沙漠研究 22-1, 1 -4 (2012)"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Evaluating the Invasion Strategic of Mesquite (Prosopis juliflora)
in Eastern Sudan Using Remotely Sensed Technique
B. HOSHINO*1), Abdelaziz KARAMALLA2), Mohamed A. M. ABD ELBASIT3), Karina MANAYEVA1), K. YODA4),
Mahgoub SULIMAN2), Mohamed ELGAMRI2), H. NAWATA5) and H. YASUDA3)
Abstract: In the Red Sea coastal area, the problems of desertification, land degradation and dust storm are still serious. Because,the invasive species Mesquite (Prosopis juliflora) has a high capacity to fix sand dunes, mesquite trees were introduced into Sudan andplanted in Khartoum and eastern Sudan. However, the tree was invaded both natural and managed habitats, including watercourses,
floodplains, highways, degraded abandoned land and irrigated areas. The weed is more of a problem within central, northern andeastern Sudan. In this study a remote sensing approach for the mesquite tree control is proposed. To monitor mesquite water use
efficiency the concept of a Normalized Difference Infrared Index (NDII), which is defined as the ratio of actual to foliar water content,
have been applied and compared with the ground measurements of stomatal conductance (mmol. m−2. s−1), field spectral, volumetricsoil water content. As results, on the base of the PALSAR L-band microwave polarimetric backscatter coefficient, the soil moistureand surface roughness could be estimated with a good accuracy for bare-soil surfaces.
Key Words: Africa, Backscatter coefficient, Mesquite (Prosopis juliflora) control, NDII

1. Introduction
The mesquite tree is native to South and North America
and was introduced into Sudan in 1917. The tree is wellknown for its high adaptability to arid and semi-arid conditions
and characterized by very high water use efficiency.
Introduction of the mesquite has caused several environmental
problems in Sudan.
Mesquite can detect even very tiny soil moisture and growto various conditions. Some of the many adaptive abilities
that allow mesquite to thrive under such conditions involve
ability of roots to adapt to a wide variety of soil conditions.
Its roots can grow upwards towards the soil surface tocapitalize on little rainfall, and can also grow to depths down to80 m and extend laterally more than 30 m. This is the mostextensive root system of any plant in the world (Thorp et al.,2001).
Remote sensing methods for mesquite control in eastern
Sudan come within a project of Human Subsistence Ecosystem
in Arab countries to combat livelihood degradation for thepost-oil Era.
The project is implemented with theco-ordination and partnership between the Research Institutefor Humanity and Nature (RIHN) in Japan and the College ofAgricultural Studies at Sudan University of Science andTechnology (SUST). The work is conducted in the Red Sea

coastal and Kassala irrigated agriculture area in eastern Sudan
using different methods of remote sensing, aiming at setting apractical methodology that could effectively help in assessing
mesquite distribution and its spreading patterns.
Studies on the plants adaptation to environment are very
important for evaluation of the vegetation response to global
climate changes. Vegetation water content is important
ecophysiological parameter that provides information on thevegetation’s environmental adaptation and used to determine
the plants water stress. An accurate estimate of the plantwater content is significant key in evaluating the strategic
expansion of the invasive mesquite in semi-arid area of Africa.
The mesquite has a problematic spreading that threatens
agriculture in vast areas of eastern Sudan, including Tokar
Delta, Gash Delta, numerous sites along Red Sea coast area
and irrigated farmland of Kassala plain. According to Brown
et al. (1929), in 1917 the mesquite tree were introduced into
Sudan from South Africa and Egypt and planted in Khartoum.
This tree has characteristics of high drought tolerance, sanddunes fixing, and could also be used as fodder for animals.
The mesquite usage, beside sand dune fixation is limited tofirewood and charcoal production. More than 90% ofmesquite is thought to be spreaded in eastern Sudan, where
livestock keeping and subsistence cultivation constitute themain source of income. Invasive mesquite tends to form
dense and impenetrable thickets. Its highly competitive* Corresponding Author: aosier@rakuno.ac.jp
1) Department of Environmental Symbiotic Science, College of Agriculture, Food and Environment Sciences, Rakuno Gakuen University, 069-8501, Japan2) Collage of Forestry Science, Sudan University of Science and Technology, Khartoum, Sudan

4) Faculty of Science and Engineering, Ishinomaki Senshu University, Japan

3) Arid Land Research Center, Tottori University, Japan

5) Research Institute for Humanity and Nature (RIHN), Japannature leads to reduce grass cover, stocking density, andthreatens the livelihood of traditional herders. Its invasion
into agricultural land, along irrigation channels and water
courses is also a major problem. The high water tables,
which contribute to the menacing of mesquite spread inaddition to the fertile soil, at the downstreams of Tokar, Gash
Delta and Kassala plain seasonal river (see Fig. 3) are expected.
Previous studies (Babiker et al., 2006) in other part of theworld clearly showed that eradication of mesquite is neither
desirable nor tenable. In this study a remote sensing approachfor evaluating the invasion strategy of mesquite tree (Prosopisjuliflora) was proposed.

2. Methodology and Result
2.1. Measurement of mesquite water use efficiency
In the eastern Sudan’s semi-arid land, adaptation of themesquite to a variety of habitats may be related to a plasticityof root system distribution. Mesquite tree uses deep groundwater with a taproot in the dry season and the soil water with alateral roots in wet season. When available, mesquite will
exploit sources of deep water by growing a taproot. Mesquite
can also persist on sites that have little or no ground water by
growing lengthy shallow lateral roots. Therefore, most of thenative plants growing around a mesquite tree become witheredand died.
Moreover the mesquite can control the leaf water
evaporation and survive even in the driest season, because themesquite water use efficiency is higher than those of nativespecies. Figure 1 shows the parameters of the stomatal
conductance (mmol m−2 s−1) in native plants and mesquite.
Beginning from 11:00 am, most of the native plants goes into
midday depression of photosynthesis and reduce
evapotranspiration (ET) when air temperatures are near 40oC.

Fig. 1. Comparison of the stomatal conductance (mmol m−2 s−1) in
native plants and invasive species (mesquite) at semi-arid areaof Kassala, Sudan (Nov. 7th, 2010).

However, mesquite is gradually reducing ET from 9:00 am,and remained low evapotranspiration during the whole day.
2.2. Detecting the invasion route of mesquite trees
Mesquite tree invasions are expanded along the riverside inAtbara River basin in eastern Sudan. However, the nativeplants and farm land products retreat by the expansion ofinvasion species of mesquite. The remote sensing method isvery useful to detect the invasion route of mesquite tress inwide area. Since plants leaf absorbs red light energy, andreflect near-infrared (NIR) light energy of sunlight. Therefore,vigorously growing healthy vegetation has low red-lightreflectance (due to its chlorophyll) and high near-infrared
reflectance (due to its total biomass). Figure 2 showsparameters of the spectral reflectance of mesquite trees and thesurrounding native plants. Mesquite has low red-lightreflectance and high near-infrared reflectance. Moreover,comparison of the mesquite’s absorption of red light and thereflection of NIR light with those of other native plants,
showed that the mesquite was more healthy and vigorously
growing. To quantify water stress level in aboriginal andinvasive species we are followed. Moran et al. (1989)
investigated the effect of water stress on canopy architecture inalfalfa (Medicago sativa L.) and the sequential effect on
canopy reflectance. They found water-stressed canopies tohave a lower spectral reflectance in the NIR wavelength andlower spectral absorption in the red wavelength whencompared with unstressed canopies. A ratio of the twowavelengths was most successful in estimating the onset ofstress. In our study, as shown in Figure 2, the native plants inthe same habitat of mesquite have more water-stressed
growing situation than the mesquite. The ratio of NIR to red
wavelength for the mesquite was 4.5 while those of a nativeplant was only 1.9.
We investigated the concept of a Normalized Difference
Infrared Index (NDII), which is defined as the ratio of actual tofoliar water content. The foliar water content is often divided
by the density of liquid water to derive the equivalent waterFig. 2. The measurement field spectral reflectance (FieldSpec®
HandHeld) of native plants and invasive species (mesquite) insemi-arid area of Kassala, Sudan (Nov. 7th, 2010).

measurements of polarimetric backscattering with a good
accuracy for bare-soil surfaces. It is well known that theretrieval of soil moisture in vegetated surface from themicrowave backscatter coefficient is affected by surface
roughness. In bare soil area of Atbara River basin (in Kassala
plain, Sudan), we found good relationship between fieldmeasure soil moisture (Mv) and ALOS/PALSAR L-bandbackscatter

Fig. 3. NDII map calculated from Landsat5-TM data (where,red-colors shows the satellite extraction of mesquite tree pixels.
Mesquite was expanded along the bank of a river; and thepolygons shows the mesquite distribution area. Dec. 11th, 2009).

thickness (EWT). EWT is useful because the canopy EWT isequal to the leaf EWT multiplied by the leaf area index (LAI).
Leaf reflectance at short wave infrared (SWIR) wavelength
increases linearly with respect to leaf reflectance at NIR for adecrease in leaf EWT. The measurement can be calculated
from remotely sensed data (NIR and SWIR). Hardisky et al.
(1983) defined the Normalized Difference Infrared Index(NDII) as:

NDII =NIR − SWIR
NIR + SWIR(1)Where, NIR is reflectance of near Infrared and SWIR isreflectance of short wave infrared.
This can be calculated from the Landsat 5 ThematicMapper (TM) bands 4 (as NIR) and 7 (as SWIR). Figure 3
shows the NDII image calculated from Landsat 5 spectral band
4 and band 7. This index exhibits the ability to evaluate theinvasion strategic of alien species mesquite. As shown inFigure 3, the field measurement and result of NDII (NDII
value > 0.1) shows that the mesquite trees expansion followeda high soil moisture area along the banks of a river.
2.3. Measurement of polarimetric backscattering
The field measures using a hand-held soil moisturemeasurement system (©Hydrosense) showed that thevolumetric soil water content ranged between 3-5% in the area
within the circle of 10 meter-radius from mesquite tree centers
including under the mesquite canopies.
As the amount of water contained in soil becomes greater,
specific inductive capacity of the whole soil increases inproportion, and as a result, backscatter strength increases. In
this way, analysis of backscatter strength leads to estimate theamount of water content in target land area. Based on theaccurate scattering model (Oh et al., 2002) both the soil
moisture and surface roughness can be retrieved from thecoefficient0( M v = 4.24s baresoil+ 126.4 ,R2=0.985) (Hoshino et al., 2011). However, for surfaces withvegetation cover, the soil moisture retrieval is a challenging
problem because of complicated scattering mechanisms in thevegetation canopy. Except for the case of dense forest
canopies, backscatter from vegetation (s0dB) can be divided
into surface (s0s), volume (s0dv) and multiple (both) scattering(s0int) (Oh et al., 2002):
000+ σ inτσ dB= τ 2σ σ0 + σ dv

(2)

A semi-empirical polarimetric backscattering model forbare soil surfaces is inverted directly to retrieve both thevolumetric soil moisture content and the surface roughness
height from multipolarized radar observations (Wang et al.,
1980; and Pathe et al., 2009). However, we found that thePALSAR L-band has had difficulties to penetrate the highdense mesquite trees. Fig. 4 shows the backscatter coefficient
subtraction value ( ∆s (0Sep −15 − 2009 ) − ( Sep −10 − 2007 ) ) calculated from[total backscatter coefficient σ 0 [dB ]= 10 log σ 0 m 2 m −2] of

PALSAR L-band HH/HV polarimetric data on September 10th,
2007 and September 15th, 2009 and computed as follows:∆VSM = f ( R, m s ) ≈ ∆s t0 = (s t02 − s t01 )(3)

Where, △VSM is seasonal or annual change of volumetric soil
moisture; R is surface roughness and ms in soil moisture.
To reduce the effect of roughness on a retrieval of soil
moisture, a ratio of backscatter coefficient for wet and dry soil
has been calculated. The backscatter coefficient subtraction
values (September 10th, 2007 and September 15th, 2009)
showed that changes in the soil moisture at the bare soil areas
were greater than that in mesquite tree ground (Fig. 4).
It has been suggested that the mesquite trees had grown up
a little during two years period (from 2007 to 2009), because
the value of △st0 was slightly positive (0 to + 4) at themesquite tree areas. Contrarily, the soil moisture of bare soilin the Atbara River basin showed a great change (△s0 was (+4 <△s0) or (0 >△s0)).

in vegetated ground in semi-arid region.
Acknowledgement
This work was supported by grants from Research Institutefor Humanity and Nature (RIHN), Japan (Project leader: Dr. H.
Nawata) and Grant-in-Aid for Scientific Research (B) (No.)
23404014 from the Japan Society for the Promotion of Science(JSPS) of Japan (Project leader Prof. H. Yasuda). The
authors would like to thank the RIHS and JSPS supports.
Fig. 4. The distribution map of PALSAR backscatter coefficient
subtraction value (△st0) between September 15th, 2009 and
September 10th, 2007 of Kassala area (where, the changes ofsurface roughness (mesquite tress pixels) ranged the values of(0 ≤ △st0 ≤+ 4) and the changes of soil moisture (bare soil orriver channel pixels)) are (+ 4 < △st0) or (0 > △st0)).

3. Discussion
The mesquite trees (Prosopis juliflora) grow in arrays ofhabitats, and characterized by very high water use efficiency.
In this study we applied ground measurement of the plants
stomatal conductance and satellite based Normalized
Difference Infrared Index (NDII) to detect the foliar watercontent, and tried to retrieve the mesquite tree distribution area.
If vegetation water content can be estimated independently
using reflectance in the SWIR, then the retrievals of soil
moisture content would be more accurate. The problem isthat SWIR reflectances are dominated by foliar water content
and are not affected by stem water content. However, plants
often have allometric relationships between foliar and stem
mass, so estimation of foliar water content from SWIR
reflectance would allow predicting of vegetation water content.
We found that the high dense mesquite trees make it difficult to
detect by using PALSAR L-band because of the penetrationdifficulties. Based on the accurate scattering model, both thesoil moisture and surface roughness can be retrieved from themeasurements of the polarimetric backscatter with a good
accuracy for bare-soil surfaces. However, for areas covered
with dense mesquite trees, the soil moisture retrieval is achallenging problem because of complicated scattering
mechanisms in the mesquite canopy. So far, additional
studies on the estimation of the mesquite biomass using
PALSAR L-band microwave data are needed. In this study,
thebackscatter
coefficient
subtraction
values( ∆s t0 = (s t02 − s t01 ) , where, t1 and t2 are September 10th, 2007
and September 15th, 2009, respectively) showed that changes
in the soil moisture in the bare soil area were greater than that

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