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Forest Ecology and Management xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco
Variability of Tamarix spp. characteristics in riparian plant communities are
affected by soil properties and accessibility of anthropogenic disturbance in
the lower reaches of Heihe River, China
⁎
Jingyi Ding, Wenwu Zhao , Bojie Fu, Shuai Wang, Hao Fan
State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, PR China
Institute of Land Surface System and Sustainable Development, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tamarix spp.
Riparian plant community
Vegetation-environment interactions
Anthropogenic disturbance
The lower reaches of Heihe River
Tamarix, one of the most widely distributed riparian shrub genera, plays an important role in ecosystem stabilization in arid zones. Various studies have assessed the effect of stressors on Tamarix spp. in riparian plant
communities in homogenous habitats. However, less is known about the factors driving variation of Tamarix spp.
characteristics across heterogeneous habitats in desert riparian zones, which would constrain effective ecological
conservation in such areas. In this study, field experiments were conducted to determine the variability of
Tamarix spp. characteristics and its driving factors across various riparian plant communities in the lower
reaches of Heihe River, China. Five riparian plant communities containing Tamarix spp. were classified, ranging
from tree–shrub–herb community to shrub dune. With the increase of height and crown diameter of Tamarix
spp., its capacity to resist drought increased, while community diversity formed a bimodal variation pattern
instead of consistently decreasing trend due to variations of vegetation–environment interactions across the
heterogeneous habitats. Among environmental factors, accessibility of anthropogenic disturbance and soil
properties, explaining 42% of vegetation variance, were deemed the key factors influencing the variation of
Tamarix spp. characteristics. By contrast, water availability (e.g., soil moisture and proximity to the river)
showed a weak relationship with Tamarix spp. characteristics due to the drought tolerance of Tamarix spp. and
its interactions with the extreme environment. As accessibility of anthropogenic disturbance and soil properties
were regarded as major environmental factors driving the Tamarix spp. characteristics, multiple measures such
as establishing protective zone, controlling cantaloupe planting and developing water-saving irrigation were
proposed to secure the vegetation structure of Tamarix spp. and maintain the resilience of riparian plant communities under intensive human activities.
1. Introduction
Riparian zone is the interfaces between land and water, characterized by a heterogonous environment and various vegetation communities (Naiman and Décamps, 1997). Tamarix (also called: saltcedar or
tamarisk, family Tamaricaceae) is one of the most widely distributed
riparian genera because of its ability to adapt to a range of saline and
drought conditions. (Ding and Zhao, 2016; Stromberg et al., 2009). It
provides multiple ecosystem services to humans and it functions as an
ecological protection against desertification, especially in arid ecosystems (Ma et al., 2009). As a result of its strong drought tolerance and
adaptive ability, Tamarix spp. grows with other species and develops
into different riparian plant communities under long-term
vegetation–environment interactions (Yang et al., 2002). Various studies have explored the interaction between Tamarix spp. and environmental factors in riparian plant communities, and the results varied
with the habitat investigated (Stromberg et al., 2009). Studies in humid
riparian areas showed that the Tamarix spp. was characterized by
strong competition for water and the ability to secrete salt into the soil,
which would alter the growth condition of native species, form a singlespecies community, and thus, decrease the biodiversity (Natale et al.,
2010). By contrast, other studies, mainly in desert oasis areas, reported
that interactions between Tamarix spp., shrubs, and environmental
factors could provide good wildlife habitats through the ‘fertile island’
effect, resulting in a higher level of biodiversity (Li et al., 2007). Although the interaction between Tamarix spp. and environmental factors
⁎
Corresponding author at: Institute of Land Surface System and Sustainable Development, Faculty of Geographical Science, Beijing Normal University, No. 19, XinJieKouWai St.,
HaiDian District, Beijing 100875, PR China.
E-mail address: [email protected] (W. Zhao).
http://dx.doi.org/10.1016/j.foreco.2017.10.003
Received 29 April 2017; Received in revised form 28 September 2017; Accepted 2 October 2017
0378-1127/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Ding, J., Forest Ecology and Management (2017), http://dx.doi.org/10.1016/j.foreco.2017.10.003
Forest Ecology and Management xxx (xxxx) xxx–xxx
J. Ding et al.
Fig. 1. The lower reaches of Heihe River basin (B) in China (A) and the location of sampling sites in the study area (C). Notes: A–E in (C) indicate the sampling transects.
greatly to variation in species diversity in desert riparian areas (Dı́Az
and Cabido, 2001).
Except for natural factors, the impact of anthropogenic disturbances
on vegetation has significantly increased in the Anthropocene (Lavorel
et al., 2015). The ‘accessibility to anthropogenic disturbance’ is a
comprehensive index which could represent the possibility of communities being disturbed by various anthropogenic activities, with mainly
negative effects on community characteristics (Liu et al., 2014b; Zeng
et al., 2011). Vegetation communities near to roads are more easily
disturbed by grazing and tourism, which in turn can alter the environmental conditions (e.g., soil properties and water availability),
leading to degradation of vegetation communities. In addition, vegetation communities near road are easily accessed by large agricultural
machines and disturbed by other human activities (e.g., collecting
firewood, expanding farmland), which may alter community structure
and decrease ecosystem resilience significantly (Daryanto et al., 2013).
Although previous studies provide some insight into the interactions
between Tamarix spp. in riparian plant communities and environmental
factors (DiTomaso, 1998; Li et al., 2007; Stromberg et al., 2009), key
factors that drive the formation of Tamarix spp. characteristics in desert
riparian zones remain unclear. In addition, previous studies mainly
concentrated on natural factors, such as water availability and soil
properties (Ma et al., 2009; Natale et al., 2010; Xu et al., 2011), which,
without consideration of anthropogenic effects, cannot provide a
comprehensive view of Tamarix spp. characteristics' formation, especially in desert riparian zones, which are characterized by heterogeneous habitats and intense anthropogenic activities.
In the current study, we surveyed multiple vegetation community
characteristics (including community floristic composition, community
structure indices, and diversity indices) and environmental factors
(including water availability, soil properties, and accessibility to anthropogenic disturbance) in the lower reaches of Heihe River. We first
classified riparian plant community types that contains Tamarix spp. in
in riparian plant communities has long been recognized, these studies
mainly focused on homogeneous habitats in the study area. However,
vegetation–environment interactions vary with habitat types and habitats in the desert riparian zone are highly heterogonous, varying from
humid riparian areas near the riverbank to drought-stressed desert
peripheries. Previous studies in desert riparian zones mainly focused on
vegetation–environment interactions in shrub dune habitats (Xie et al.,
2015). However, less is known about the factors determining the
variability of Tamarix spp. characteristics in riparian plant community
across heterogeneous habitats in desert riparian zones, which inhibits
effective ecological restoration of Tamarix spp. in riparian plant communities in this crucial ecosystem (Lü et al., 2011).
The variation in Tamarix spp. community characteristics (e.g.,
community structure, floristic composition and diversity) results from
interactions between vegetation and multiple environmental factors
(Flato et al., 2013). Water availability is a key factor that positively
influences vegetation characteristics, particularly in arid and semiarid
ecosystems (Fang et al., 2016). Tamarix spp., as a drought tolerance
species, depends on multiple water resources to maintain its growth.
Soil moisture, especially a depth of 40–60 cm is the direct water resource for Tamarix spp. in riparian plant communities; by contrast,
groundwater, to a depth of 10 m, representing the survival threshold for
Tamarix spp., is its stable water resource (Long et al., 2014). In addition, as both groundwater and soil moisture are recharged by rivers or
streams, the distance from river channel can be regarded as a proxy for
water availability, given that the influence of river will weaken with the
distance from river channel (Hao et al., 2010). Soil properties (e.g., soil
physical properties and soil chemical properties), as the foundation of
vegetation growth, also shape the community floristic composition and
species vitality by influencing ecological and hydrological processes
(Stirzaker et al., 1996). Previous studies have reported that soil heterogeneity is one of the main factors determining the dominant species
in vegetation communities, whereas changes in soil nutrition contribute
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J. Ding et al.
2.3. Data collecting
the study area and then we explored the variation in community
characteristics among the different community types. Based on these
results, we determined the key factors driving the variability of Tamarix
spp. characteristics. The objectives of this study were to: (1) explore the
variation of Tamarix spp. characteristics across heterogeneous habitats;
(2) discern key factors shaping Tamarix spp. characteristics formation;
and (3) suggest possible management suggestions to protect Tamarix
spp. in riparian plant communities against changing environmental
conditions.
The distance of each site from the river channel and the nearest road
was recorded by mobile GPS (version eTrex 301, Garmin Electric
Business Management Co., Ltd., Shanghai, China). Three shrub quadrats
(10 × 10 m) in each site were established in triangle with 30 m away
from each other. In each quadrat, the height and species in each layer
were recorded. The crown diameter was measured by averaging the
diameter of each crown in an east-west and north-south direction.
Coverage of each species was calculated by the percentage of crown
area to the sampling area and plus the canopy coverage which was
measured by photographing from the base looking up using a fish-eye
lens (Lei and Wen, 2008). The specific leaf area (SLA) of Tamarix spp.
was measured in each site: 20 Tamarix spp. leaves were randomly
sampled from four directions (e.g., east, south, west, and north) on each
plant and stored in an ice chest immediately after collection. In the
laboratory, leaves were removed from twigs and placed without overlap
onto an optical scanner bed. WinSEEDLE image analysis software was
used to evaluate leaf area (S/cm2). After scanning, the sampled foliage
was dried and weighed (M/g), and then the SLA was calculated using
Eq. (1):
2. Material and methods
2.1. Study area
The study area was located in the lower reaches of Heihe River
(40°20′–42°30′N; 99°30′–102°00′E), in the Ejina Oasis, Northwest China
and covered an area of 3 × 104 km2 (Fig. 1). The topography of the
basin inclined from the southwest to the northeast, with an average
slope of 1–3‰ (Akiyama et al., 2007). The region has a typical continental arid climate, with maximum and minimum temperatures
usually occur in July (41 °C) and January (−36 °C), respectively. The
long-term annual average precipitation at the study site was less than
42 mm, while the annual pan evaporation rate was approximately
2300–3700 mm (Wen et al., 2005). The Heihe River, rising in the Qilian
Mountains, is the main water resource for vegetation in the Ejina Oasis.
The plant communities in the Ejina oasis grow mainly along the riverbanks and on the fluvial plain, with dominant vegetation including
Populus euphratica in the tree layer; Tamarix spp., Lyceum ruthenicum in
the shrub layer; and Karilinia caspica and Peganum harmala in the herbaceous layer (Zhao et al., 2016). Sparse and drought-tolerant desert
species, such as Reaumuria soongorica and Zygophyllum xanthoxylon, are
mainly distributed in the Gobi desert. The main soil types in the study
site are grey desert soils and grey–brown desert soils (Chen et al.,
2014).
Local agriculture mainly depends on cantaloupe plantations cultivated during May–September in each year. In addition, the Ejina Oasis
is one of most famous tourist attractions in China, with almost 200,000
visitors visit per year (Hochmuth et al., 2014). Two primary roads and
one highway were built parallel to the river channel and across the
south of the oasis to support the development of agriculture and
tourism in this region.
SLA = S/M (unit: cm2/g)
(1)
At each sampling site, bulk density (BD) was measured using a
stainless-steel cutting ring (100 cm3 in volume) and oven dried at
105 °C to a constant weight. Three replicates of soil moisture samples
and soil samples were collected along the diagonal line of each quadrat
(i.e. on the two opposite angles and the center of each quadrat). Soil
moisture samples were collected using an auger (5 cm in diameter). Soil
gravimetric water content (SWC) was determined every 10 cm at a
depth of 0–80 cm, and every 20 cm at a depth of 80–200 cm. Samples
were collected in aluminum boxes and weighed at the time of sampling
as well as after oven drying at 105 °C for 48 h. Soil samples were collected to a depth of 100 cm in each quadrat to determine the soil
composition and chemical properties. After any roots had been removed, the soil samples were air-dried and passed through a 2-mm
sieve; surface soil samples (from a depth of 0–20 cm) were subsequently
analyzed in the laboratory to determine their clay (< 0.002 mm), silt
(0.002–0.05 mm), sand (0.05–2 mm), and gravel (> 2 mm) content
using a Malvern Mastersizer 2000. Soil organic matter (SOM) was
measured using the K2Cr2O7 method (Liu, 1996). Total nitrogen (TN)
was determined using the Kjeldahl method (ISSCAS, 1978). Total
phosphorus (TP) was determined using a UT-1810PC spectrophotometer (PERSEE, Beijing, China) after H2SO4–HClO4 digestion
(ISSCAS, 1978). Total salt content (TS) was determined by oven method
(Liu, 1996).
2.2. Sampling design
The field experiment was conducted in July 2015 and July 2016. As
riparian plant communities containing Tamarix spp. are mainly distributed in land-water transition zone (i.e. riparian zone) and their
characteristics vary with environmental gradient (Fu et al., 2014),
transect method was used to sample sites in this study under the
principle of random sampling (Wheater et al., 2011). To fully cover the
distribution pattern of riparian plant communities that contain Tamarix
spp., five 3500-m long transects (A–E) were randomly established
northward across the study area (Fig. 1C). As the distance from river is
the major environment gradient in the low reaches of Heihe river (Fu
et al., 2014), transects were established along the distance from river.
On each transect, systematic sampling was used to sample each site
(Yates, 1981). Sampling started at the distance of about 100 m from
river which was randomly sampled across the region and the sampling
sites (50 × 50 m) were systematically sampled with about 500 m intervals at each transect. Transects were not strictly in line shape or
perpendicular to river as sites were sampled far from villages and large
farmlands. A total of 38 sites were sampled during the field sampling.
2.4. Statistical analysis
Importance value (IV) is a comprehensive index that used to determine the overall importance of each species in the community
structure (Odum and Barrett, 1971; Ruiz and Lugo, 2012; Ross et al.,
2016) and it was used in community classification in this study. The
importance value (IV) of a tree was calculated based on the average of
relative density, relative dominance, and relative height, whereas the
IV of shrubs and herbs in each site was calculated based on the average
of relative density, relative dominance, and relative coverage (Zhang
and Dong, 2010).
The total diversity index of each Tamarix spp. community was used
to depict the community diversity in each site. Based on the community
vertical structure, the total diversity index for each community was
measured using the weight of indices in different growth types, using
Eq. (2) (Gao et al., 1997):
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J. Ding et al.
Wi = (Ci/ C + hi / h)/2
(2)
significance. The statistical analyses were performed using SPSS software (version 20.0; SPSS lnc., Chicago, IL, USA). The CCA analysis and
Monte Carlo forward selection in RDA were performed using CANOCO
(version 4.5; Microcomputer Power, Ithaca, NY, USA).
where C was the total coverage of community (C = ∑ Ci ); i = 1, tree
layer; 2, shrub layer; 3, herb layer; h was the thickness of the leaf layer
for various growth types (h = ∑ hi ), Wi was the weighted parameter of
diversity index of ith growth type, Ci was the coverage of the ith growth
type, and hi was the average thickness of the leaf layer of the ith growth
type. The thickness of the tree leaf layer was calculated as being 33.3%
of the height of the tree layer, whereas the shrub layer was 50% and the
herb layer was 100%.
The total diversity index of the community (A) was calculated using
Eq. (3):
A=
∑ Wi Ai
3. Results
3.1. Classification of riparian plant communities that contain Tamarix spp.
In the lower reaches of Heihe River, five types of riparian plant
communities that contain Tamarix spp. were determined based on
TWINSPAN classification and DCA ordination (Fig. 2). The composition
of each sampling site is detailed in Table 1 and the spatial location of
each community is shown in Fig. A.1. Five communities were mainly
different in community structure and floristic composition and the
characteristics of each communities is listed as below.
Community I was Ass. Populus euphratica–Tamarix spp. + herbs.
This community type was distributed within 1600 m of the river
channel with low coverage (36.09% on average). Characterized by treeshrub-herb layers, this community was dominated by Popolus euphratica
in the tree layer. Tamarix spp. were mainly sparsely distributed (coverage 0.67–22%) under the Popolus euphratica canopy, while Sophora
alopecuroides was the only herb species in the community, growing
sparsely in the understory (average coverage < 5%).
Community II was Ass. Tamarix spp.–Lycium ruthenicum + herbs.
This community was distributed widely, ranging from 1050 m to
3200 m from the river channel, with a total community coverage of
15.54–85.42%. This community was primarily dominated by Tamarix
spp. (importance value = 0.48–0.93), accompanied with Lycium ruthenicum in the shrub layer (importance value = 0.07–0.52). Sophora
alopecuroides and Karelinia caspia were the major species in the herb
layer with the latter being the more dominant of the two species (importance value = 0.51–0.97).
Community III was Ass. Lycium ruthenicum–Tamarix spp. + herbs.
This community was distributed within 1020–2870 m of the river
channel, with a community coverage of 28.50–88.79%. The shrub layer
was dominated by Lycium ruthenicum (importance value = 0.54–0.77),
accompanied with a high coverage of Tamarix spp. (3.5–60%). In the
herb layer, most sites were dominated by Sophora alopecuroides with
importance value range from 0.16 to 0.65.
Community IV was Tamarix spp. + sparse herbs. This community
was primarily distributed more than 2000 m from the river channel.
Tamarix spp. dominated in this community with a high importance
value (0.76–1) and coverage (19.67–86%). Lycium ruthenicum and
Reaumuria songarica (a typical desert shrub) occurred in some sites, but
with low importance value (< 0.25). Herb grows sparsely in only five
sites in this community with most of sites dominated by Sophora alopecuroides.
Community V was Ass. Tamarix spp. shrub dune. This community
was mainly distributed within 960–2780 m of the riverbank. Tamarix
spp. were the dominant species of the shrub layer (importance
value = 0.55–0.71), and mainly occurred in shrub dune form. Lycium
ruthenicum
also
grows
in
this
community
(importance
value = 0.29–0.45) and Reaumuria songarica grows in some sites.
Sophora alopecuroides herbs dominated the herb layer, but were sparsely
distributed and only existed in one site.
(3)
where A is the total diversity index of the community; i = 1 for the tree
layer, 2 for the shrub layer and 3 for the herb layer; Wi is the weighted
parameters of the diversity index for the ith growth type; Ai is the diversity index of the ith growth type.
Species diversity indices of each layer were determined (Liu et al.,
1997) using Shannon-Wiener diversity index (Eq. (4)):
s
H = − ∑ (Pi lnPi )
i=1
(4)
Simpson dominance index (Eq. (5)):
s
D = 1− ∑ Pi2
i=1
(5)
Pielou evenness index (Eq. (6)):
Jsw = H /(ln(S ))
(6)
and Patrick richness index (Eq. (7)):
R=S
(7)
where Pi was the relative importance value of species i, and S was the
total number of species in the ith site. All the diversity indices were
calculated based on the above equations using Excel software (Microsoft Office, version 2010).
We used Two-way Indicator Species Analysis (TWINSPAN, in
WinTWINSPAN, version 2.3; a method of hierarchical classification)
and Detrended Correspondence Analysis (DCA, in CANOCO, version
4.5; an ordination axes partitioning) to classify the possible Tamarix
spp. community types. The importance value data for all plant species
were used in these analyses.
Least significant difference (LSD) test in the one-way analysis of
variance (ANOVA) was used to determine the significance of differences
in vegetation characteristics and environmental factors among the
classified communities. Pearson correlation analysis and canonical
correspondence analysis (CCA) were used to determine the strength of
possible relationships between vegetation indices and environmental
factors. The statistical significance level was P < .05. Stepwise multiple linear regression analyses between Tamarix spp. characteristics
and environmental factors were used to explore the explanation of each
environmental factor for the vegetation variance. To further separate
the key influencing environment variables, marginal and conditional
effects of various variables were calculated via Monte Carlo forward
selection in a redundancy analysis (RDA). Marginal effects reflect the
effects of environmental variables on the community characteristics,
whereas conditional effects reflect the effects of environmental variables on the community characteristics after the anterior variable had
been eliminated by the forward selection method. Once the redundant
variables were eliminated and a group of key environmental factors was
determined through the forward selection, this method enabled key
variables to be determined through the strength of their effects and
3.2. Variation of Tamarix spp. characteristics
The height and crown diameter of Tamarix spp. showed a similar
pattern, increasing from Community I to Community V, with significantly (P < .05 or P < .01) higher values in Communities IV or V
compared with Communities I to III. With the increase in height and
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J. Ding et al.
Fig. 2. The dendrogram of sampling sites based on the TWINSPAN classification (a) and DCA ordination diagram (b). Notes: Numbers 1–38 represent the number of the sampling sites. D
indicates the classification level and N indicates the number of sampling sites in each classification. I–V represent community I–V. Arrows indicate that all the sites were divided into five
major groups after the fourth classification.
characteristics and environmental factors, CCA (Canonical
Correspondence Analysis) ordination diagram was conducted for these
variables (Fig. 4). A Monte Carlo permutation test for all canonical axes
showed P < .05 which indicated the significance of CCA analysis result. In Fig. 4, the coverage of Tamarix spp. was distributed closely with
soil moisture, especially the 30–100 cm soil moisture (SWC2), while the
height of Tamarix spp., which was situated opposite to coverage,
showed a close relationship with bulk density and coarse particle
composition (e.g., sand and gravel). Crown diameter was distributed at
the left of diagram and showed a positive relationship with the distance
from road. The specific leaf area and diversity indices were grouped
together in the diagram, which showed a close relation with bulk
density and soil particle composition.
Regression analyses between Tamarix spp. characteristics and environmental factors were used to further explore the influence of environmental factors on the Tamarix spp. characteristics (Table 3). The
regression analyses indicated the distance from river as predictors for
Pielou evenness index. Soil properties (e.g., bulk density, clay, sand,
and total phosphorus), explaining 11.9–31.7% variance, were selected
as the predictors for coverage, specific leaf area, and community diversity (e.g., Shannon–Wiener diversity index, Simpson dominance
index and Patrick richness index). Distance from road, explaining
14.7–38.1% of the variance, was mainly related to vegetation height
and crown diameter.
crown size of Tamarix spp. (i.e. from Community I to Community V),
the community coverage also increased, before a slight decrease in
Community V. The specific leaf area, which represents the droughtresistant ability of a plant, decreased from Community I to Community
V, being significantly (P < .01) lower in the Community V.
Community diversity differed significantly among the five communities
(Fig. 3e, f). Shannon-Wiener diversity index, Patrick richness index and
Pielou evenness index were all high in Communities II, III, and V. By
contrast, Simpson dominance index reached high value in Communities
I and IV, whereas the other diversity indices were low.
3.3. The relationship between Tamarix spp. characteristics and water
availability, soil properties, and accessibility of anthropogenic disturbance
To explore how environmental factors affect the Tamarix spp.
characteristics, 14 environmental indices including water availability
(i.e., 0–30 cm soil moisture, 30–100 cm soil moisture, 100–200 cm soil
moisture, and distance from river), soil properties (i.e., bulk density,
clay, silt, sand, gravel, soil organic matter, total nitrogen, total phosphorus, and total salt content) and accessibility of community to anthropogenic disturbance (i.e., distance from road) were selected based
on 38 sampling sites in the field experiment. The result of Pearson
correlation analysis between Tamarix spp. characteristics and environmental indices is shown in Table 2. Water availability showed a weak
relationship with the vegetation characteristics, with only the
30–100 cm soil moisture positively correlated with Tamarix spp. coverage, whereas the distance from river was positively correlated with
the diversity indices. Soil properties showed a significant relationship
with vegetation characteristics. Bulk density and soil composition (e.g.,
clay and gravel) negatively correlated with vegetation structure (e.g.,
coverage, height crown diameter, and specific leaf area) and positively
correlated with community diversity (e.g., Shannon–Wiener diversity
index and Patrick richness index), whereas total nitrogen was only
significantly correlated with crown diameter. Distance from road, representing the accessibility of anthropogenic disturbance, was positively correlated with community structure, especially crown diameter.
To visually display the relationship between vegetation
3.4. Key factors affecting Tamarix spp. characteristics
To examine key factors that affect the vegetation indices in communities that contain Tamarix spp., redundant variables were eliminated based on the marginal and conditional effects of 14 variables
determined using Monte Carlo forward selection (Table 4). All the environmental factors explained 56% of the total variance. Distance from
road, total nitrogen and bulk density were regarded as the key environmental factors (P < .1) shaping the characteristics of the Tamarix
spp. Distance from road, representing the accessibility of anthropogenic
activity, accounted for the most explanation (50%), whereas total nitrogen and bulk density, representing the effect of soil properties
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Table 1
The information of sampling sites in riparian plant communities that contain Tamarix spp.
Plot number
Distance from river (m)
Tamarix spp. coverage (%)
Community coverage (%)
Importance value of major species in tree/shrub/herb layer
S1
S2
S3
S4
S5
S6
Community I
1
2
8
14
21
30
220.00
630.00
490.00
60.00
140.00
1530.00
8.00
22.00
20.00
1.67
1.00
0.67
9.00
80.42
41.00
29.00
33.17
23.92
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
–
–
–
–
–
–
–
–
–
–
–
–
1.00
1.00
–
–
0.62
–
–
–
–
–
–
–
Community II
3
4
6
7
19
20
1050.00
1380.00
2850.00
3200.00
2700.00
3200.00
43.00
50.00
27.00
35.00
15.00
4.50
85.42
52.25
80.50
84.37
31.70
15.54
–
–
–
–
–
–
0.93
0.91
0.64
0.71
0.88
0.48
0.07
0.09
0.36
0.29
0.12
0.52
–
–
–
–
–
0.10
0.49
0.19
–
0.33
–
–
0.51
0.70
0.97
0.59
0.84
0.87
Community III
9
10
11
12
13
16
17
36
1020.00
1470.00
1860.00
2510.00
2870.00
1250.00
1700.00
2870.00
13.00
23.00
7.80
15.67
8.33
60.00
3.50
15.00
32.00
81.54
41.22
82.67
61.35
88.79
28.50
57.20
–
–
–
–
–
–
–
–
0.43
0.40
0.33
0.46
0.32
0.44
0.23
0.57
0.57
0.60
0.67
0.54
0.68
0.56
0.77
0.43
–
–
–
–
–
–
–
–
–
0.38
0.65
–
0.37
0.16
–
0.25
–
–
–
–
0.20
0.23
–
–
Community IV
5
15
18
22
24
28
29
31
32
33
34
38
2290.00
480.00
2200.00
440.00
1580.00
110.00
580.00
1770.00
2350.00
2560.00
2840.00
2290.00
73.30
26.00
55.00
70.00
86.00
19.67
69.00
78.33
81.33
22.67
49.33
45.00
76.63
95.00
55.00
70.00
86.23
98.67
69.00
78.33
81.33
23.33
49.83
67.94
–
–
–
–
–
–
–
–
–
–
–
–
0.84
1.00
1.00
1.00
0.87
0.88
1.00
1.00
1.00
0.89
0.82
0.76
0.16
–
–
–
0.13
0.12
–
–
–
–
0.11
0.24
–
–
–
–
–
–
–
–
–
0.11
0.08
–
0.50
0.17
–
–
–
0.51
0.26
–
–
–
–
0.26
0.07
–
–
–
–
0.11
0.10
–
–
–
–
–
Community V
23
25
26
27
35
37
960.00
1920.00
2370.00
2780.00
1470.00
2510.00
53.33
70.00
48.33
75
40.00
10.00
82.17
98.33
64.83
92.00
34.87
28.34
–
–
–
–
–
–
0.56
0.55
0.61
0.53
0.64
0.71
0.44
0.45
0.39
0.42
0.29
0.29
–
–
–
0.06
0.07
–
0.14
–
–
–
–
–
–
–
–
–
–
–
Notes: S1, Populus euphratica; S2, Tamarix spp.; S3, Lycium ruthenicum; S4, Reaumuria songarica; S5, Sophora alopecuroides; S6, Karelinia caspia.
leaf area, reflecting the resource utilization strategy of a species under
drought stress (Anyia and Herzog, 2004), decreased with the increase in
the height and crown size of Tamarix spp. and reached a significantly
low value at the Community V. This indicates that the community became more tolerant towards the drought environment with the increasing dominance of Tamarix spp., especially in Community V which
was located at the periphery of oasis and desert.
Species diversity is often used to depict variation in vegetation-environment interactions (Isbell et al., 2015). Previous studies showed
that the growth of Tamarix spp. could either increase or decrease the
biodiversity of community in different habitats (Whitcraft et al., 2007;
York et al., 2011). In addition, it has also been reported that species–environment interactions in stressed environments can change the
fundamental response shape of the community (Oksanen and Minchin,
2002). In the current study, instead of forming a consistent decreasing
or increasing trend with the enhanced dominance of Tamarix spp.
accounted for 25% of the total explanation.
4. Discussion
4.1. Variation of Tamarix spp. characteristics
Five riparian plant communities that contain Tamarix spp. were
classified by TWINSPAN and DCA in the lower reaches of Heihe River.
From Community I to III, community mainly varied in community
structure and floristic composition, transiting from a tree-dominated
community (Community I) to a shrub-dominated community
(Community III), with diminishing tree coverage and increasing shrub
and herb coverage (Fig. 3a). Community coverage fluctuated slightly
from Community III to V, with increasing dominance of Tamarix spp.
(e.g., represented by a significant increase in the height and crown
diameter) (Fig. 3a, c, d) and a rapid decrease in herb coverage. Specific
6
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J. Ding et al.
Fig. 3. Differences in the coverage (a), specific leaf area (b), height (c), crown diameter (d) of Tamarix spp. and community diversity indices (e, f) in the five riparian plant communities
that contain Tamarix spp. Notes: Values represent mean ± SE. Values with the same letter are not significantly different at the .05 significance level (LSD test). Values of an index
followed by different lower-case letters differ significantly at the .05 significance level, while, values of an index followed by different upper-case letters differ significantly at the .01
significance level.
7
Forest Ecology and Management xxx (xxxx) xxx–xxx
−0.071
−0.182
0.162
Fig. 4. CCA ordination diagram between vegetation characteristics and environmental
factors. Notes: CD, crown diameter; SLA, specific leaf area; Shannon, Shannon–Wiener
diversity index; Simpson, Simpson dominance index; Patrick, Patrick richness index;
Pielou, Pielou evenness index; SWC1, 0–30 cm soil moisture; SWC2, 30–100 cm soil
moisture; SWC3, 100–200 cm soil moisture; BD, bulk density; SOM, soil organic matter;
TN, total nitrogen; TP, total phosphorus; TS, total salt content; Df River, distance from
river; Df Road, distance from road.
Table 3
Stepwise multiple linear regression of Tamarix spp. characteristics with environmental
factors.
Notes: Significant correlations (P < .05) are shown in bold and significant correlations (P < .01) in bold with underline.
0.586
0.22
Distance from road (m)
Accessibility of human
disturbance
0.359
−0.299
−0.181
0.311
0.077
0.194
−0.226
0.259
−0.01
0.07
0.05
−0.039
0.338
0.113
0.095
−0.155
0.324
−0.242
−0.186
−0.232
−0.055
−0.342
−0.137
−0.199
0.238
−0.22
0.071
−0.016
0.043
0.016
0.428
0.35
−0.346
0.347
−0.327
0.061
0.212
−0.165
−0.287
−0.13
−0.39
−0.149
−0.038
0.154
−0.393
−0.198
0.268
0.034
0.101
0.123
0.099
0.231
0.114
−0.375
0.095
0.121
−0.084
−0.337
−0.118
−0.319
−0.14
−0.095
0.136
−0.523
0.096
0.263
−0.223
−0.246
0.05
−0.218
−0.051
0.133
Bulk density (g/cm3)
Clay (%)
Silt (%)
Sand (%)
Gravel (%)
Soil organic matter (g/kg)
Total nitrogen (g/kg)
Total phosphorus (g/kg)
Total salt content (%)
0.205
Soil properties
−0.06
−0.103
0.35
0.157
0.189
−0.239
0.258
−0.128
−0.042
−0.102
−0.026
0.112
−0.153
−0.044
0.106
0.22
0.154
0.155
0.214
−0.078
−0.307
−0.036
0.041
0.014
0.186
0.328
0–30 cm soil moisture (g/g)
30–100 cm soil moisture (g/
g)
100–200 cm soil moisture
(g/g)
Distance from river (m)
Water availability
0.064
0.132
0.13
0.063
0.289
−0.0003
0.21
0.031
Patrick richness
index
Simpson dominance
index
Shannon–Wiener diversity
index
Specific leaf area
(cm2/g)
Crown diameter
(cm)
Height (cm)
Coverage (%)
Environmental factors
Table 2
Pearson correlation analysis of the relationship between community characteristics, water availability, soil properties, and accessibility of anthropogenic disturbance.
Pielou evenness
index
J. Ding et al.
Variable
Predictor
Adjust R2
P value
Pielou evenness index
Coverage
Specific leaf area
Shannon–Wiener diversity
index
Distance from river
Bulk density
Clay
Bulk density
0.131
0.242
0.129
0.128
< .01
< .001
< .05
< .05
Bulk density, Sand
Bulk density
Bulk density, Sand
Bulk density
Bulk density, Sand
Bulk density, Sand, Total
phosphorus
Distance from road
Distance from road
0.316
0.119
0.302
0.12
0.218
0.317
< .001
< .05
.001
< .05
< .01
< .001
0.147
0.381
< .05
< .001
Simpson dominance index
Patrick richness index
Height
Crown diameter
(DiTomaso, 1998), community diversity indices (i.e. Shannon-Wiener
diversity index, Patrick richness index and Pielou evenness index)
formed a bimodal pattern, peaking in Community II, III, and Community V. In the Communities II and III, the community was mainly
characterized by diverse species in the shrub and herb layer (e.g., Tamarix spp., Lycium ruthenicum, Karelinia caspia, and Sophora alopecuroides). Compared with communities dominated by a single, highly
competitive species (e.g., Popolus euphratica in Community I or Tamarix
spp. in Community IV), less competitive environments enable to balance resource at different layers and make it possible for the growth of
other species (Marshall and Baltzer, 2015). In addition, high water
availability at all soil depths in Community III (Table A.1) provided an
ample water resource for both deep- and shallow-rooted species, thus
contributing to the high community richness (Fig. 3f) in this waterlimited ecosystem. Despite the high diversity recorded in Communities
II and III, the diversity indices peaked again in Community V. Although
this site was characterized by low soil moisture, the frequent interactions between wind erosion and shrubs create a suitable environment
for diverse species (Ravi et al., 2010). The large crown diameter of
Tamarix spp. in Community V enables sand to be trapped in the foliage
(Cheng, 2007), resulting in the deposition of fine soil particles onto the
shrub dunes (5.67% clay and 44.42% silt) (Table A.1), leading to
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Table 4
Key influencing factors selected based on the marginal and conditional effects obtained from the Monte Carlo test of forward selection.
Environmental factors
Distance from road
Total nitrogen
Bulk density
Gravel
Silt
Sand
Total phosphorus
Soil organic matter
Distance from river
100–200 cm soil moisture
30–100 cm soil moisture
Clay
0–30 cm soil moisture
Total salt content
Marginal effects
Percentage of variance explained (%)
28.3
13.7
11.8
8.8
3.8
2.3
2.1
1.6
1.1
8
7
5
4
4
Conditional effects
Percentage of variance explained (%)
P value
Relative explanation (%)
Environmental factors
Distance from road
Total nitrogen
Bulk density
30–100 cm soil moisture
100–200 cm soil moisture
0–30 cm soil moisture
Silt
Gravel
Distance from river
Clay
Sand
Soil organic matter
Total phosphorus
Total salt content
Total
28
8
6
3
3
2
2
2
2
0
0
0
0
0
56
0.006
0.03
0.064
0.174
0.148
0.278
–
0.308
0.324
0.884
0.742
–
–
0.734
50.00
14.29
10.71
5.36
5.36
3.57
3.57
3.57
3.57
0
0
0
0
0
–
also have an important role in shaping Tamarix spp. characteristics.
Bulk density and soil composition, explaining 24.2% of the variance in
coverage and 12.8–30.2% of the variance in community diversity, respectively (Table 3), were key factors that determined soil structure and
indirectly influenced its water-holding capacity (Stirzaker et al., 1996).
In humid area, soil with high bulk density often composed by high
content of fine particle and the water holding capacity usually increases
as the texture become finer (Black, 1968; Ravi et al., 2010). However,
in hyperarid zone, soil of high bulk density usually characterized by
relatively low fine particle as the soil contains a part of coarse fragment
in the study area (e.g., the gravel content in the sites of Communities II
and III) (Table A.1), which would result in a low water-holding capacity
in soil, thus inhibit the growth of dominant species in the community
and negatively shaping the Tamarix spp. structure (e.g., coverage,
height, and crown diameter). By contrast, inhibiting the growth of
Tamarix spp. could alleviate the strong competition and allelopathic
effects on soils (Zhang et al., 2002). This would encourage the growth
of diverse species and thus promote community diversity and weaken
community dominance by a single species. Total nitrogen explained 8%
of the variance in Tamarix spp. characteristics in our study and has been
reported as one of the main factors influencing riparian vegetation in
the Heihe River Basin (Xu et al., 2011). Although it is an essential
nutrient supply for vegetation growth, we found that total nitrogen was
negatively correlated with crown diameter, which differs from the positive relationship commonly reported between total nitrogen and
crown diameter in humid areas (Wang et al., 2010). In hyperarid areas,
Tamarix spp., which is often the dominant species in the shrub layer, is
characterized by strong competition for water and nutrition resources.
With the increasing dominance of Tamarix spp., indicated by the increase in Tamarix spp. coverage, height and crown diameter, the resource competition and utilization of the shrub layer increased, which
could lead to a reduction in the quality of the soil resource. This would
prevent the growth of the herb layer, the main resource of soil organic
matter in the hyperarid zone, thus further reduce the amount of total
nitrogen in the soil (Xu et al., 2011).
Although water availability has been defined as a crucial factor
determining vegetation growth in arid zones (Lü et al., 2014), current
study found that the variation in Tamarix spp. characteristics showed a
weak relationship with water availability. Soil moisture is the direct
water resource for vegetation in arid zones. Approximately 60% of
Tamarix spp. roots were concentrated at a depth of 20–60 cm (Imada
et al., 2013), indicating the high potential for soil moisture utilization
at this depth. However, only Tamarix spp. coverage was positively
changes in the deposition of soil in vegetated patches. This can lead to
the development of water and nutrient sinks in vegetated patches,
rendering them suitable habitats for the growth of xerophytic herbs and
shrubs, thus contributing to the accumulation of soil nutrients (8.09 g/
kg soil organic matter) (Table A.1) and high diversity recorded in
Community V (D'Odorico et al., 2007).
4.2. Impact of soil properties and accessibility of anthropogenic disturbance
on Tamarix spp. characteristics
The spatial variation in Tamarix spp. characteristics was the result of
interactions between vegetation and environmental factors. Previous
study has shown that variation in water and nutrient availability associated with soil properties are likely to be important selective forces
shaping ecosystem stability in arid zones (Rosenthal et al., 2005). In
this study, we found that accessibility of anthropogenic disturbance
(i.e., distance from road) and soil properties (e.g., bulk density and total
nitrogen), explaining 28% and 14%, respectively, of the variance in
community characteristics and were the key factors affecting the variation of Tamarix spp. characteristics (Table 4).
The distance from road had significant influence on the structure of
the Tamarix spp., explaining 14.7% and 38.1% of the variance in height
and crown diameter, respectively (Table 3) (Liu et al., 2014a; Zenner
et al., 2012). Tamarix spp. is often used as fuel wood in the desert riparian zone and the communities distributed near the road, such as
Communities I, II, III, and IV (Table A.1), which were more easily being
affected by fuel wood collection. The habitats of Tamarix spp. communities, especially Communities III and IV, were also more likely to be
utilized as farmland, because they were characterized by suitable
agricultural resources (e.g., high soil moisture and fine particle soils) as
well as being located near to the road, making it easier to be accessed
by humans and agriculture machinery (Yang, 2013). In addition, the
herb species in the understory of riparian plant communities containing
Tamarix spp. are used for grazing (Thiele et al., 2008) and the Ejina
Banner is a predominantly pastoral region, raising 62% of the livestock
(e.g., approximately 178,000 sheep livestock unit) of the whole area
(Gala et al., 2007). Heavy pastoral pressure threatens the herb layer and
the vegetation community structure, especially in those communities
located near the road, which makes them more easily accessed by the
livestock and herdsman. Thus, being located near a road results in a
high risk of disturbance by anthropogenic activities, which can affect
the growth of Tamarix spp., especially the crown diameter.
Soil properties, which explained 14% of the variance in vegetation,
9
Forest Ecology and Management xxx (xxxx) xxx–xxx
J. Ding et al.
Heihe River, the farmland also largely expanded (Fig. A.2), especially
for cantaloupe cultivation, which is characterized by high water consumption. The conveyance water, which was intended to be used for
ecological recovery, is instead used for the expansion of irrigation
farmlands. The increasing water requirements of the expanding farmland will results in drought stress and threatens species survival,
especially for herb species, which have low drought tolerance but
contribute significantly to the community diversity (Thuiller et al.,
2006). Thus, in order to maintain ecosystem resilience and avoid decrease in community diversity, measures such as controlling cantaloupe
planting and developing water-saving irrigation practice are suggested,
particularly around the high diversity communities (e.g., Community II,
III and V).
correlated with the soil moisture at a depth of 30–100 cm, and this
explained only 3% of the variance in vegetation (Tables 2 and 4). This
could be due to the variation in water-use strategies in different Tamarix spp. habitats. When located close to the riverbank, Tamarix spp.
mainly utilizes a combination of river water and soil water; as the
distance from the riverbank increased, Tamarix spp. mainly replies on
the groundwater (Fu et al., 2012). This change in water resource is
likely to explain the weak relationship between soil moisture and variance in vegetation, as well as between soil moisture and Tamarix spp.
characteristics. In addition, although previous studies showed that
community diversity decreased with the weakening of water influence,
the species richness and evenness were positively correlated with the
distance from river. This is likely to result from the variation in vegetation–environment interactions with the distance from river. As the
distance from river increased, the interaction between Tamarix spp. and
wind erosion became more prominent, leading to finer particles (e.g.,
clay) being deposited on the surface soil, benefitting the growth of diverse species (D'Odorico et al., 2007). In addition, Tamarix spp. could
also benefit the growth of shallow-rooted species (e.g. herbs) by redistributing the deep soil water to the shallow layer (via hydraulic
uplift) in drought environments (e.g., Community V) (Yin et al., 2010).
Thus, this mutualistic effect of Tamarix spp. as well as vegetation–environment interactions in extreme environments, could result in
increased community richness and evenness even in communities located further away from river.
5. Conclusion
Based on the variation of Tamarix spp. characteristics, we found that
Tamarix spp. characteristics formed significant pattern along heterogeneous habitats investigated in this study. With the increase in height
and crown size, Tamarix spp. coverage and drought tolerance increased,
whereas community diversity displayed a bimodal pattern due to the
variation of species-environment interactions in the resource limited
area. Among environmental factors, accessibility of anthropogenic
disturbance and soil properties, which explained 42% vegetation variance of Tamarix spp. and accounted for 75% of the total explanation,
were deemed to be key environmental factors that affecting the variation in Tamarix spp. community characteristics. The variation of
Tamarix spp. community characteristics showed a weak relationship
with water availability because of the ability of Tamarix spp. to draw up
water from the deep soil and the vegetation-environment interactions
in extreme environments. Based on these results, management such as
establishing a protective zone, could be put in place to protect the vegetation structure and secure the ecosystem services provided by these
communities (e.g. carbon storage, firewood, and sand fixation) against
aggravated anthropogenic disturbance. In addition, management that
reduce drought stress (e.g. controlling cantaloupe planting and developing water-saving irrigation practice) were also recommended to help
secure the vegetation structure of Tamarix spp. and maintain the resilience of riparian plant communities under intensive human activities.
4.3. Applications for management
Our study illustrates that vegetation structure of Tamarix spp. (e.g.,
height and crown diameter), contributing to the ecosystem service
provisions (e.g., firewood provision and sand fixation) (Hierro et al.,
2000), were negatively influenced by the accessibility of anthropogenic
disturbance and soil physical properties. However, with ecological recovery, the seasonal tourism, which is concentrated between September
and October, has significantly increased (Hochmuth et al., 2014). Exposure to intense trampling by tourists might potentially reduce the
water holding ability in soils and thus negatively affecting the vegetation structure of Tamarix spp. as well as the ecosystem services provision (e.g., firewood provision and sand fixation). Among five communities, Communities I to IV were located nearest to the road, which
makes these communities more likely to be affected by the diverse effects of intensive tourism. Thus, for Community I to IV, management
measures, such as establishing protective zones near the roads is recommended in future ecological restoration to secure ecosystem service
provisions in this desert riparian zone.
Riparian plant communities containing Tamarix spp. also strongly
influence the resistance of ecosystem against disturbance in hyperarid
zone. Communities characterized by high diversity (e.g., Community II,
III and V) could maintain ecosystem functions during stress or disturbance due to the complementary of function traits and ecological
redundancy (Isbell et al., 2015). Although ecological water conveyance
has significantly improved the water condition in the lower reaches of
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 91425301) and State Key Laboratory of Earth
Surface Processes and Resource Ecology (No. 2017-FX-01(2)). We are
grateful for reviews from the editor, Dan Binkley, guest editors and two
anonymous reviewers, who significantly improved the quality of this
article. We are thankful to the Alashan Research Station of Cold and
Arid Region Environment and Engineering Research Institution,
Chinese Academy of Sciences for their support and contributions to the
fieldwork.
Appendix A
A.1. The spatial location of five classified riparian plant communities that contain Tamarix spp. in the lower reaches of Heihe
In the lower reaches of Heihe, 38 sampling sites were classified into five types of riparian plant communities that contain Tamarix spp. based on
Two-way Indicator Species Analysis (TWINSPAN) and Detrended Correspondence Analysis (DCA). The spatial location of each community was
displayed in different symbol based on the classification result (Fig. A.1).
10
Forest Ecology and Management xxx (xxxx) xxx–xxx
J. Ding et al.
Fig. A.1. The spatial location of five classified riparian plant communities
that contain Tamarix spp. in the lower reaches of Heihe.
A.2. The temporal variation of sown area in the lower reaches of Heihe during 2000 to 2015
The temporal variation of sown area in the lower reaches of Heihe during 2000 to 2015 is displayed in the Fig. A.2 based on the statistical
yearbook of Ejina Banner obtained from National Library, China. The total sown area significantly increased since the implement of ecological water
conveyance project in 2000, rising from 1514 hm2 in 2000 to 5136 hm2 in 2015. Among different types of crops, the cantaloupe characterized with
high economic value, has been widely planted and accounted for 79.6% of the total sown area after 15 years’ ecological water conveyance.
Fig. A.2. The temporal variation of sown area in the lower reaches of
Heihe during 2000 to 2015.
11
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A.3. The characteristics of environmental factors in the five classified riparian plant communities that contain Tamarix spp.
Table A.1 summarizes the characteristics of environmental factors in five classified communities. In all communities, the soil moisture increased
with the depth of soil layer, while the coefficient of variation value decreased along the depth with the highest variation at the surface layer
(88.93%). Community III had the highest soil moisture in all layers, while Community II and V characterized with relative low soil moisture among
the five communities.
Bulk density and soil particle composition were used to depict the soil physical property and all of these indices generally did not differ
significantly among the five communities. Characterized with the highest proportion of fine soil particle, the Community IV had the lowest bulk
density, while Community II and III, containing certain percentage of gravel, had higher bulk density. Compared with soil particle composition, there
was relative low degree of variability (29.92–37.63%) in soil nutrient and there was no significantly difference in soil nutrient among five communities. Total salt content, did not differ significantly among five communities, and reached the highest value in Community III.
The distance from river represents the impact of river on each community. Community I, located nearest to the river bank (511.67 m), differed
significantly from other communities. Community III, IV, V located in the same range of distance (1500–2000 m) from the river channel, while
Community II located farthest from the river channel (2396.67 m). The distance from road depicted the accessibility of human disturbance to certain
communities. Community II, III, IV located near to the road, with the similar distance from the road (217.50–255.00 m). This was then followed by
the Community I (578.33 m) and the Community V distributed remotest (861.67 m) from the road.
Table A.1
The characteristics of environmental factors in the five classified riparian plant communities that contain Tamarix spp.
Factor
SWC1 (g/g)
SWC2 (g/g)
SWC3 (g/g)
Clay (%)
Silt (%)
Sand (%)
Gravel (%)
BD (g/cm3)
SOM (g/kg)
TN (g/kg)
TP (g/kg)
TS (%)
Distance from river (m)
Distance from road (m)
Community
CV(%)
I
II
III
IV
V
0.06 ± 0.01a
0.10 ± 0.01a
0.14 ± 0.01a
5.50 ± 1.63a
18.00 ± 4.77a
76.67 ± 6.10a
0
1.28 ± 0.03a
7.52 ± 0.46a
0.52 ± 0.02a
0.48 ± 0.03a
1.02 ± 0.14a
511.67 ± 90.64a
578.33 ± 58.24a
0.06 ± 0.01a
0.08 ± 0.01a
0.11 ± 0.01a
2.17 ± 0.56a
23.53 ± 6.12a
67.75 ± 5.91a
5.72 ± 1.10a
1.39 ± 0.04a
7.15 ± 0.43a
0.53 ± 0.03a
0.40 ± 0.02a
0.77 ± 0.19a
2396.67 ± 156.97b
255 ± 40.02a
0.08 ± 0.01a
0.11 ± 0.01a
0.23 ± 0.01b
7.81 ± 1.60a
20.69 ± 3.64a
65.90 ± 4.51a
5.63 ± 0.89a
1.41 ± 0.05a
5.77 ± 0.20a
0.48 ± 0.02a
0.51 ± 0.01a
1.93 ± 0.29a
1943.75 ± 90.41b
238.75 ± 24.06a
0.08 ± 0.005a
0.13 ± 0.005a
0.17 ± 0.01a
6.88 ± 0.68a
37.39 ± 3.16a
55.73 ± 3.59a
0
1.19 ± 0.01a
7.09 ± 0.27a
0.47 ± 0.02a
0.43 ± 0.01a
1.12 ± 0.07a
1624.17 ± 80.31b
217.50 ± 16.10a
0.08 ± 0.01a
0.09 ± 0.01a
0.16 ± 0.01ab
5.67 ± 0.83a
44.42 ± 6.20a
49.92 ± 6.83a
0
1.34 ± 0.04a
8.09 ± 0.56a
0.48 ± 0.04a
0.50 ± 0.02a
1.40 ± 0.26a
2001.67 ± 114.85b
861.67 ± 134.02b
88.93
62.85
47.42
140.66
128.19
62.83
121.16
17.76
37.63
36.33
29.92
109.29
55.31
83.47
Notes: Values represent mean ± SE. Values with the same letter are not significantly different at the 0.05 significance level (LSD test), while values of an index followed by different
letters differ significantly at the 0.05 significance level (LSD test). CV, coefficient of variation; SWC1, 0–30 cm soil moisture; SWC2, 30–100 cm soil moisture; SWC3, 100–200 cm soil
moisture; BD, bulk density; SOM, soil organic matter; TN, total nitrogen; TP, total phosphorus; TS, total salt content.
Appendix B. Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foreco.2017.10.003.
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