沙漠研究 24-1, 17-20 (2014)
Ἃ₍◊✲ 24-1, 17 - 20 (2014)
Journal of Arid Land Studies
Journal of Arid Land Studies－ DT11 Refereed Paper －㸫DT11 Refereed Paper㸫

Dynamics of Canopy Tree of Riparian Forest, Populus euphratica,
and Their Relationship with Environmental Conditions in Ejina Oasis,
Inner Mongolia, China
Xiaogang LI*1), Yuling LI2), Guosheng ZHANG3), Linhe WANG3) and Ken YOSHIKAWA1)
Abstract: In order to elucidate the ecological mechanisms sustaining the riparian forest under unreliable environmental conditions,replicate censuses were conducted to clarify the structure and dynamics of Populus euphratica forest in Ejina Oasis. Judging fromthe changes in stem length, the leaf biomass remained at the same level under different weather conditions. Dieback phenomenon iscommon in Ejina Oasis and spatial distribution of well growing and dieback canopy trees indicated some aspect of canopy dynamics
with frequent dieback was not caused by the shortage of water supply but by the competition for water among canopy trees.
Key Words: Canopy tree, Ejina Oasis, Forest dynamics, Populus euphratica, Spatial distribution

1. Introduction
There are many kinds of ecosystems in dryland
characterized by small biomass under a lack of water.
Therefore, if sufficient amount of water is supplied, a highly
productive ecosystem can be established even in dryland.
Oasis is an unique habitat superabundant in water resource andcan support diverse and a fertile ecosystem in it. As water inoasis is mainly supplied by river and/or groundwater comingfrom outside water reservoirs, the vegetation in oasis isvulnerable to irregular hydrological conditions, especially theamount of rainfall and its temporal distribution. Therefore,most of plants being able to grow in arid and semi-arid regions
have high drought tolerance in physiological and/or ecologicaltraits. However, some oasis species, such as Populus
euphratica Oliv, are not markedly drought-tolerant and are able
to persist only in riverine sites with high water availability.
P. euphratica is a famous deciduous arbor in Salicaceae as
a riparian forest in arid and semi-arid regions between
Morocco and Mongolia, and between 30 and 47 degrees
northern latitude. Heihe River originated from Qilian
Mountains and reached the west and east Juyanhai Lake has
the second largest inland river basin in the arid region ofNorthwestern China. In the most lower reach of Heihe River(Ejina Oasis), large riparian Populus euphratica forests are
established, which have an important role to support ecologicalconditions of Ejina Oasis. It is also the first barrier tosandstorms in the middle of Heihe River valley andNorthwestern China (Xu et al., 2003). As the population* Corresponding Author: gev421214@s.okayama-u.ac.jp

along Heihe River basin had been increasing in recent decades,
river water and groundwater coming from the mountains has
been overused. Especially, as lots of reservoirs had been built
in the middle reaches, the amount of water supply in Ejina
Oasis decreased year by year, and even was cut off. As theenvironmental conditions had changed as above, most of the P.
euphratica forest were damaged in Ejina Oasis and led to landdesertification (Wang and Cheng, 1999; Qi and Luo, 2007;
Zhao et al., 2004).
To protect and restore the P. euphratica forests in Ejina
Oasis, Ecological Water Conveyance Project (EWCP) had
been carried out since 2000 by the Chinese government.
After the project, the groundwater and the vegetation have afavorable change in a small scale (Guo et al., 2009; Wang et al.,2011). But how P. euphratica forests maintain themselves inthis unreliable condition is not clearly understood.
To promote protection and restoration of P. euphratica
forests in Ejina Oasis, we undertook a study to examine thedynamics of forest structure and also determined the effects ofsite conditions on the spatial structure of these riparian stands atthe canopy level.

2. Materials and Methods
2.1. Study area
Study area is an oasis in the lower reaches of Heihe Riverin the county of Ejina, Inner Mongolia Autonomous Region,China (39°52’-42°47’N, 97°10’-103°7’E) (Fig. 1). It is atypical hyper-arid desert with the mean annual precipitation of42 mm and the mean annual potential evaporation of 3,755
㸦Received, September 12th, 2013; Accepted, February 12th, 2014㸧

Okayama University, 700-8530 1-1-1, Tsushima-naka, Kita-ku, Okayama City, Japan
1) Graduate School of Environmental and Life Science, Okayama University, Japan
2) Agricultural University of Hebei, China

3) Inner Mongolia Agricultural University, China

aabbbDensity (N/ha)

Average height (m)18Fig. 1. Study area.

mm. More than 60% of the rainfall occurs during July toSeptember. The basin elevation ranges from 898 m to 1598
m above sea level (Wen et al., 2005). Three types ofgroundwater (young groundwater, regional background waterand groundwater in Gurinai) were found in this area by Qin etal. (2012).
2.2. Tree census
A permanent experimental plot with the size of 100 m ×100 m was established in the western part of Ejina Oasis in2005. All trees were numbered and the location, height ofdead and living top, and DBH and crown width, weremeasured every year from 2009 to 2012. Topography andsoil EC were measured by mini compass and EC meter
respectively in 2010.
2.3. Data analysis
Index of aggregation (Ia) was used to analyze spatial
distribution pattern of canopy trees (contagious, random anduniform distribution, if Ia was greater than, equal to and smaller

Fig. 2. Frequency distribution of tree height.

Change in Number

Fig. 3. Temporal changes in height and density of canopy trees.
Height values are means ± SD. Different letters indicate thesignificantly differences (P<0.05).

Fig. 4.

Changes in number of canopy trees.
canopy;: dieback;:dead.

: recruit tothan unity) and spatial associations of trees and environmentalconditions (ground level and soil EC) were analyzed by using
index of local association (X) (negative, independent andpositive association, if X was smaller than, equal to and greaterthan zero). Both indices were calculated by Spatial Analysis
by Distance IndicEs (SADIE) (Perry, 1998; Perry and Dixon,2002) and significant difference from unity and zero for Ia andX, respectively, was determined by Į = 0.05.

3. Results
3.1. Tree height distribution
Frequency distribution of tree height in 2009 (Fig. 2)
shows a bimodal distribution indicating two height groups
classified at 2 m height, namely canopy trees and saplings.
3.2. Dynamics of canopy trees
The density of canopy trees showed a gradual decrease in
the last four years (Fig. 3) because the number of dead trees
was greater than that of recruit trees in every year (Fig. 4).
Meanwhile the average height of canopy trees gradually
increase in this period (Fig. 3) as the height in 2011 and 2012were significantly higher than that in 2009.
Dieback phenomenon is common in our research area.
Figure 5 shows changes in stem length of well growing (opencircle) and dieback (solid circle) canopy trees and the heights

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ChangeStemin Stemlengthlength(m)
(m)

(m)

(a)

Ia = 1.44*

(m)

(b)

Ia = 1.94*

(m)

(m)

Tree height (m)Length(m)Changein Length(m)Fig. 5. Relationship between stem lengths and heights of canopytrees in 3 years. ‫ۑ‬: well growing trees; ‫ە‬: dieback trees; :dead trees.

Fig. 7. Spatial distribution of well growing canopy trees; (a) Wellgrowing trees for all census period, (b) Dieback trees.
*: significant for contagious distribution.
Table 1. Average height (m) of well growing trees and dieback trees.
2009
2010

Well growing trees
6.15 ± 1.58 a
6.45 ± 1.52 a

Dieback trees
6.09 ± 1.58 a
6.32 ± 1.63 a

2011
2012

6.66 ± 1.55 a
7.04 ± 1.72 a

6.38 ± 1.52 a
6.42 ± 1.67 bFig. 6. Total growing length and dieback length of canopy trees. ‫ڧ‬:
growth of stem; ‫ڦ‬: dieback length.

of dead trees are shown on the abscissa axis with cross marks.
As shown in this Figure 5, most of dead trees’ heights wereshorter than those of living trees, resulting in a gradual increasein average height of canopy trees as shown in Figure 3. No
significant relationships were detected between tree height andboth growth and dieback length.
Figure 6 shows the total growth length and dieback lengthof canopy trees. Although growth length was longer thandieback length in 2009 to 2010, the total growth length anddieback length were almost balanced from 2010 to 2012, andno were not related to weather conditions, namely 11.2 mm,26.2 mm and 40.7 mm of annual precipitation from 2009 to2012, respectively.
3.3. Spatial distribution
Although both well growing and dieback trees show
significant contagious distribution of their spatial arrangement
as shown in Figure 7, trees with dieback (Fig. 7b) had thehigher degree of aggregation (Ia = 1.94), indicating spatial
unevenness of site conditions under which dieback occurs.
Heights of well growing trees increased gradually; theseindividuals became significantly taller than trees with diebackin 2012 (Table 1). Therefore, the taller trees grew well andCPA(m2/m2)

Height values are means ± SD. Different letters in the column indicate
the significant differences (P<0.05).

Fig. 8. Changes in canopy projection area (CPA) of canopy trees.
Table 2. Index of local association (X) among canopy tree of P.
euphratica, soil salinity and ground undulation.
ECTopography
Topography
Well growing trees

0.4663*0.2581*0.3685*

Dieback trees

0.1592

0.3461**: Significantly positive association, P<0.05.

increased in height, while smaller trees suffered someenvironmental stress, which resulted in dieback. Changes intotal CPA (canopy projection area) of canopy trees are shownin Figure 8 indicating almost sustainable canopy cover for fouryears.
To analyze the spatial association among canopy trees, soil
salinity and ground undulation, index of local association (X)were calculated by SADIE (Table 2). Well growing trees
were positively associated with increasing soil salinity andelevation, and dieback trees were positively associated withelevation. Thus, elevated ground promoted tree survival, andtree growth became well even under high soil salinity.

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4. Discussion
Although the dieback phenomena is the effective response
to drought stress, the total length of height growth in this forest
balanced with the sum of dieback length from 2010 to 2012
irrespective of changes in weather conditions (Fig. 6). The
stable canopy area (Fig. 8) with immediate replacement ofdieback top canopy by equivalent stem growth (Fig. 6)
suggests that the leaf biomass has remained almost at the samelevel in the study area.
If the water shortage is the main cause of dieback, high rateof dieback trees are expected in the area of high tree density.
However, the degree of aggregation of dieback trees washigher than that of well growing trees indicating that some
aspect of canopy dynamics with frequent dieback was notcaused by the shortage of water supply but by the competition
for some environmental factor(s) among canopy trees. As theaverage height of healthy trees was higher than that of dieback
trees (Table 1), it seems that taller individuals likely had
competitive advantages over smaller conspecifics.
On the other hand, high mortality of smaller trees (Fig. 5)
without any remarkable increase in newly joined trees (Fig. 4)
resulting in gradual increase in the average height (Fig. 3)
suggests a gradual change in forest canopy structure from thematured forest with wide range of tree size to the over-matured
forest with a small number of giant trees as reported by Monda
et al. (2008).
Death of some small canopy trees and high proportion of
dieback trees in bigger trees suggested the effects of soil watercondition. Gries et al. (2003) suggested that P. euphratica
can maintain a contact with stable groundwater by deep tap
root and wide spread lateral root system. Thus, monitoring
the changes in groundwater level is necessary for our furtherresearch.

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