Structure, reproduction and flood-induced dynamics of riparian ...

Structure, reproduction and
fl ood-induced dynamics of riparian
Tugai forests at the Tarim River in
Xinjiang, NW China
NIELS THEVS 1 * , STEFAN ZERBE 1 , MARTIN SCHNITTLER 1 ,
NURBAY ABDUSALIH 2 and MICHAEL SUCCOW 1
1 Institute of Botany and Landscape Ecology, University Greifswald, Grimmer Strasse 88, D-17487 Greifswald,
Germany
2 Institute of Resource and Environmental Sciences, Xinjiang University, Shengli Lu 14, 830046 Urumqi, China
* Corresponding author. E-mail: niels.thevs@uni-greifswald.de
Summary
Tugai forests are the riparian forests along the rivers in the continental desert regions of Central
Asia, i.e. the Tarim River, Amu Darya and Syr Darya. They mainly consist of Populus euphratica
Oliv., Populus pruinosa Schrenk. and Elaeagnus oxycarpa Schltdl. As a consequence of land opening
campaigns, large areas of Tugai forests were destroyed after the 1950s. Due to excessive use of
water for irrigation, the remaining Tugai forests are under severe threat. Near natural Tugai forests
still exist along the Tarim middle reaches in the Tarim Huyanglin Nature Reserve, Xinjiang, NW
China. There is a gap in understanding, how the seedlings of P. euphratica establish as trees which
continuously connect to the groundwater. Therefore, the set of conditions which must be met for
germination and successful establishment, i.e. formation of Tugai forests, was investigated along
a representative transect still under natural conditions. P. euphratica seedlings germinate in belts
during the retreat of the summer fl ood on freshly deposited sites bare of other vegetation. Such
germination sites are formed by river dynamics. While germination takes place regularly in the
study area, successful establishment is restricted to few germination events. Seedlings face dropping
groundwater levels during spring and early summer of the second year after germination. Therefore,
for successful establishment, it is essential that the fl ood of the second year starts in time and is high
enough, in order to replenish the groundwater. Furthermore, clayey soil layers in the subsoil may
play a role for successful establishment, too, as they store water better than sandy soil layers.
Introduction
Tugai forests are the riparian forests along the rivers
in the continental, winter-cold deserts of Central
Asia, i.e. along the Tarim River in the Tarim
Basin as well as the Amu Darya and Syr Darya in
the Aral Sea Basin. They mainly consist of Populus
euphratica Oliv., Populus pruinosa Schrenk. and
Elaeagnus oxycarpa Schltdl. ( Tian, 1991 ; Ogar,
2003 ). Tugai forests are associated with shrub
communities (mainly Tamarix species) and grassland
vegetation, i.e. Phragmites australis Trin.
© Institute of Chartered Foresters, 2008. All rights reserved. Forestry, Vol. 81, No. 1, 2008. doi:10.1093/forestry/cpm043
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46
ex Staud.( Ogar, 2003 ; Thevs et al. , 2007 ; Thevs
et al. , in press ). The plant species of the Tugai
forests and associated plant communities are socalled
phreatophytes. They depend on groundwater
or phreatic water along riverbanks, piedmont
springs and lake shores. Along the middle and
lower reaches of the Tarim River, the Tugai forests
almost exclusively consist of the tree species
P. euphratica ( Wang et al. , 1996 ). Consequently,
P. euphratica is a keystone species ( Bond, 1994 ) of
these riparian forests. We refer to the term Tugai
forests commonly used by researchers who work
on riparian ecosystems of Central Asia.
From a global perspective, Tugai forests are a
unique and threatened fl ood plain ecosystem. In
the desert regions of Central Asia, these forests
are an important habitat for plant and animal
life and contain the highest biodiversity in these
regions ( Thevs, 2005 ; Thevs et al. , 2005 ; Thevs,
2006 ). Additionally, these fl ood plain forests are
a major natural resource for their human populations
and provide environmental benefi ts such as
landscape preservation, wind protection, stabilization
of moving sand and soil and riverbank stabilization
( Weissgerber, 1994 ; Zerbe et al. , 2005 ;
Thevs et al. , 2007 ). At present, the world ’ s largest
contiguous Tugai forests occur along the Tarim
River and its tributaries in the Tarim basin ( Wang
et al. , 1996 ). However, the total area of Tugai forests
in the Tarim basin declined from ~ 500 000 ha
in 1958 to ~ 200 000 ha in 1978 ( Huang, 1986 ).
In the Aral Sea Basin, the area of Tugai forests
shrunk from ~ 500 000 ha in 1950 to ~ 70 000 ha in
1998 ( Treshkin, 2001 ). Compared with the other
plant communities of the Tugai vegetation, the
Tugai forests are the most threatened ecosystem
( Wang et al. , 1996 ; Treshkin, 2001 ). The Tugai
forests were destroyed directly in order to gain
new farmland and indirectly as a consequence of
excessive use of the water resources for irrigation.
The restoration of such unique forest ecosystems
is of worldwide signifi cance ( Zerbe, 2002 ; van
Andel and Aronson, 2005 ).
Comprehensive surveys on the biology and
ecology of P. euphratica are given by Huang
(1986) , Xinjiang Linkeyuan Zaolin Zhisha
Yanjiusuo (1989) , Xinjiang Ziyuan Kaifa Zonghe
Kaocha Dui (1989) , China Ministry of Forestry
(1990) , Liu et al. (1990) and Wang et al. (1996) .
Populus euphratica is an obligate phreatophyte
( Zeng et al. , 2002 ; Gries et al. , 2003 ; Foetzki,
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2004 ), which means it must continuously have
groundwater contact for survival. The maximum
groundwater depth for the survival of P. euphratica
is estimated as 10 – 13 m, according to a literature
review by Thevs (2005) . Populus euphratica
follows two reproductive strategies, i.e. generative
and clonal reproduction ( Xinjiang Linkeyuan
Zaolin Zhisha Yanjiusuo, 1989 ; Xinjiang Ziyuan
Kaifa Zonghe Kaocha Dui, 1989 ; China Ministry
of Forestry, 1990 ; Wang et al. , 1996 ; Saito et al. ,
2002 ; Bruelheide et al. , 2004 ; Westermann et al. ,
in press ).
The seeds are light, have pappus-like hairs, and
are dispersed by wind and water. The main fruiting
period is during the fl ooding period of the
Tarim River, i.e. between July and September.
Germination and establishment depend on freshly
deposited riverbanks, mostly accreting riverbanks.
Experiments confi rmed that optimal germination
occurs under conditions of high light, a temperature
between 25°C and 30°C and water-saturated
soils with a salt content lower than 0.2 per cent.
Under such conditions, the germination rate exceeds
80 per cent ( Xinjiang Linkeyuan Zaolin
Zhisha Yanjiusuo, 1989 ). Germination occurs in
lines or narrow strips marking fl ood water lines
at the riverbank and depends on a moving river
channel ( China Ministry of Forestry, 1990 ; Liu
et al. , 1990 ). As river courses move, the sites continuously
change from wet (frequently fl ooded) to
dry. These different groundwater levels are indicated
by different plant species ( Thevs, 2005 , 2006 ;
Zerbe and Thevs, 2007 ; Thevs et al. , in press ).
Riparian forests in the arid and semiarid western
USA (so-called cottonwoods) as described by
Braatne et al. (1996) appear similar to the Tugai
forests considering basic mechanisms of regeneration
and the environmental conditions: the
main tree species of these forests are also phreatophytes
( Smith et al. , 1998 ). The seeds are dispersed
also during the fl ood and germinate on
riverbanks after the fl ood. The fl ood peak occurs
in April – May, when water level rises by 2 – 3 m.
Seedling survival is signifi cantly enhanced by
clayey soil layers in the subsoil and by summer
precipitation. Ice scoring during the winter may
erode a signifi cant number of seedlings ( Rood
et al. , 1998 ; Cooper et al. , 1999 ; Stromberg,
2001 ). Thus, a set of conditions must be met for
germination and survival of the seedlings in the
cottonwoods: coinciding time of fl ood retreat

and seed dispersal, freshly deposited riverbanks
as germination sites, additional summer rainfall,
clayey soil layers in the subsoil and germination
sites located high enough to avoid ice scoring.
The Tugai forests face harsher environmental
conditions than the cottonwoods. The water
level during fl oods rises 3 – 4 m. The fl ood season
and the time of seed dispersal occur from July to
September ( Song et al. , 2000 ). Seedlings thus
have to survive the winter shortly after germination.
Furthermore, there are no signifi cant additional
rainfall events, as the annual precipitation
does not exceed 50 mm ( Weili Xian Difangzhe
Bianzuan Weiyuanhui, 1993 ).
The patterns of reproduction and forest formation
of the cottonwoods therefore cannot be
directly applied to the Tugai forests and there
is a gap in understanding how do seedlings of
P. euphratica establish as trees which continuously
connect to the groundwater. Our objective was to
identify the set of conditions which must be met
for germination and successful establishment, i.e.
FLOOD-INDUCED DYNAMICS OF TUGAI FORESTS 47
formation of Tugai forests, based on the knowledge
inherited from the cottonwoods. As a case
study, we investigated the river course movement
of the Tarim during the past decades, recent fl ood
events, soil texture and the structure and spatial
distribution of vegetation types along a representative
transect still under natural conditions.
This study has to be seen against the background
of recent constructions of lateral dykes
along the middle reaches of the Tarim River in
2004. Water is channelled through these dykes
down to the drought-prone lower reaches of the
Tarim River ( Song et al. , 2000 ; Giese et al. , 2005 ,
Westermann et al. , in press ).
Study area
The transect chosen for this study is located in the
western part of the Tarim Huyanglin Nature Reserve
at the middle reaches of the Tarim River near
the settlement Iminqäk ( Figure 1 ). The transect
Figure 1 . Location of the investigation area in South Xinjiang, NW China with the Tarim River and the
Tarim Huyanglin Nature Reserve, location between the latitude of 40° 55 ′ N and 41° 15 ′ N and longitude
of 84° 15 ′ E and 85° 30 ′ E.

48
stretches between 41° 12.1 ′ N, 84° 22.9 ′ E and
41° 12.5 ′ N, 84° 22.0 ′ E.
The Tarim river system is located at the northern
edge of the Taklamakan Desert, which is
337 600 km 2 and the second largest sand desert
of the world ( Zhu, 1986 ). The Tarim basin is
equivalent to the southern part of the Xinjiang
province, NW China. It is bordered by the mountain
ranges of the Tian Shan in the north and
northeast, the Pamir in the west and the Kunlun
(Kurum) Mountains and Tibetan Plateau, respectively,
in the south and southeast ( Figure 1 ). Some
of the surrounding mountain ranges exceed 7000 m
above sea level. Consequently, all humid air currents
are cut off and the climate is continentalarid
with a mean annual precipitation of less than
50 mm and a mean annual potential evapotranspiration
of more than 2500 mm. In January, the
average air temperature is − 9°C and 25°C in July,
thus also refl ecting the strong continental character
of the climate ( Weili Xian Difangzhe Bianzuan
Weiyuanhui, 1993 ; Yuan and Li, 1998 ; Xinjiang
Weiwuer Zizhiqu Shuili Ting and Xinjiang Shuili
Xuehui, 1999 ).
The Tarim River, with a total length of 1321
km, is fed by melt water and precipitation in the
mountains through its three tributaries Aksu,
Hotan and Yarkant. The river used to end in the
Taitema Lake, but it has not reached this lake for
the last 30 years, because all water entering the
Tarim lower reaches was diverted for irrigation.
About 75 per cent of the annual run off of the
Tarim River is concentrated in the months July,
August and September ( Figure 2 ) resulting in annual
summer fl ood periods. The annual discharge
varies from year to year. Each fl ood event transports
huge loads of sediments into the large alluvial
plain and thus continuously raises the river’s
base level ( Xia, 1998 ). Every fl ood changes the
river course, erodes land and forms new riverbanks
and opens new river branches or closes existing
ones. Extraordinary fl oods can even open
entirely new river courses or close river courses
( Figure 3 ; see also Zhou, 1989 ).
In the rather level fl ood plain along the middle
reaches, the Tarim River has formed a wide inland
delta with a web of braided branches. However,
due to diversion of water for irrigation and river
management, the fl ood dynamics have lost much
of its former strength. The most dynamic part of
the Tarim River now lies in the western part of
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Figure 2 . Monthly runoff of the Tarim River at Yengi
Bazar from 1992 to 2005 (data provided by Tarim
Watershed Administration Bureau, Korla, China).
Figure 3 . Changes of the Tarim River course along
the middle reaches of the Tarim according to Hedin
(1905) and Zhonghua Renmin Gongheguo Guojia
Tuciju (1959) and Landsat Multispectral Scanner
(path 155, row 31), 20 October 1973, Landsat Enhanced
Thematic Mapper (path 144, row 31), 06
July 2000 images.
the Tarim Huyanglin Nature Reserve around
Iminqäk ( Figures 1 and 4 ), because no lateral dyke
(cf. Introduction) has been built there ( Yuan and
Li, 1998 ; Xinjiang Talimu Huyang Ziranbaohuqu
Guanlizhan, 2002 ; Thevs, 2005 ).
Methods
The study was conducted on a representative transect
of 900 m length perpendicular to the Tarim

River near Iminqäk (Bügür County) in the Tarim
Huyanglin Nature Reserve. We have chosen a
representative reach of the Tarim River which has
maintained its natural dynamic ( Figure 4 ). River
course changes were traced through satellite images
and personal observations in the fi eld ( Table 1 ).
The satellite image from 2001, Landsat Enhanced
Thematic Mapper (ETM) was rectifi ed based on
street crossings as reference points. The rectifi ed
image was projected into the Universal Transverse
Mercator coordinate system (zone T 45). The
geographical positions of the street crossings were
measured using a Garmin 12 Global Positioning
System. The other satellite images were aligned to
the rectifi ed satellite image from 2001.
To characterize the soils, seven profi les were
drilled in July 2004. The locations of the soil profi
les are shown in Figure 5 . The soil profi les were
drilled as deep as the groundwater layer at base
Figure 4 . River course of the Tarim River around
Iminqak in 1973, 1992 and 2001 derived from
Landsat Multispectral Scanner (path 155, row 31,
20 October 1973), Landsat Thematic Mapper (path
144, row 31, 26 September 1992) and Landsat Enhenced
Thematic Mapper (path 144, row 31, 10
August 2001) images.
FLOOD-INDUCED DYNAMICS OF TUGAI FORESTS 49
fl ow of the Tarim River in July 2004, i.e. between
155 and 500 cm. Soil texture was recorded in the
fi eld for all horizons according to the fi eld estimation
key for soil texture of the Boden (1994) . This
fi eld estimation key for soil texture is based on
the stickiness and plasticity of a moist soil sample
and, on principle, is a similar approach as by
Brady and Weil (1999) . The key employed here
( Boden, 1994 ) is based on the following ranges of
grain sizes for sand, silt and clay: sand 2000-63
μ m, silt 63-2 μ m and clay below 2 μ m. Additionally,
in July 2003, the elevation was measured
along a line, connecting all seven soil profi les
using a levelling device and a rod. The zero elevation
base was the base fl ow in July 2004.
Along the transect, vegetation was mapped in
a 100-m wide strip in September 2004. The vegetation
map was digitized using the cartography
software Polyplot 5.4 ( www.polyplot.de ). The
map units of the vegetation map were vegetation
types, which were derived from their dominant
species and which are commonly used by Chinese
scientists (according to the literature survey by
Thevs, 2005 ). The nomenclature of the species
followed Hudaberdi and Xu (2000) . The transect
cuts through rows of P. euphratica trees, which
we consider originated from germination events.
The age of tree rows was determined by analysing
tree cores, taken at a height of 35 cm with a
Suunto tree corer of 5 mm diameter in September
2004. The 10 tallest trees were chosen from the
tree row in the belt of the vegetation map. The
presence of P. euphratica seedlings was mapped
every September from 2004 until 2006.
In 2002, 2003 and 2004, the area fl ooded
during the summer fl ood was recorded in the
fi eld. Data on daily water runoff at Yengi Bazar
(about 30 km upstream of the transect) from
Table 1 : Data sources for tracing the river course changes of the Tarim River near the settlement of Iminqäk in
the western part of the Tarim Huyanglin Nature Reserve
Dating of fl oods Source
Flood in 1973 Landsat MSS (path 155, row 31), 20 October 1973, retrieved from www.landsat.org
Flood in 1992 Landsat TM (path 144, row 31), 26 September 1992, retrieved from www.landsat.org
Base fl ow in 2000 Landsat ETM (path 144, row 31), 06 July 2000, purchased from GAF, Germany
Flood in 2001 Landsat ETM (path 144, row 31), 10 August 2001, purchased from GAF, Germany
Base fl ow in 2004 N. Thevs, personal observation
Base fl ow in 2005 Quickbird image, 15 June 2005, purchased from GAF, Germany
MSS = Multispectral Scanner, TM = Thematic Mapper, ETM = Enhanced Thematic Mapper.

50
Figure 5 . The transect investigated, with its seven
soil profi les and the location of the Tarim River
course during fl ood 1973, 1992 and 2001 and base
fl ow 2000, 2004 and 2005, cf. Table 1 .
1992 to 2005 were kindly provided by the Tarim
Watershed Bureau. Data on annual runoff were
available from 1957 to 1991. At 10 August 2002,
the fl ood line was recorded at the transect between
sites no. 3 and 5 ( Figure 5 ). Furthermore,
the riverbank at site no. 2 was fl ooded too. The
corresponding runoff at Yengi Bazar was 446 m 3
s − 1 at 10 August 2002. Through this threshold,
we were able to roughly trace back how long the
parts of the transect with a low elevation were
fl ooded in each year after 1991.
Results
Reconstructed river course changes
The location of the Tarim River main course during
the fl ood periods in 1973, 1992 and 2001 as
well as the base fl ow in 2000, 2004 and 2005 in
relation to the transect is shown in Figure 5 . The
channel changed its course from the location of
sites no. 4 and 5 towards the south between the
years 1973 and 1992. Since then, it has moved
gradually southward and relocated the meander
shown in Figure 5 , forming a wide accreting riverbank.
In 1992, the Tarim River fl owed between
sites no. 2 and 3. Site no. 3 was situated close
to the northern bank of the Tarim River and site
no. 2 close to the southern bank. Currently, site
no. 2 clearly is located on the northern riverbank
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and the levee with the highest elevation had been
formed there. The Tarim River migrated signifi -
cantly during the past 30 years ( Figures 5 and 6 ).
Soil properties and relief
The soil horizons of all seven profi les with their
texture are shown in Figure 6 . The soil profi les at
the sites no. 1 and 2 consist of sediments from the
Tarim River which were deposited after 1992 (cf.
the relocation of the Tarim River from 1992 onwards
in Figure 6 ) and mainly consist of sand and
silt. In profi le no. 1, no horizon of fi ne sediment
like clay or clayey silt were recorded, while in profi
le no. 2, a horizon of clayey silt not exceeding
10 cm was found. In soil profi le no. 3, we observed
a rather thick clayey horizon of 85 cm in the subsoil,
but silt in the topsoil. The soil profi les of sites
no. 4 and 5 had a top horizon with clayey silt and
clay, respectively, and showed fi ne-grained horizons
of clayey silt and clay in the subsoil. In particular
in profi le no. 5, fi ne-grained horizons were
characteristic throughout the whole profi le.
The levee at site no. 2 and the terrace next to
site no. 5 are the highest points of the transect,
not considering the dunes around site no. 7. From
the levee at site no. 2 towards site no. 3, the elevation
decreases slightly. Site no. 3 lies in between
two fl at levees. Between site no. 3 and the terrace
at site 5, the transect is bowl shaped ( Figure 6 ).
Flood plain vegetation
The distribution of the vegetation along the transect
from the current river course towards the hinterland
can be described as follows ( Figure 6 ):
1 Myricaria pulcherrima Batalin. reed: this vegetation
type formed a belt along the current
levee on the accreting riverbank ( Figure 7 ).
Here, narrow belts (0.6 – 1 m wide each) of
P. euphratica seedlings and saplings were
found. One group of seedling belts had germinated
in 2002, but did not survive until
autumn 2004. Site no. 2, sampled in 2004,
was located on the levee within a belt of P. euphratica
seedlings. Another strip of seedlings
was found in 2006, which had germinated in
2005. These seedlings survived till autumn
2006.

2 Calamagrostis pseudophragmites Link ex
Rchb. reed: towards the hinterland, the
M. pulcherrima reed is replaced by C. pseudophragmites
( Figure 7 ). Within this vegetation,
a few single shrubs of the genus Tamarix and
small P. euphratica specimens occurred.
3 Glycyrrhiza infl ata Batalin. Halimodendron
halodendron Druce P. euphratica Tugai
forest with rows of low and tall growing
P. euphratica specimens: the C. pseudophragmites
reed is bordered by P. euphratica
shrubs and a row of P. euphratica trees
next to site no. 3. The age, i.e. minimum
age, was determined to be 11 – 12 years
( n = 10 trees). Right along the transect, three
more P. euphratica tree rows, which were
taller than 6 m in height, were found. The
space in between is fi lled with P. euphratica
shrubs, H. halodendron and G. infl ata .
FLOOD-INDUCED DYNAMICS OF TUGAI FORESTS 51
Figure 6 . Three-dimensional profi le of the transect at the middle reaches of the Tarim with its elevation, soil
profi les and vegetation types. The closed capillary fringe refers to the soil depth at which all soil pores are
fi lled with water through capillary rise. Numbers indicate soil profi les. Letters indicate plant communities
after Thevs (2006) : (A): Myricaria pulcherrima community, (B): Calamagrostis pseudophragmites reed, (C):
Halimodendron halodendron Glycyrrhiza infl ata Tugai forest (bush-like Populus euphratica and tree rows
from P. euphratica ), (D): Phragmites australis Tugai forest, (E): H. halodendron G. infl ata Tugai forest
( P. euphratica trees), (F): Tamarix ramosissima Tugai forest.
4 Phragmites australis Tugai forest: within
the depression around sites no. 4 and 5,
the density of P. euphratica shrubs decreases
slightly. Lotus tenuis Waldst. &
Kit. ex Willd and P. australis increase in
coverage while H. halodendron disappears
( Figure 8 ).
5 Glycyrrhiza infl ata Batalin. H. halodendron
Druce P. euphratica Tugai forest: this vegetation
type covers the area of the fi rst terrace
( Figure 8 ), i.e. corresponding with the former
northern riverbank of the Tarim. This forest
type stretches across site no. 6 towards the
dunes around site no. 7.
6 Tamarix – P. euphratica Tugai forest: this forest
community grows on the dunes in the hinterland
of the transect starting at site no. 7,
which is ~ 800 m from the river course.
We observed in 2002 that the current levee and
the area between levee and site 3 were slightly

52
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Figure 7 . Accreting riverbank of the Tarim at the transect investigated, with Myricaria pulcherrima community
(site 2) and Calamagrostis pseudophragmites reed on the accreting riverbank and Tugai forest in the
background. Site 1 is located on the bare riverbank (photograph: N. Thevs, July 2004).
Figure 8 . Young bush-like Populus euphratica at site 5 with the terrace and the older Tugai forest in the
background (photograph: N. Thevs, July 2004).

above the fl ood line on 10 August 2002. At that
day, a discharge of 446 m 3 s − 1 was measured at
Yengi Bazar. Later in 2002, the water level rose
further and fl ooded the levee. The discharge
measured at Yengi Bazar increased, too, after 10
August 2002. Figure 9 shows the time span with a
fl ow of 446 m 3 s − 1 and more at Yengi Bazar from
1992 to 2005, i.e. the time span during which
the lower parts of the transect were fl ooded. This
time span ranges from 0 to 39 days, indicating
that the part of the transect between the levee and
site 3 is not fl ooded annually.
Discussion
The satellite images and our observations show
that the riverbank south of site no. 3 has been
formed since 1992. This is in accordance with the
soil texture of these two profi les near the current
river course, i.e. the sedimentation of sand and
silt. These sediments were deposited from more
or less rapidly fl owing water as the stream only
deposits larger grain sizes here ( Graf, 1988 ). The
lower part of the transect (between site no. 3 and
site no. 5) is fl ooded more often and longer than
the levee and, additionally, it is fl ooded by stagnant
water. Therefore, the profi les no. 4 and 5
have clayey top layers. Profi le no. 5 lies in the
Tarim River course from 1973. Since then clayey
material has deposited there.
Figure 9 . Number of days with a discharge of
446 m 3 s − 1 and more per year, measured at Yengi
Bazar from 1992 to 2005.
FLOOD-INDUCED DYNAMICS OF TUGAI FORESTS 53
We conclude that the Tarim changed its riverbed
after a single fl ood event rather than through
gradual river course changes to a position close
to the course of 1992. Since then, it continuously
moved southward forming the accreting
riverbank with sites no. 1, 2 and 3. Such sudden
river course changes were also reported by Zhou
(1989) for the Tarim River and stated by Graf
(1988) for dry-land rivers. If the Tarim River had
moved gradually all the way from the 1973 course
down to the 1992 course, the clayey soil horizons
in the soil profi les sites no. 3 and 4 would have
been washed away and replaced by sandy and
silty sediments as at sites no. 1 and 2.
A characteristic feature of the Tugai forests is
that P. euphratica tree rows alternate with shrubby
P. euphratica . These rows developed after seed
germination from a former seedling belt immediately
adjacent to the river course. Similar forest
structures were observed by Scott et al. ( 1996 ,
1997) in cottonwoods. The shrubs between the
P. euphratica tree rows are root suckers as revealed
by investigations on the age structure of
Tugai forest in the Tarim basin by Westermann
et al. (in press) . The small shrubby poplars are
connected by horizontal roots instead of having
taproots like the seedlings in the strip at site no. 2.
As P. euphratica has a very high light demand
for germination ( Huang, 1986 ; China Ministry
of Forestry, 1990 ; Liu et al. , 1990 ; Wang et al. ,
1996 ), generative reproduction can only occur
outside established fl ood plain forests.
Though the Tarim River has gradually moved
southward since 1992 and deposited sediments on
the accreting riverbank, we only observe two distinct
groups of P. euphratica stands which originate
from germination events, i.e. the seedling
belt at site no. 2 and the tree row at site 3. We did
not observe trees which are the consequence of
annually repeating germination events. The seedling
belts at site no. 2 germinated in autumn 2002
and autumn 2005, respectively. The trees in the
fi rst row (site no. 3) are at least 12 years old and
thus are older than 1992. These two seedlings
belts and the tree row thus refl ect germination
and successful establishment. Such tree belts as
a consequence of single germination events and
successful establishment in distinct years were
also observed by Xinjiang Linkeyuan Zaolin
Zhisha Yanjiusuo (1989) for a site between Yengi
Bazar and Iminqäk.

54
The time gaps between germination events according
to our current state of knowledge can be
explained by three possible causes:
1 Flood events differ in time and peak level
and are not closely related to the main seed
ripening time of P. euphratica , which are the
months September and October. Some fl oods
may expose the sites suitable for germination
too early, others too late.
2 In principle, seed germination can occur continuously
every year. However, the establishment
of seedlings may be a bottleneck. Thus,
we only can observe the successfully established
trees and saplings and those seedlings which
still are alive. For cottonwoods, ice scouring,
drowning or erosion during the fl ood of the
year following germination as well as drought
were reported as factors infl uencing the mortality
of seedlings ( Rood et al. , 1998 ; Cooper
et al. , 1999 ; Stromberg, 2001 ). If seeds germinate
on a low position of the riverbank, they,
on the one hand, are close to the groundwater,
but, on the other hand, have to face the risk
of being eroded away by the winter ice or the
subsequent fl ood. When seeds germinate on a
higher position, the risk of erosion is lower,
but the groundwater level falls deeper relative
to the seedlings ’ position during the base fl ow
period. Thus, the risk of mortality during the
base fl ow period is higher on a high elevation
position. During base fl ow, i.e. before the
onset of the summer fl ood the soil commonly
dries out in such an extreme continental-arid
climate as typical for the Taklamakan desert.
This risk of drought can be limited by fl oods,
which recharge the groundwater in time and
as high as into the horizons rooted by the
seedlings. The fl oods in 2002 and 2005 apparently
were high enough to deposit seeds on levees
at site no. 2. The fl oods in 2003 and 2006
started in time, i.e. July. The fl ood 2003 was
as high as 2002 and the fl ood 2006 was even
higher than in 2005 (N. Thevs, personal observation).
Consequently, the fl oods in 2003
and 2006 recharged the soil in time. The fl ood
in 2004 ( Figure 2 ) apparently did not moisten
the soil suffi ciently and the seedlings were not
able to survive.
3 The soil texture might be another factor
which infl uences the successful establishment
FORESTRY
of P. euphratica trees. Cooper et al. (1999)
reported that Populus deltoides seedlings
survived at signifi cantly higher rates on sites
with loam layers >10 cm thick in the upper
45 cm of the soil, because it stores water
better than sandy soils. In our transect, the
clayey horizons in the subsoil of site no. 3
thus may have enhanced the successful establishment
of the trees there. This area is
fl ooded during fl ood peaks when the levee at
site no. 2 is fl ooded, too, as the elevation and
traces of fl owing water indicate.
The rivers Syr Darya and Amu Darya, which next
to the Tarim River once harboured large areas of
Tugai forests, have lost all their natural fl ood regime.
Only occasional fl ood pulses occur depending
on the water consumption of the irrigation
schemes and the management of the upstream
water reservoirs. Mostly, those fl ood pulses do
not coincide with seed dispersal and they are too
weak to relocate the river course ( Giese et al. ,
2004 ). Thus, the Tugai forests of the Aral Sea
Basin reproduce only vegetatively ( Wang et al. ,
1996 ).
Conclusions
The river dynamics and fl ood events play a key
role in the formation of Tugai forests. The river
dynamics create site suitable for the germination
of P. euphratica , i.e. ‘ safe sites ’ after Urbanska
(1997) . Such sites, not covered by dense vegetation
(e.g. reeds), only can be formed when the
river moves laterally. Floods drift the seeds to
germination sites as well as recharge the soil so
that seeds can germinate. But in the following
year at least, a fl ood is essential, in order to recharge
the soil moisture for the seedlings. This
is the main difference between Tugai forests and
the North American cottonwoods. In the cottonwoods,
the seeds germinate in spring and the
seedlings thus can grow during nearly the whole
vegetation season, which is supported by rainfall
in summer. Seedlings in Tugai forests have to
survive the winter a short time after germination
and right after the winter they have to face severe
droughts. Additionally, clayey horizons can
enhance the successful establishment. Such sites
are created if the river erodes a new bed and cuts

through sediments deposited in backswamps,
i.e. sediments containing clayey layers deposited
under conditions of stagnant water.
Accordingly, those parts of the Tarim River with
prevailing natural river dynamics are crucial for the
continuous natural regeneration and sustainable
conservation of the Tugai forests. Additionally,
generative reproduction will provide genetic diversity
within the Tugai forests, which has to be
considered essential for their conservation ( Meffe
and Ronald Carrol, 1997 ). Such generative reproduction
today is restricted to the middle reaches
of the Tarim River. Therefore, the Tarim middle
reaches must become a core area for Tugai forest
conservation. The dykes which were constructed
in the past years will decrease the dynamics of
the river system and consequently will be a threat
for conservation and regeneration of the Tugai
forests at the middle reaches of the Tarim River.
Within the conditions of the regulated Tarim,
water managers must contribute to the conservation,
maintenance and development of the Tugai
forests by providing water for fl oods.
Funding
The Volkswagen Foundation (1/78636); the Heinrich-
Böll-Foundation; the Deutscher Akademischer Austauschdienst
(D/05/06946).
Acknowledgements
We thank the Tarim Watershed Administration Bureau,
Korla, China, for providing us with hydrological data
on the Tarim River. We also thank the editors and
reviewers who helped to improve the manuscript.
Confl ict of Interest Statement
None declared.
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Received 21 February 2007