function, it can
not be achieved only fencing, especially in the high shrub cover area
of\emph{C. microphylla} . It may require some human interference
practices that inhibit \emph{C mircophylla} growth (Xiong et al., 2004;
Alados et al., 2019) and incorporates biotic perspectives into the
framework of restoration (Koyama et al., 2015; Pierce et al., 2019).

\textbf{ACKNOWLEDGMENTS}

We acknowledge the staff of the Inner Mongolia Grassland Ecosystem
Research Station (IMGERS) for their support. This work was supported by
the National Key Research and Development Program of China
(2017YFD0200808; 2016YFC0500706) and National Natural Science Foundation
of China (31570427).

\textbf{CONFLICT OF INTEREST}

The authors declare that we have no known competing financial interests
or personal relationships that could have appeared to influence the work
reported in this paper.

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Table 1 Results of community stability analysis.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Plot & Fitting formula & R\textsuperscript{2} & Intersection coordinate
& Stability index\tabularnewline
\midrule
\endhead
2003 & y = -0.0108x2 + 1.9364x + 12.686 & 0.99 & (33.98, 66.02) &
5.06\tabularnewline
1983 & y = -0.0115x2 + 1.8767x + 23.604 & 0.94 & (30.20, 69.80) &
6.93\tabularnewline
1979 & y = -0.0126x2 + 2.0686x + 14.552 & 0.98 & (32.07, 67.93) &
5.86\tabularnewline
Low & y = -0.0129x2 + 2.0761x + 16.495 & 0.97 & (31.24, 68.76) &
6.29\tabularnewline
Medium & y = -0.0102x2 + 1.7441x + 23.978 & 0.97 & (31.36, 68.64) &
6.22\tabularnewline
High & y = -0.0145x2 + 2.2532x + 13.357 & 0.96 & (30.88, 69.12) &
6.50\tabularnewline
\bottomrule
\end{longtable}

Table 2 The results of general linear models for evaluating the effect
of fencing duration, shrub cover and their interactions on species
richness and phylogenetic structure. IS: null model based on independent
swap algorithm; TL: null model based on taxa labels algorithm.
SES\textsubscript{MPD}: standard effect size of mean pairwise distances;
SES\textsubscript{MNTD}: standard effect size of mean nearest taxon
distance.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
Variable & Fencing duration (df=2) & Fencing duration (df=2) & Fencing
duration (df=2) & Shrub cover (df=2) & Shrub cover (df=2) & Shrub cover
(df=2) & Fencing duration \selectlanguage{ngerman}× Shrub cover (df=4) & Fencing duration \selectlanguage{ngerman}×
Shrub cover (df=4) & Fencing duration \selectlanguage{ngerman}× Shrub cover (df=4) & Model
R\textsuperscript{2}\tabularnewline
\midrule
\endhead
& \emph{F} & \emph{P} & \selectlanguage{greek}ω\selectlanguage{english}\textsuperscript{2} (\%) & \emph{F} & \emph{P}
& \selectlanguage{greek}ω\selectlanguage{english}\textsuperscript{2} (\%) & \emph{F} & \emph{P} & \selectlanguage{greek}ω\selectlanguage{english}\textsuperscript{2}
(\%) &\tabularnewline
SES\textsubscript{MPD} (TL) & 12.574 & \textbf{0.000} & 18.03 & 11.843 &
\textbf{0.000} & 16.98 & 2.548 & \textbf{0.046} & 7.31 &
0.434\tabularnewline
SES\textsubscript{MPD} (IS) & 0.789 & 0.458 & 1.77 & 1.229 & 0.298 &
2.76 & 1.464 & 0.221 & 6.58 & 0.112\tabularnewline
SES\textsubscript{MNTD} (TL) & 3.017 & 0.055 & 5.87 & 5.698 &
\textbf{0.005} & 11.10 & 1.312 & 0.273 & 5.11 & 0.231\tabularnewline
SES\textsubscript{MNTD} (IS) & 1.222 & 0.300 & 2.61 & 3.751 &
\textbf{0.028} & 8.02 & 1.001 & 0.412 & 4.28 & 0.156\tabularnewline
\bottomrule
\end{longtable}

Table 3 Results of analyses of phylogenetic signal, comparing the
K-statistic and its associated p-value for seven measured species
functional traits.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Trait & Type of variable & Range & \emph{K} & p\tabularnewline
\midrule
\endhead
Vegetation height (VH) & Continuous (cm) & 13.00-44.48 & 0.32 &
\textbf{0.048}\tabularnewline
Specific leaf area (SLA) & Continuous (m\textsuperscript{2}/kg) &
11.58-26.37 & 0.22 & 0.225\tabularnewline
Leaf nitrogen concentration (LNC) & Continuous (g/kg) & 2.32-4.73 & 0.31
& \textbf{0.046}\tabularnewline
Leaf carbon concentration (LCC) & Continuous (g/kg) & 35.37-47.17 & 0.53
& \textbf{0.008}\tabularnewline
Leaf area (LA) & Continuous (cm\textsuperscript{2}) & 0.56-113.07 & 0.15
& 0.746\tabularnewline
Leaf dry matter content (LDMC) & Continuous (mg/g) & 93.01-404.19 & 0.33
& \textbf{0.035}\tabularnewline
Leaf thickness (LT) & Continuous (mm) & 0.11-0.93 & 0.34 &
\textbf{0.021}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Figure captions}

\textbf{Figure 1} Species richness of different fencing duration and
shrub cover communities.

\textbf{Figure 2} Non-metric multidimensional scaling (NMDS) ordination
of community taxonomic composition. A) Fencing duration treatments; B)
Shrub cover treatments. Each point represents the species composition in
a given quadrat. Lines represent the confdence ellipse at the 0.95
level.

\textbf{Figure 3} Phylogenetic diversity of different fencing duraition
and shrub cover communities.

\textbf{Figure 4} Relationship between phylogenetic diversity and
species richness.

\textbf{Figure 5} The standardized effect sizes SES of mean pairwise
distance (SES\textsubscript{MPD}) and mean nearest taxon distance
(SES\textsubscript{MNTD}). The significant difference from the null
model is shown by asterisks. The same English letters indicate
non-significant differences (\emph{p} \textgreater{}0.05) among
different fencing duration treatments; and the same Greek letters
indicate non-significant (\emph{p} \textgreater{}0.05) differences among
different shrub cover conditions.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1/Figure-1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-2/Figure-2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-3/Figure-3}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-4/Figure-4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-5/Figure-5}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

