沙漠研究 24-1, 29-32 (2014)
Ἃ₍◊✲ 24-1, 29 -32 (2014)
Journal of Arid Land Studies
Journal of Arid Land Studies－ DT11 Refereed Paper －㸫DT11 Refereed Paper㸫

Estimation of Water Sources of Invasive Tree Species in Arid Environments
by Oxygen Stable Isotope Analysis
Tadaomi SAITO*1), Mayu TSUKUMO2), Mohamed A. M. Abd ELBASIT3), 9), Hiroshi YASUDA4)
Takayuki KAWAI5), Naoko MATSUO6), Koji INOSAKO1), Kumud ACHARYA7), Amir E. BABIKER3)
Abubakr A. HAMD3) and Hiroshi NAWATA8)
Abstract: The objective of this study is to estimate water resources of native and invasive tree species in arid environments byoxygen stable isotope analysis. The target trees were Prosopis juliflora (invasive) in Sudan and Tamarix ramosissima (invasive) andProsopis pubescens (native) in USA. Three sampling sites were established in Khartoum, Sudan and one site was established in
Nevada, USA in 2012. Stems of target trees, soils at various depths, groundwater and rainwater were collected in each site.
Meteorological conditions and soil-stem water content have been monitored in these sites. For the plant and soil samples, water was
extracted using a vacuum distillation system. The į18O content of the groundwater, rainfall, stem water and soil water were measured
using an isotope ratio mass spectrometer. The results from oxygen stable isotope analysis showed that Prosopis juliflora in Sudan did
not use only groundwater but also used the soil water in the rainy season, although this species is generally thought to use groundwater
through deep tap roots. Similar results were shown from stem water content monitoring in the sites. Both Tamarix ramosissima
and Prosopis pubescens in USA used shallow saline groundwater, suggesting that not only Tamarix but also Prosopis pubescens are
salt tolerant species.
Key Words: Invasive species, Mesquite, Oxygen stable isotope analysis, Tamarix, Xylem water

1. Introduction
Trees in arid and semi-arid regions have several
mechanisms to make efficient use of limited water resources tosurvive harsh conditions. Recently, approaches using stable
isotopes have been developed as a powerful tool forinvestigating processes in plant-water relations such asrecognizing plant water use and response to different types ofwater sources, better understanding water utilization processes,
water use efficiency, and the pattern, mechanism, and ability
to adopt to arid environments (Yang et al., 2010). In nature,
there exist two stable isotopes of hydrogen (1H - protium and2
H - deuterium) and three stable isotopes of oxygen (16O, 17O,
18
O). The concentrations of 2H and 18O in meteoric waters
exhibit a broad range of variations, both in time and space.
The isotopic composition of water within the water cycle
varies during phase changes (evaporation and condensation).
For example, 18O has two extra neutrons and is a bit heavier
than 16O, which results in a preference for evaporating thelighter 16O containing water and leaving more of the 18O water
behind in the liquid state (called isotopic fractionation).
* Corresponding Author: tadaomi@muses.tottori-u.ac.jp

However, water is not isotopically fractionated when taken up
by the plant (Thorburn and Walker, 1994). As a result, waterin plant tissues carries the same isotopic signal as the source
water (e.g. rainwater, groundwater, soil water, etc.) until itreaches the sites of evaporation or transpiration, generally inthe leaves. Therefore, the isotope ratio of stem water can beused as a measure of the isotopic signature of the water being
utilized (Craig, 1966). The objective of this study is toestimate water sources of native and invasive tree species inarid environments by oxygen stable isotope analysis. The
target trees were Prosopis juliflora (invasive) in Sudan andTamarix ramosissima (invasive) and Prosopis pubescens(native) in US.

2. Materials and Methods
2.1. Target trees
Prosopis (mesquite) is a genus of “multi-purpose” tree,
native to South and Central America and the Caribbean,
which has been introduced widely into arid and semi-arid
regions of the world (Gallaher and Merlin, 2010; Pasiecznik etal., 2001). Nilsen et al. (1983) reported that Prosopis
㸦Received, November 10th, 2013; Accepted, February 1st, 2014㸧

Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori, 680-8553, Japan
1) Faculty of Agriculture, Tottori University6) Graduate School of Bioresources, Mie University

2) Graduate School of Agriculture, Tottori University

7) Center for Environmental Remediation and Monitoring, Desert Research Institute, USA

3) Environment and Natural Resources and Desertification Research Institute, Sudan

8) Research Institute for Humanity and Nature, Japan

4) Arid Land Research Center, Tottori University
5) Research Institute for Natural Hazards and Disaster Recovery, Nigata University

9) Faculty of Science, University of Johannesburg

30

glandulosa in the Sonoran Desert of southern California avoids
water stress under severe drought conditions by tapping waterfrom the groundwater table using deep tap roots, and acquires
greater drought tolerance with osmotic or stomatal
conductance.
Such morphological and/or physiologicalproperties facilitate rapid invasion by Prosopis in arid andsemi-arid environments, posing a threat to human subsistence(Berhanu and Tesfaye, 2006; Bosu et al., 2009; Milton and
Dean, 2010; Mwangi and Swallow, 2008).
Tamarix ramosissima (saltceder) is native to Southeastern
Europe and Asia and well known as a major invasive species in
the Southwestern and arid regions in US, causing biodiversity
reduction and increasing risk for wildlife species in riparianzones (Bateman and Paxton, 2010). Tamarix is salt tolerant
species and rapidly spreading in river systems using shallow
saline ground water (Imada et al., 2012).
2.2. Site description and sample collection
2.2.1. Sudan sites
Three sampling sites (Al Rawakeeb, Soba and Al Kadaro)for isotope analysis were established in Prosopis juliflora(mesquite) communities in Khartoum, Sudan in May 2012.
Annual precipitation of the sites is about 150 mm and rainyseason is from June to September. The groundwater levels of
Al Rawakeeb, Soba and Al Kadaro were about 80, 20 and 23
m, respectively. Two Prosopis trees were selected in each site
and two stems (or branches) were cut per one tree about every
month. The branches were chopped and then placed in glass
bottles. Soil (0-30 cm) and groundwater samples were also
taken using bottles about every month in each site. Rainwater
samples were taken only in Al Rawakeeb site. There were
five rainfall events in 2012 in the Al Rawakeeb area andrainwater samples were taken at every event. In Soba and Al
Kadaro, monitoring of sap flow, soil-stem water content,
groundwater level and meteorological conditions have also
been conducted (Saito et al., 2013).
2.2.2. USA site
One sampling site (Virgin River) was established in amixed community of Tamarix ramosissima and Prosopis
pubescens in Nevada, USA. Annual precipitation of the sites
is about 400 mm. The site was located near Virgin River andthe groundwater level was shallow (about 1 m). In addition,salinity of groundwater was high (electrical conductivity was
18 dS/m) and salt accumulation was observed at the soil
surface in the site. Four branch samples from two Tamarix
trees and two Prosopis trees, soil samples at 10, 20, 50 and 80
cm depths, and groundwater samples were taken in April 2012.
2.3. Laboratory analysis and data analysis
For the plant and soil samples, water was extracted using a

Fig. 1. Relationship between amount of precipitation andį18O ofrainwater in Al Rawakeeb in 2012. Higher į18O value means
heavier water. When the į18O value is equal to zero, the weightof the water is same as the Standard Mean Ocean Water,
V-SMOW.

vacuum distillation system.
The 18O content of thegroundwater, rainfall, stem water and soil water were measured
using an isotope ratio mass spectrometer (DELTA V: Thermo
Fisher Scientific) at Tottori University, Japan.
The isotopic ratio of 18O is expressed by į18O (‰) asfollows:

G 18O

Rsample  Rsmow
Rsmow

u 1000(‰)(1)

where R is the ratio of 18O/16O in the sampled water (Rsample) orin Standard Mean Ocean Water, V-SMOW (RSMOW), according
to the International Atomic Energy Agency (IAEA). Overall
analytical precision of the spectrometer was ±0.2‰ for į18O.

3. Results and Discussion
3.1. Results in Sudan
Relationship between amount of precipitation and į18O
values of rainwater in Al Rawakeeb was shown in Figure 1.
The į18O contents of precipitation decreased with increase in
the amount of the precipitation. This relationship is called the“amount effect” (Dansgaard, 1964).
Variations in į18O and amount of precipitation in Al
Rawakeeb were shown in Figure 2. The į18O values of thegroundwater were low and almost constant. The į18O valuesof the rainwater were basically higher than those of thegroundwater and varied with the amount of the precipitation asshown in Figure 1. The į18O values of the soil water werehigh except for Aug. 9 because light water molecule tends toevaporate and heavy water molecule tends to remain at or near
the soil surface. On Aug. 9, the į18O value of the soil water
was similar to the rainwater because the soil seemed to be
filled with low į18O rainwater after two heavy rainfall events.
The į18O value of the stem water on June 25 (before the rainyseason) was much higher than that of the groundwater,

31

2
2

Aamount of precipitation
Aamount
Rainwaterof precipitation
Rainwater
Soil water (0-30cm)
Soil water (0-30cm)

80
80

Groundwater
Groundwater
Stem water
Stem water

70
70

į18
į18
OO(permil)(permil)

1
1

60
60

0
0

50
50

Ͳ1
Ͳ1

40
40

Ͳ2
Ͳ2

30
30

Ͳ3
Ͳ3

20
20

Ͳ4
Ͳ4

AmountAmountofofprecipitation(mm)precipitation(mm)

3
3

10
10

Ͳ5
Ͳ5
Ͳ6
Ͳ6
25ͲJun25ͲJun10ͲJul10ͲJul25ͲJul25ͲJul

9ͲAug
9ͲAug

24ͲAug
24ͲAug

8ͲSep
8ͲSep

23ͲSep23ͲSep

8ͲOct
8ͲOct

0
0

4
80
80
4
70
3
70
3
60
2
60
2
50
1
50
1
40
0
40
0
30
Ͳ1
30
Ͳ1
20
Ͳ2
20
Ͳ2
Ͳ3
10
Ͳ3
10
Ͳ4
0
Ͳ4
0
31ͲMay 20ͲJun 10ͲJul 30ͲJul 19ͲAug 8ͲSep 28ͲSep 18ͲOct 7ͲNov
31ͲMay 20ͲJun 10ͲJul 30ͲJul 19ͲAug 8ͲSep 28ͲSep 18ͲOct 7ͲNov
Amountofprecipitation Groundwater Stemwater Soilwater(0Ͳ30cm)
Amountofprecipitation Groundwater Stemwater Soilwater(0Ͳ30cm)

AmountAmountofofprecipitation(mm)precipitation(mm)ɷ18ɷ18O(permil)O(permil)

Fig. 2. Variations in į18
O of groundwater, rainwater, stem water andFig. 2. Variations in į18O of groundwater, rainwater, stem water andsoil water and amount of precipitation in Al Rawakeeb sitesoil water and amount of precipitation in Al Rawakeeb site
(Sudan) in the rainy season in 2012.
(Sudan) in the rainy season in 2012.

O of groundwater, stem water and soil water
Fig. 3. Variations in į18
Fig. 3. Variations in į18O of groundwater, stem water and soil waterand amount of precipitation and in Soba site (Sudan) in theand amount of precipitation and in Soba site (Sudan) in therainy season in 2012 .
rainy season in 2012 .

suggestingsuggesting mesquite
mesquite trees
trees dodo notnot useuse only
only groundwater
groundwater asas their
their
water
water source
source even
even inin thethe dry
dry season.
season. Originally,
Originally, thethegroundwater
level
of
this
site
was
about
80
m
and
it
seems
groundwater level of this site was about 80 m and it seems totobebe difficult
difficult forfor trees
trees toto useuse thethe groundwater.
groundwater. After
After thethe heavy
heavy
18
rainfallO valuevalue ofof thethe stemstem water
water
rainfall events
events (on(on Aug.
Aug. 24),24), thethe įį18O
wassimilartotherainwater,suggesting
mesquite
was similar to the rainwater, suggesting mesquite trees
trees usedused
surfacesoilwater.
surface soil water.
18
Similar
Similar tendencies
tendencies were
were seen
seen inin Soba
Soba (Fig.
(Fig. 3).
3). TheThe įį18O
O
valuesvalues ofof thethe groundwater
groundwater were
were low
low andand almost
almost constant.
constant.
18
The
O values
values ofof thethe soil
soil water
water were
were highhigh duedue toto thetheThe įį18O
18
influence
influence ofof evaporation.
evaporation. The
The įį18O
O values
values ofof thethe stemstem water
water
were
were higher
higher than
than those
those ofof thethe groundwater,
groundwater, suggestingsuggesting
mesquite
trees
do
not
use
only
groundwatermesquite trees do not use only groundwater even
even thoughthough thethegroundwater
level
in
this
site
was
sufficiently
shallow(aboutgroundwater level in this site was sufficiently shallow (about
20
20 m)m) forfor mesquite
mesquite trees.
trees. Similar
Similar tendencies
tendencies were
were also
also seen
seen
in
in Al
Al Kadaro
Kadaro site.
site.
To
To obtain
obtain aa better
better understanding
understanding ofof thethe results
results fromfrom thetheisotopeapproach,variations
of
stem
water
content,
soil
isotope approach, variations of stem water content, soil water
water
content
content andand daily
daily precipitation
precipitation inin rainy
rainy season
season inin 20122012 were
were

-1
show
show inin Figure
Figure 4.
4. We
We focus
focus onon thethe stemstem ofof treetree-1.. On
On Jul
Jul
30th,
10.4
mm
of
precipitation
wasobserved
and
only
the
soil
30th, 10.4 mm of precipitation was observed and only the soil
moisturemoisture sensor
sensor at
at 55 cmcm depth
depth responded
responded toto thisthis event;
event; other
other
soil
and
stem
water
contents
did
notrespond.
OnAug
1stsoil and stem water contents did not respond. On Aug 1st andand
2nd,
2nd, 47.0
47.0 mmmm ofof precipitation
precipitation in
in total
total waswas observed
observed andand the
the
soil
water
content
at
15
and
30
cm
depth
drasticallyincreased,soil water content at 15 and 30 cm depth drastically increased,-1
andstarted increasing.
increasing. ThisThis
and thenthen stem
stem water
water content
content ofof treetree-1 started
suggestssuggests thatthat mesquite
mesquite uses
uses soil
soil water
water below
below 1515 cm
cm depth.
depth.
-1The
linearly increased
increased with
with time
time
The stemstem water
water content
content ofof treetree-1 linearly
3
-3
and
the
maximumvaluereached0.46m
3 m-3 on Aug 29th.
and the maximum value reached 0.46 m m on Aug 29th.
The
The stemstem water
water content
content started
started decreasing
decreasing fromfrom thethe next
next day
day
and
the
soil
water
content
at
15cm
depth
of
this
day
was
and the soil water content at 15cm depth of this day was 0.19
0.19
3
-3
m
m3 m
m-3.. This
This soil
soil water
water content
content value
value gives
gives close
close agreementagreementwiththeprimary
wilting
point
(pF
3.8)
obtainedfromwith the primary wilting point (pF 3.8) obtained from thethe waterwater
retention
retention curve
curve ofof thethe soil
soil inin thisthis study
study site.
site. This
This result
resultindicates
that
the
stem
water
content
starteddecreasing
becauseindicates that the stem water content started decreasing becauseroot
root water
water uptake
uptake became
became difficult
difficult due
due toto drying
drying ofof thethe soil.
soil.
The
resultsfromboth
sensor-based
approachandisotopic
The results from both sensor-based approach and isotopic
approachapproach suggested
suggested thatthat Prosopis
Prosopis juliflora
juliflora used
used thethe soil
soil water
water
after
the
heavy
rainfallevents.
This
means
Prosopisjuliflora
after the heavy rainfall events. This means Prosopis juliflora
reliesrelies heavily
heavily on
on surface
surface soil
soil water
water inin rainy
rainy seasons,seasons, althoughalthoughthis
this species
species isis generally
generally regarded
regarded as
as using
using groundwater
groundwater throughthrough
deep
tap
roots
(Nilsenetal.,1983).
deep tap roots (Nilsen et al., 1983).

3.2.
3.2. Results
Results inin USA
USA
18
The
į
O
18 values of the soil water at 20-80 cm depths wereThe į O values of the soil water at 20-80 cm depths were
almostalmost constantconstant andand similar
similar toto thethe value
value ofof thethe groundwater
groundwater
(Fig.
5).
This
was
becausethegroundwaterlevel(Fig. 5). This was because the groundwater level ofof thisthis sitesite
waswas quite
quite shallow
shallow (about(about 11 m)m) andand thethe composition
composition ofof thethe soilsoil
water
water was
was influenced
influenced byby capillary
capillary rise
rise ofof thethe groundwater.
groundwater.
18Theį
18O value of the soil surface (10 cm) was high due to theThe į O value of the soil surface (10 cm) was high due to the18
influence
influence ofof evaporation.
evaporation. The
The įį18O
O values
values ofof thethe stemstem water
waterofmesquiteandTamarix
wereconstantandsimilartothoseof mesquite and Tamarix were constant and similar to those ofofthethe groundwater
groundwater andand soil
soil water
water (20-80(20-80 cm),cm), indicating
indicating both
both
species
species use
use thethe groundwater
groundwater and/or
and/or soil
soil water
water at
at 20-80
20-80 cmcm
depths.
depths. Since
Since thethe salinity
salinity ofof thethe groundwater
groundwater andand soil
soil water
water
was
was highhigh inin thisthis site,
site, this
this result
result suggests
suggests thatthat notnot only
only Tamarix
Tamarix
ramosissima
but
also
Prosopis
pubescensareramosissima but also Prosopis pubescens are salt
salt tolerant
tolerant
species.
species.

4.
4. Conclusion
Conclusion
The
The waterwater sources
sources ofof native
native andand invasive
invasive tree
tree species
species in
in
Sudan
and
Americawereestimatedbyoxygenstableisotope
Sudan and America were estimated by oxygen stable isotope
analysis.
analysis. Prosopis
Prosopis juliflora
juliflora inin Sudan
Sudan isis generally
generally regarded
regarded asasusing
groundwater
throughdeeptaproots.
Our
studies,using groundwater through deep tap roots. Our studies,
however,
however, suggest
suggest thatthat Prosopis
Prosopis juliflora
juliflora is
is notnot usingusing only
only thethegroundwater
groundwater but
but also
also soil
soil water
water after
after heavy
heavy rainfall.
rainfall. Both
Both
Tamarix
Tamarix ramosissima
ramosissima andand Prosopis
Prosopis pubescens
pubescens inin USAUSA used
used
shallowsalinegroundwater,suggesting
that
not
only
shallow saline groundwater, suggesting that not only Tamarix
Tamarix

32

Fig.
Fig. 4.
4. Variations
Variations ofof stemstem water
water content
content ofof mesquite,
mesquite, soil
soil water
water content
content andand daily
daily precipitation
precipitation inin rainy
rainy season
season inin 20122012 inin Soba
Soba site,
site, Sudan
Sudan (Saito(Saitoetal.,2013).
Dielectricmoistureprobeswere
installedtotwomesquite
trees
(4
and
7
cm
in
diameter,respectively).
Soil
water
et al., 2013). Dielectric moisture probes were installed to two mesquite trees (4 and 7 cm in diameter, respectively). Soil water content
content waswas
measuredmeasured at
at 5,
5, 15,
15, 3030 cm
cm depths.
depths.
22įį1818OO(permil)(permil)

0
0
Ͳ2
Ͳ2
Ͳ4
Ͳ4
Ͳ6
Ͳ6
Ͳ8
Ͳ8

Ͳ10
Ͳ10
Ͳ12
Ͳ12

䕺Groundwater
䕺Groundwater

䕦Stem
䕦Stem water
water

䖃Soil
䖃Soil water
water

18 of groundwater, stem water (mesquite and Tamarix) andFig.
O of groundwater, stem water (mesquite and Tamarix) and
Fig. 5.
5. į
į18O
soil
soil water
water (10,(10, 20,
20, 50,50, 8080 cm
cm depths)
depths) inin Virgin
Virgin River
River site
site (US)
(US)
in
April
2012.
in April 2012.

but
but also
also Prosopis
Prosopis pubescens
pubescens areare salt
salt tolerant
tolerant species.
species.
Acknowledgements
Acknowledgements
This
This work
work waswas supported
supported byby JSPSJSPS KAKENHI
KAKENHI Grant
Grant
Numbers
24780233,
23404014,
and
“A
Study
of
Human
Numbers 24780233, 23404014, and “A Study of Human
Subsistence Ecosystems
Ecosystems inin ArabArab Societies:
Societies: To
To Combat
CombatSubsistence
Livelihood
Degradationforthe
Post-oil
Era”(Project
Livelihood Degradation for the Post-oil Era” (Project Leader:
Leader:Hiroshi Nawata,
Nawata, Research
Research Period:
Period: 2008-2013),
2008-2013), andand Joint
Joint
HiroshiResearch
Program
of
Arid
Land
ResearchCenter,Tottori
Research Program of Arid Land Research Center, Tottori
University.
University.

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