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\begin{document}

\title{The vegetation N:P ratio stoichiometric is a driver of negative density
dependence in a succession series of a semi-arid area}



\author[1]{Xing Dong He}%
\author[1]{Jinglei Zhang}%
\author[1]{Xiangxiang Yang}%
\affil[1]{Nankai University}%


\vspace{-1em}



  
  \date{May 25, 2024}


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\selectlanguage{english}
\begin{abstract}
Plant negative density dependence is the result of interactions between
plants themselves and between plants and the environment. We selected a
succession series comprising early successional, mid-successional and
late successional stages (Artemisia ordosica, Sophora alopecuroides and
Stipa bungeana communities, respectively) in a semi-arid area. We
investigated plant density and biomass and determined the nitrogen (N)
and phosphorus (P) contents of each plant species in each of 225
quadrats, and we calculated the N and P contents of vegetation using
biomass as a weighted coefficient. We found that the total plant density
of the A. ordosica community increased with the increase in vegetation
N:P ratio while the total plant density of the S. bungeana community
decreased with the increase in vegetation N:P ratio, with the latter
(late successional stage) community exhibiting negative density
dependence. In the communities representing the early successional and
mid-successional stages, the vertex coordinate of the quadratic function
relationship between plant total density and vegetation N/P ratio was
(16.6, 353.3); that is, if the N:P ratio of the vegetation was greater
than 16.6, the community was characterized by negative density
dependence. The analysis showed that the negative density dependence was
due to P limitation. These findings reveal that the vegetation N:P ratio
in a semi-arid region is the driver of negative density dependence.%
\end{abstract}%



\sloppy


\textbf{KEY WORDS}

semi-arid area, plant density, vegetation N:P ratio, negative density
dependence, nutrient limitation, ecosystem, plant community succession

\textbf{1 INTRODUCTION}

Negative density dependence in a plant community is mainly reflected in
the density dependence, distance dependence, and community compensation
effect, which is not only a reflection of intraspecific or interspecific
competition but also a reflection of light, soil moisture and nutrient
status after competition. Negative density dependence represents a
self-thinning type of attenuation and the embodiment of the realized
niche. In the past 20 years, ecologists have studied the patterns and
strengths of conspecific - and heterogeneous negative density dependence
(Adler et al., 2018; Chisholm \& Fung, 2018) in various plant taxa,
including grassland and woodland plant species (Johnson et al., 2012;
Detto et al., 2019; Forrister et al., 2019) and especially tropical
rainforest species (Bagchi et al., 2011; Johnson et al., 2017; Kellner
et al., 2018). Some ecologists have explored the relationship between
negative density dependence and biodiversity. For example, abundant
species exhibit weaker negative density dependence than rare species,
and species-rich regions show stronger negative density dependence than
species-poor regions (Johnson et al., 2012). However, the driving
mechanisms of negative density dependence continue to attract the
interest of ecologists.

Plant density is one of the most important community characteristics
(Grime, 2001) and is the basis of plant community biodiversity (Tilman,
2000). Density dependence is a very common property of plant populations
or communities. Generally, plant density exhibits a Malthusian or
logistic growth process in the growth process of plant populations. When
density dependence occurs, plant density is followed by negative
feedback of growth. Whether the competition resulting from this negative
feedback is consistent with the intraspecific (Verhulust, 1838) or
interspecific competition model (Volterra, 1926), plant populations are
all manifestations of the exploitive competition of plants for
resources. In particular, plant density is strongly affected by
competition for soil moisture and nutrients, which can influence plant
stoichiometry, e.g., the N:P ratio.

The law of minimum of Liebig (1840) suggests that plant growth depends
on the availability of the scarcest resource. The Shelford's (1913) law
of tolerance suggests that a species has tolerance limits for certain
factors, beyond which it cannot survive. These two laws reveal the
mechanism by which a plant's stoichiometric N:P ratio can affect
vegetation. The stoichiometric N:P ratio in plant tissues can predict
the supply of nutrients such as N and P (Koerselman \& Meuleman, 1996;
Kranabetter, Harman-Denhoed, \& Hawkins, 2019; Tian et al., 2019). Some
studies have shown that the nutrient contents of leaves can reflect the
soil nutrient supply (Luo et al., 2017; Liu et al., 2018), and plant
stoichiometric N:P ratio has been used as an indicator of the
environmental supply of nutrients to plants (Aerts \& Chapin, 2000;
G\selectlanguage{ngerman}üsewell, 2004; Matzek \& Vitousek, 2009). Shaver \& Chapin (1995) and
Bedford, Walbridge, \& Aldou (1999) point out that the stoichiometric
N:P ratio in plant tissues can serve as an indicator of whether
vegetation is restricted or plant growth is affected by N or P,
conditions that are significant for maintaining species richness in
ecosystems and biodiversity. Güsewell, Koerselman, \& Verhoeven (2003)
point out that the relative utilization rate of N and P in plant
communities could be reflected by the stoichiometric N:P ratio in plant
tissues and that deficiencies in N and P could be predicted.

Although plant density is related to productivity (Grace, 1999; Grime,
2001), it is also affected by the plant N:P ratio. Some studies have
shown that the stoichiometric N:P ratio in plants reflects the degree of
species endangerment. For example, Venterink et al. (2003) measured the
N:P ratio in vascular plant tissues at 150 wet-point sites in Poland,
Belgium, and the Netherlands to determine whether each site was
restricted by N or P by examining the growth of plant communities at
various sites and compiling data on endangered species. They found that
endangered species were growing predominantly in phosphorus-limited wet
sites and that the abundance and productivity of endangered species
decreased with increasing P. At present, there is a lack of a clear
statistical relationship between plant N:P ratio and plant density.

Theoretical analysis indicates that the plant N:P ratio can affect plant
density (Elser et al., 2000), especially in arid and semi-arid regions.
Here, we hypothesize that in arid and semi-arid regions, the transition
to the late stage of a population or a community is accompanied by an
increase in the vegetation N:P ratio due to P consumption by plants.
Furthermore, we hypothesize that the higher vegetation N:P ratio leads
to a self-thinning type of attenuation of plant density, which occurs
during the negative density dependence. In this study, we investigated
the effect of changes in the stoichiometric N:P ratio of vegetation and
associated characteristics on plant density in a succession series in a
semi-arid region. Our goal was to determine whether the vegetation N:P
ratio is the mechanism driving negative density dependence.

\textbf{2 METHODS}

\textbf{2.1 Field investigation}

We chose the Ningxia Habahu National Nature Reserve, China, as the
research area. The Reserve belongs to a semi-arid region with
geographical coordinates of 106°53\selectlanguage{english}'10\selectlanguage{english}"E--107deg38\selectlanguage{english}'10\selectlanguage{english}"E,
37deg38\selectlanguage{english}'36\selectlanguage{english}"N--38deg02\selectlanguage{english}'00\selectlanguage{english}"N. The Reserve has an elevation of 1,300--1,622 m,
an average annual temperature of 7.7degC, an average annual precipitation
of 282 mm and an average annual evaporation of 2,250 mm.

In the research area, a successional series exists because of grazing
disturbance (He et al., 2016). The plant communities in early
successional, mid-successional and late successional stages of the
succession series were \emph{Artemisia ordosica} , \emph{Sophora
alopecuroides} and \emph{Stipa bungeana} communities, respectively. The
soil below the \emph{A. ordosica} community included semi-fixed and
fixed sandy soil, and the main plant species in the community
included\emph{Corispermum declinatum} , \emph{Heteropappus altaicus}
,\emph{Cynanchum thesioides} , \emph{Inula salsoloides} ,
\emph{Astragalus melilotoides} , \emph{Thermopsis lanceolate} ,
\emph{Allium mongolicum} ,\emph{Aneurolepidium dasystachys} ,
\emph{Calamagrosis epigeijos} ,\emph{Ixeris chinensis} , \emph{Bassia
dasyphylla} , \emph{Salsola collina} and \emph{Hedysarum mongolicum.}
The soil below the \emph{S. alopecuroides} community was fixed aeolian
sandy soil, and the main plant species in the community included
\emph{A. scoparia} , \emph{Poa sinoglauca} , \emph{Pennisetum
centrasiaticum} , \emph{A. dasystachys} ,\emph{Echinops gmelini} ,
\emph{Chenopodium glaucum} , \emph{C. acuminatum} , \emph{Oxytropis
psammocharis} , \emph{Tribulus terrestris} ,\emph{I. chinensis} ,
\emph{Setaria viridis} , \emph{Peganum nigellastrum} , \emph{Lactuca
tatarica} , \emph{A. tenuissimum} ,\emph{Euphorbia humifusa} , \emph{E.
kozlovi} var\emph{. angustifolia}and \emph{E. esula} . The soil in the
area of the \emph{S. bungeana}community was light sierozem and sierozem,
and the main plant species included \emph{Cleistogenes squarrosa} ,
\emph{P. sinoglauca} ,\emph{Glycyrrhiza uralensis} , \emph{Agropyron
mongolicum} , \emph{O. aciphylla} , \emph{Gypsophila davurica} ,
\emph{Lespedeza davurica} ,\emph{Hedysarum brachypterum} ,
\emph{Haplophyllum davuricum} ,\emph{Linum perenne} , \emph{Polygagla
tenuifolia} , \emph{Iris tenuifolia} , \emph{Laguchilus ilicifolius} ,
\emph{Potentilla bifurca}and \emph{Cynanchum hancockianum.}

In July 2016, we designed a 2x10 km belt transect to investigate
features of the three communities.

For the \emph{A. ordosica} community, we established 25 sampling plots
along the belt transect. In each plot, a 50 m sampling line was randomly
established, and a 4x4 m quadrat was set at distances of 0 m, 25 m, and
50 m along the sampling line to investigate the number and biomass of
the \emph{A. ordosica} plants in each quadrat. The plants were cut along
the root stock, and a balance was used to measure the biomass of\emph{A.
ordosica} . Then, a 1x1 m subquadrat was randomly established within
each 4x4 m quadrat, and the number and biomass of each herb was
recorded. Each herb plant in each subquadrat was cut along the root
stock, and a balance was used to measure the biomass. The branches and
leaves of each plant species in each subquadrat were collected; in each
4x4 m quadrat, a soil sample to 20 cm depth was collected.

For each of the \emph{S. alopecuroides} and \emph{S.
bungeana}communities, 25 sampling plots were established in the belt
transect, respectively. In each plot, a 50 m sampling line was randomly
established, and a 1x1 m quadrat was set at was set at distances of 0 m,
25 m, and 50 m along the line. The number and biomass of each herb plant
in each quadrat were measured. Each herb was cut along the root stock,
and a balance was used to measure biomass. Samples of branches and
leaves of each plant in each quadrat were collected and brought back to
the laboratory. In every 1x1 m quadrat, a soil sample to 20 cm depth was
collected.

\textbf{2.2 Tissue assays}

The dried plant samples were determined at Nankai University. To
determine N and P concentrations in plant leaves, the leaf samples were
first digested by the concentrated
H\textsubscript{2}SO\textsubscript{4}-H\textsubscript{2}O\textsubscript{2}method
(Bao, 2000). After digestion, the solution was diluted and precipitated,
and the supernatant was collected. N content was determined by SKD-800
Kjeldahl nitrogen analyzer, and P content was analyzed by the
molybdenum-antimony anticolorimetry method (Bao, 2000). The analysis of
each sample was repeated three times. The concentrations of N and P in
soil were determined in the same manner as those in leaves.

\textbf{2.3 Calculations and statistics}

The contents of N and P in vegetation were calculated as the weighted
means of the contents of N and P, respectively, in branches and leaves
of all plants in each investigated quadrat, and the biomass of branches
and leaves of each species was used as the weighting coefficient. After
the N and P contents of vegetation were calculated, the N:P ratios of
vegetation were determined.

To calculate the species diversity of the \emph{A. ordosica}communities,
the number of \emph{A. ordosica} plants in each 4x4 m quadrat was
transformed into the number per of 1x1 m subquadrat. Then, the number of
\emph{A. ordosica} plants per square meter and the number of herb plants
per square meter were used to calculate the species diversity of
\emph{A. ordosica} communities.

The Shannon-Wiener index (\emph{H} ') was used to characterize the
species diversity of the community. The formula used to calculate this
index was as follows:

where \emph{N} is the total number of individuals of a species in the
quadrat, \emph{S} is the number of plant species in the quadrat,
and\emph{n\textsubscript{i}} is the number of individuals of the
\emph{i} th species.

For the regressions of variables of the \emph{A. ordosica} and \emph{S.
alopecuroides} communities, SPSS 20.0 software was used, and Excel 2016
was used to plot the results. For the regression of variables of
the\emph{S. bungeana} communities, the component + residual plot of the
regression of vegetation N:P ratio and total plant density was performed
in R (version 3.5.3; R Core Team, 2019).

\textbf{3 RESULTS}

The N:P ratios of vegetation and soil for the \emph{A. ordosica}
,\emph{S. alopecuroides} and \emph{S. bungeana} communities are
presented in Table 1.

From the data distribution, the vegetation N:P ratios of the \emph{A.
ordosica} communities were lower than those of the \emph{S.
alopecuroides} and \emph{S. bungeana} communities (Table 1, Figure 1).
The vegetation N:P ratios generally decreased in the order \emph{S.
bungeana} communities \textgreater{} \emph{S. alopecuroides} communities
\textgreater{} \emph{A. ordosica} communities. As a leguminous
plant,\emph{S. alopecuroides} has strong nitrogen fixation capacity. The
vegetation N:P ratio was higher at sites where \emph{S.
alopecuroides}dominated. In \emph{S. alopecuroides} communities, the
minimum N:P ratio was 7.56, and the maximum was 23.88.

\textbf{TABLE 1} N:P ratios of vegetation and soil in communities of
different successional stages in a semi-arid area

\textbf{FIGURE 1} Distribution of vegetation N:P ratios in a
successional series in a semi-arid area

In the early successional stage, represented by the \emph{A.
ordosica}communities, there was a significant power function regression
relationship between plant total density and vegetation N:P ratio
(\emph{p} \textless{}0.05). As the vegetation N:P ratio increased, plant
total density increased (Figure 2). Furthermore, the aboveground biomass
of the \emph{A. ordosica} communities showed an increasing trend with
increasing total plant density, although the regression relationship was
not significant (\emph{F} = 1.274, \emph{R} \textsuperscript{2} =
0.053,\emph{p} = 0.271).

\textbf{FIGURE 2} Relationship between total density and vegetation N:P
ratio in an \emph{Artemisia ordosica} community representing the early
successional stage in a semi-arid area

In the mid-successional stage represented by the \emph{S.
alopecuroides}community, there was no significant regression
relationship between plant total density and the vegetation N:P ratio
(Figure 3). However, the aboveground biomass of the community showed a
significant power function regression relationship with total plant
density (\emph{p} \textless{}0.05). With increasing total plant density,
aboveground biomass increased (Figure 4).

\textbf{FIGURE 3} Relationship between total density and the vegetation
N:P ratio in a \emph{Sophora alopecuroides} community representing the
mid-successional stage in a semi-arid area

\textbf{FIGURE 4} Relationship between aboveground biomass and total
density in a \emph{S. alopecuroides} community representing the
mid-successional stage in a semi-arid area

\textbf{FIGURE 5} Relationship between total density and the vegetation
N:P ratio in an \emph{A. ordosica} community representing the early
successional stage and a \emph{S. alopecuroides} community representing
the mid-successional stage in a semi-arid area

Upon pooling the vegetation N:P ratio and total density data of
the\emph{A. ordosica} and \emph{S. alopecuroides} communities, i.e., the
early successional and mid-successional stages, a significant quadratic
function relationship was observed between total density and vegetation
N:P ratio, and the vertex coordinate was (16.6, 353.3) (Figure 5).

In the late successional stage represented by the \emph{S.
bungeana}community, a significant negative regression relationship
between plant total density and the vegetation N:P ratio was observed
(\emph{p} \textless{}0.05). As the vegetation N:P ratio increased, plant
total density decreased (Figure 6). Regression is based on a correlation
between variables and does not imply causation, but for this late
successional stage \emph{S. bungeana} community, a negative correlation
between plant total density and vegetation N:P ratio was observed. In
addition, the aboveground biomass of the \emph{S. bungeana} community
decreased with increasing total density, although the regression
relationship was not significant (\emph{F} = 0.843,\emph{R}
\textsuperscript{2} = 0.035, \emph{p} = 0.368).

\textbf{FIGURE 6} Relationship between total density and the vegetation
N:P ratio in a \emph{Stipa bungeana} community representing the late
successional stage in a semi-arid area

\textbf{4 DISCUSSION}

The plant N:P ratio not only is a link factor among different scales of
molecular, cellular, individual, population, community and ecosystem
(Elser et al., 2000; Sterner \& Elser, 2002; Elser \& Hamilton, 2007)
but is also an important indicator of nutrient supply (Koerselman \&
Meuleman, 1996) and dynamic nutrient limitations of the environment
(Gusewell, 2004; Vitousek et al., 2010; Hu et al., 2018). Since species
richness is always associated with productivity (Grime, 2001), nutrient
availability first affects productivity, then plant density over long
time. Therefore, the vegetation N:P ratio affects plant density. In the
present study of a successional series in a semi-arid area, as the
vegetation N:P ratio increased, plant total density increased
significantly in the \emph{A. ordosica} community (early successional
stage) but decreased significantly in the \emph{S. bungeana} community
(late successional stage). This suggests that the vegetation N:P ratio
was the mechanism driving negative density dependence.

In the natural world, no organism is isolated; rather, each organism is
a member of a population consisting of many homologous species.
Individuals of the same species have similar requirements for growth,
reproduction and survival. Thus, when their demand for resources exceeds
the supply, conspecifics compete for resources. Plants cannot move on
their own; thus, nutrient limitation will affect their physiology and
biochemistry and limit plant biomass. Strong nutrient competition
increases nutrient stress and affects individual reproduction and
mortality, thereby affecting plant density. The vegetation N:P ratio is
an important indicator to of plant nutrient limitation (Gusewell, 2004;
Vitousek et al., 2010). When nutrient limitation is severe, the
vegetation N:P ratio can affect not only productivity but also the plant
density of the community, especially in arid and semi-arid areas.

In the present study of a successional series in a semi-arid area, as
the vegetation N:P ratio increased in the early successional-stage
community of \emph{A. ordosica} , total density increased linearly and
significantly, whereas biomass increased, but the relationship was not
significant. For sand-dune plants in the early successional stage, soil
nitrogen varies to a greater extent that does soil phosphorus (He, You,
\& Yu, 2016). This difference exists because of the deposition of
atmospheric nitrogen and the transformation of organic matter via litter
decomposition, which causes soil nitrogen to increase rapidly. Plant
nitrogen then increases accordingly, which eventually leads to a larger
vegetation N:P ratio. However, due to the extremely low nitrogen content
in dune soil for the early successional stage community, the absolute
nitrogen contents of plants were low. Therefore, the total density of
the \emph{A. ordosica} community increased with increasing vegetation
N:P ratio. In the mid-successional stage, \emph{S. alopecuroides}
community, soil phosphorus began to accumulate, and soil nitrogen
continued to accumulate. There was no significant statistical
relationship between total density and vegetation N:P ratio; however,
the biomass of the \emph{S. alopecuroides} community increased
significantly with increasing total density. In the late successional
stage, phosphorus was consumed in large quantities due to its
physiological and biochemical effects on plant drought resistance, and
the plant communities were restricted by phosphorus. In the \emph{S.
bungeana} community, representing the late successional stage, total
density decreased significantly with increasing vegetation N:P ratio,
which indicated significant negative density restriction in this stage.
At this stage, biomass decreased with increasing total density of the
plant community, reflecting phosphorus limitation; however, the decrease
was not significant.

Koerselam \& Meuleman (1996) indicated that when the vegetation N:P is
\textless{}14, plant growth at the community level is mainly restricted
by N, whereas when N:P is 14-16, plant growth is restricted by N and P,
and when N:P is \textgreater{}16, plant growth is mainly restricted by
P. In the present study, we demonstrated that the vegetation N:P ratios
of the \emph{A. ordosica} , \emph{S. alopecuroides} and \emph{S.
bungeana} communities were 11.27+-0.97, 17.08+-0.86 and 20.84+-1.01. These
results indicated that the \emph{A. ordosica} community was limited by
N, whereas the \emph{S. alopecuroides} and \emph{S. bungeana}communities
were limited by P. Interestingly, in the early and middle successional
stages, there was a significant quadratic function relationship between
total density and vegetation N:P ratio in the communities, and the
vertex coordinate was (16.6, 353.3). These findings revealed that the
community would face negative density constraints when the vegetation
N:P ratio was close to 16.6. In the middle and late successional stages,
plant growth was mainly restricted by P, which led to negative density
dependence.

For populations and communities, the vegetation N:P ratio is of core
interest when predicting nutrient limitation, whether N limitation or P
limitation (Vitousek et al., 2010). For instance, in our previous
research we established an 80 x 80 m sample plot in an \emph{A.
ordosica} community on fixed sand-dunes in the Alxa Desert. We
investigated plant density in 400 quadrats (4x4 m) in the plot and
determined total N and available P in the 20 cm soil layer of each
quadrat. Finally, we regressed the density of \emph{A. ordosica} against
the soil N:P ratio. We found that \emph{A. ordosica} density had no
significant regression relationship with soil N or P but had a
significant, negative linear relationship with the soil N:P ratio. In
previous work, regression results of soil nutrients and plant density
were similar between a sampling scale of 40x20 m and a sampling scale of
4x4 m (Wu et al., 2009). The above observations suggest that the N:P
ratio is more informative than N or P content alone.

Negative density dependence is a very common phenomenon in nature. The
factors affecting negative density dependence are water, nutrients and
salt. Regarding nutrients, density is not limited so much by N or P but
by the vegetation N:P ratio. The results of the present study indicate
that the vegetation N:P ratio in a semi-arid area is the factor driving
negative density dependence.

With the development of a plant community, the vegetation N:P ratio
continuously changes. From the early successional stage to the late
successional stage, the vegetation N:P ratio gradually increases. In the
early successional stage \emph{A. ordosica} community, total plant
density increased with increasing vegetation N:P ratio; however, in the
late successional stage \emph{S. bungeana} community, total plant
density decreased with increasing vegetation N:P ratio. Thus, the
vegetation N:P ratio is a driving factor of plant community variation.

\textbf{ACKNOWLEDGEMENTS}

The field survey of this study was completed in the Ningxia Habahu
National Nature Reserve. Thanks to Mr. Wan Xue You and Mr. Dian Yu of
the Reserve Administration for their strong support. At the same time,
we are also very grateful to Professor Song Gu, Associate Professor Qing
Hua Nie, Professor Hui Fen Liu and Professor Jin Long Wang for
participating in some field investigations. Thanks to my graduate
students Hong Juan Jing, Yu Ting Liang, Hong Tao Xie, Xuan Zhao, Meng Qi
Li, Ping Yi Xu and Jia Yue Zhang for participating in some field
investigations and experimental analysis. This research was supported by
the National Key Research and Development Program of China
(2016YFC0500706).

\textbf{CONFLICT OF INTEREST}

None declared.

\textbf{AUTHOR C ONTRIBUTIONS}

XDH conceived and designed the research, analyzed the data, wrote the
manuscript; JTG and JLZ took part in field investigation and contributed
data.

\textbf{DATA AVAILABILITY STATEMENT}

All data used in the production of this article are available via Dryad:

https://doi.org/10.5061/dryad.4qrfj6q65

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\textbf{TABLE 1} N:P ratios of vegetation and soil in communities of
different successional stages in a semi-arid area\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Community & Soil & Soil & Soil & Vegetation & Vegetation &
Vegetation\tabularnewline
\midrule
\endhead
& Total N & Total P & N:P ratio & N & P & N:P ratio\tabularnewline
Ao & 0.177\selectlanguage{ngerman}±0.016 & 0.098\selectlanguage{ngerman}±0.007 & 1.816\selectlanguage{ngerman}±0.114 & 21.627\selectlanguage{ngerman}±0.656 &
2.193\selectlanguage{ngerman}±0.153 & 11.271\selectlanguage{ngerman}±0.974\tabularnewline
Sa & 0.160\selectlanguage{ngerman}±0.020 & 0.183\selectlanguage{ngerman}±0.015 & 0.836\selectlanguage{ngerman}±0.063 & 25.154\selectlanguage{ngerman}±0.564 &
1.560\selectlanguage{ngerman}±0.078 & 17.077\selectlanguage{ngerman}±0.865\tabularnewline
Sb & 0.303\selectlanguage{ngerman}±0.027 & 0.229\selectlanguage{ngerman}±0.018 & 1.436\selectlanguage{ngerman}±0.141 & 14.422\selectlanguage{ngerman}±0.619 &
0.716\selectlanguage{ngerman}±0.040 & 20.842\selectlanguage{ngerman}±1.007\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

Note: Ao-\emph{Artemisia ordosica} community; Sa-\emph{Sophora
alopecuroides} community; Sb-\emph{Stipa bungeana} community.\selectlanguage{english}
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