Landscape changes of desertification in arid areas: the case of south-west Algeria

18 Pages·2010·1.08 MB·English

EnvironMonitAssess
DOI10.1007/s10661-010-1744-5
Landscape changes of desertification in arid areas:
the case of south-west Algeria
AzizHirche·MostefaSalamani·
AbdelkaderAbdellaoui·SalimaBenhouhou·
JaimeMartínezValderrama
Received:4February2010/Accepted:4October2010
©SpringerScience+BusinessMediaB.V.2010
Abstract This study aims to monitor the arid matic maps that have been carried out during
Algerian High Plateaus, a key region for pas- 30 years. Available data for the study are veg-
toral activities which has suffered harsh and etation maps derived from aerial photographs
widespread degradation from the eighties. This (1975–1978) and from satellite imagery (2006).
area is not sufficiently known by the interna- The parameters considered include vegetation,
tional scientific community. For this purpose, we flora, and soil surface properties. The study area
considered phytoecological inventories and the- is part of the ROSELT/OSS (ROSELT: Réseau
d‘Observatoires de Surveillance Ecologique à
Long Terme (Long Term Ecological Monitor-
A.Hirche(B)·M.Salamani ing Observatories Network); OSS: Observatory
LaboratoryofVegetalEcology,FacultyofBiological of the Sahara and the Sahel) network obser-
Sciences,UniversityofSciencesandTechnology vatory (OSS 2008). To assess land degradation,
HouariBoumediene,BP32ElAlia,
weusedlandscapeecologyparameters.Thesein-
BabEzzouar,1611,Algiers,Algeria
clude the number and surface area of vegeta-
e-mail:[email protected]
tion units, synthesized by the large patch index
A.Hirche·M.Salamani and the Shannon landscape diversity index. All
ScientificandTechnicalResearchCenteronArid
parameters reflect an increase in landscape het-
◦
Regions,Frontdel’oued,BPn 1682R.P,
erogeneity. The largest decline is observed for
07000,Biskra,Algeria
Stipatenacissimavegetationunitsconstituting2/3
A.Abdellaoui of the landscape in 1978 and occupied just 1/10
UFRHumanSciencesLaboratory,Geography
in 2006. Vegetation units linked to degradation,
Department,UniversityofParisXII,61Avenuedu
such those dominated by Salsola vermiculata, in-
GénéraldeGaulle,94010,Créteil,France
existent in 1978, now dominate the steppe. An-
S.Benhouhou other result of the ongoing landscape degrada-
EcoleNationaleSupérieureAgronomique,
tion on the plateaus between 1975 and 2006 is
HacèneBadi-ElHarrach,16200,Algiers,Algeria
the decrease of vegetation cover. In 1978, 1/3 of
J.M.Valderrama rangelands only had low vegetation covers, in-
AridZoneExperimentalStation(Estación ferior to 15%. Presently 9/10 present the same
ExperimentaldeZonasAridas)/CSIC(Consejo
classcover.Thiscanbeexplainedbyseverespells
SuperiordeInvestigacionesCientíficas),Carreterade
of drought combined by an exponential rise of
Sacramentos/nLaCañadadeSanUrbano,
04120,Almeria,Spain livestock during the last 30 years. This has in
EnvironMonitAssess
turn greatly undermined the fodder potential of geographicinformationsystems(GIS)andremote
thesteppe.Resultssuggestthatthe“greening-up” sensing tools. The interest of the approach is to
describedbyseveralauthorsintheSaheloverthe considerpotentiallandscapeheterogeneityasthe
last 40 years is not observed in the Algerian, nor resultoflanddegradation.
intheNorthAfricansteppes.Onthecontrary,the In 1975, the Centre de Recherches sur les
desertificationisstillongoingandthethresholdof Ressources Biologiques et Terrestres (CRBT)
irreversibilityseemstobeimminent. carried out a major vegetation monitoring pro-
graminthesouthOransteppeproducingthematic
Keywords Changedetection·Heterogeneity· mapsoftheentirearea.Also,permanentobserva-
Landscapeecology·Desertification·Remote tionsiteshavebeensetuptocollectmoreaccurate
sensing data.Thelatterhavebeengatheredfor30yearsof
observations.
Resultsfromsimilarstudieswillprovideagen-
Introduction eral overview for the North African countries
and it would be possible to carry out rigorous
Landdegradation,longanimportantenvironmen- comparisons with landscape changes within the
tal issue in arid and semi-arid lands, is now acute Sahelianregion.ResearchersofboththeSahelian
in Algeria’s high plateaus. The Algerian steppes and North African regions collaborate presently
are the most widespread rangeland of the North in the same major research project, “DeSurvey”,
African countries. They occupy a pivotal posi- whichconstitutesanidealframeworktoexchange
tion between the hilly and humid north, called scientificdataandexperiences.
the Tell, composing 5% of Algeria’s surface, and
the south formed by the Sahara covering 86% of
the nation’s territory. The Sahara is the largest Materialandmethods
desert of the planet (Tucker et al. 1991). Never-
theless, relatively few scientific works had been Thestudyarea
edited in English up to now (Benhouhou et al.
2003).Earlystudiesbasedonremotesensingdata The study area occupies two thirds of Algeria’s
and techniques undertaken with the objective of south-westernHighPlateaus.Itisadministeredby
monitoring land conditions found no evidence thewilayaofNâama(Fig.1).Theaveragealtitude
for extensive degradation in the Sahelian region is around 1,000 m and it is primarily occupied
(Helldén1984,1991;Princeetal.1998;Tuckerand by Quaternary polygenic glacis. It includes saline
Nicholson 1999; Rasmussen et al. 2001). On the depressions such as Chott Chergui. The area is
contrary,recentstudiesoftheSahelianvegetation surrounded by chain of mountains such as the
based on remote sensing data even emphasize Antardjebelwhichculminatesat1,700m.
“the greening-up” of the Sahel (Anyamba and
Tucker 2005; Helldén 2008; Helldén and Tottrup Climate
2008). At the North African area, the results are
globally in contradiction with these results. The Climatic data was gathered from the National
aim of this study is to contribute some elemental MeteorologyOfficeandcoveralongperiodfrom
reflectionstothisissuebyevaluatingqualitatively 1907 to 2004 (ONM 2008). The climate is char-
and quantitatively the phenomenon of land evo- acterized by low annual rainfall with averages
lution in a representative area in the south Oran ranging from 263 mm in Mecheria (the center
steppe(about10millionhectares).Thestudyarea of the study area) to 287.5 mm in El Bayadh
covers approximately 400,000 ha and the time (amountainousareasurroundingthenorth-east).
horizon considered is 30 years. Data analyses are The distribution of rainfall is very irregular.
based on indices borrowed from landscape ecol- The coefficient of variability reached 43.2% in
ogy: the large patch index (LPI), the Shannon Mecheria and 31.3% in El Bayadh. Winters are
landscape diversity index (SHDI), and the use of cold and harsh while summers are hot and dry.
EnvironMonitAssess
Fig.1 Generallocating
ofsurveyzone
To illustrate the climatic context, the evolution the1983–1985periodafterthefirstdroughtperiod
of rainfall since the beginning of the century is thatbeganin1968.
graphed(Fig.2). Since the end of the 1970s, the mobile aver-
The rainfall evolution shows an alternation of ages are almost always below the average rain-
dryandhumidperiodswithnocleartrendsduring fall. Temperatures show average minimum val-
approximatelythefirsthalfofthecentury.Atthe ues for the coldest month (January) of 1.5◦C
end of the second half, dry periods become no- and −1.8◦C for Mecheria and El Bayadh, re-
tablymorefrequentandintense,especiallyduring spectively, and average maximum values of the
the 1981–1988 years (Hirche et al. 2007) consid- hottestmonth(July)of35.1◦Cand33.5◦C.Forall
eredtobethedriestperiodofthelastcentury.It stations, the dry period (Bagnouls and Gaussen
is remarkable to note that this drought period is 1953) extends over 6 months and the bioclimate
partiallythesameasfortheSahelwhichhasbeen belongstothemedium-aridclasswithcoldwinters
affected by a notable large-scale drought during variant.
Fig.2 Evolutionof 700 MECHERIA BAYADH
annualrainfallsincethe
MEDIAN MECHERIA MEDIAN BAYADH
beginningofthecentury
600 MOBILE AVERAGE 7 Per MOBILE AVERAGE 7 Per
BAYADH MECHERIA
500
m)
m 400
all (
nf
ai 300
R
200
100
0
1907191119151919192319271931193519391943194719511955195919631967197119751979198319871991199519992003
EnvironMonitAssess
Sand storms are rare in El Bayadh by the CRBT (1978), Djebaili (1978), Aidoud-
(3.1 days year−1) but are more frequent in Lounis(1997),andKadiHanifi-Achour(2003)de-
Mecheria(7.8daysyear−1).Duststormsaremuch scribethevegetationdynamicsatworkinsteppic
more frequent with an increase in the number of regions. A synthesis based upon these different
dust days since 1990 (Nouaceur 2001). Between studiesisgiveninthefollowingflowchart(Fig.3).
1990and1997,anaverageof30daysperyearwas It appears that the Algerian steppe was ini-
recordedinElBayadhreaching119daysperyear tially colonized by forests. Their gradual dete-
between2001and2004.Thesamewasobservedin rioration led to the colonization of xerophytic
Mecheria, where 58 days were recorded between vegetation consisting of a mixture of herbaceous
1990 and 1997 and 136 days between 2001 and andperennialplants.Themostemblematicisthe
2004. This increase seems to be important, even herbaceousStipatenacissima,whichcolonizedthe
majority of flat zones, corresponding to the ge-
if the period considered (2001–2004) is too short
omorphologic units called “glacis”. The plant
forstatisticalsupport.Nevertheless,whateverthe
is also observed on craggy landscapes. Lygeum
stationanyyearbetween2001and2004hasmore
spartum, another herbaceous graminoid, appears
than 100 days of dust storms. In any event, if
where sand accumulates, often in depressions.
theseresultsareprovenoveralongerseries,they
When degradation occurs on rich silt soils the
wouldbeveryalarmingbecausetheyrelatetothe
chamaephytic Seriphidium herba-alba appears.
magnitudeofsoildenudation.Theweakrecovery
Thesethreespeciesconstitutedtheframeworkof
of vegetation would then give free way to winds
theAlgerian,andeventheNorthAfricansteppe.
overvastareasoftheAlgeriansteppe.
The studies also showed that pressure on the en-
vironment, mainly by overgrazing, led these for-
Vegetation mations to disappear. They have been replaced
by plant units resulting from a dynamic degra-
With regard to the vegetation, several phytoeco- dation with less functional performances, such
logical and phytosociological studies undertaken as Atractylis serratuloides, Salsola vermiculata,
Fig.3 Flowchart
establishingthedynamic
ofvegetation
EnvironMonitAssess
Peganum harmala, and Astragalus armatus. The in paper support and then we scanned, georef-
conversion of arid grassland to shrublands due erenced, and digitized them using GIS software
to overgrazing and/or drought is a well-known (MapInfo).Followingdigitaljoiningofthemaps
phenomenon. Allington and Valone (2010) re- (El Kreider and Mecheria), we extracted a win-
view the main concerned models and conclude dowfilecorrespondingtothestudyarea.
that“currentconceptualmodelsofdesertification Corrections were also undertaken on Landsat
predict that arid grasslands exists in one of two ThematicMapperimage(2006),with30mresolu-
alternate stable states over timescales relevant to tion.Thepre-processingworkconsistsofgeomet-
management: perennial grasslands or desertified ric corrections (neighborhood re-sampling) with
shrubland”. When sand accumulation becomes 33pointsandanacceptableerror;theRMSisless
prominent, psammophytes such as Thymelea mi- than one pixel (0.92). All the maps were finally
crophyllatakehold. re-projected in UTM WGS 84. The final layouts
foroutputwerepreparedwithArcViewsoftware
Datacollectionandprocessing andthefinalscalewasthesameasthe1978maps
(1:200,000).
Themethodologyproposedinthisworkissimilar The classification process is an image-
to that followed by Hanafi and Jauffret (2007) in interpretation of the corrected TM images. We
Tunisia and by Hirche et al. (2008) in Algeria. It followedthesamplingstrategyofGounot(1969),
requires the comparison of vegetation maps es- and established 140 ground geo-referenced
tablishedfromtwowidelyseparatedtimeperiods samples distributed throughout the study area.
in order to assess the intensity of desertification. It is related to “mixture sampling” developed
Thedefinitionofdesertificationconsideredinthis by Gounot (1969) that uses all the information
work is taken from the Convention to Combat sources.Itresultedinadelimitationofvegetation
Desertification: “land degradation in arid, semi- unitsandsub-units(facies).Everyunitisassessed
arid, and dry sub-humid areas resulting from by taking a phyto-ecological releve, quadrats
various factors, including climatic variations and points for measuring vegetation and sand cover
human activities” (UNEP 1994). Hence, deser- (Daget and Poissonet 1971). Fieldwork for this
tification results in a reduction in the biological taskrequired2yearsofintensivesampling.
potential (Dregne 1995). The aim of the present Foreachsamplewerecordeddominantspecies,
work is precisely to assess the loss of vegetation thevegetationcover,andthesandcover(Fig.4).
cover.Thefirstmapwasproducedin1978andthe Is our approach feasible, considering that the
secondonein2006.Thespecificationsofmapping natureandscaleofsupportsaredifferentforboth
andsamplingareinaccordancewiththeenviron- the two periods: aerial photography and satellite
mental monitoring requirements established by images?
Godron et al. (1968), repeated and confirmed by Inordertoanswerthisquestion,itisofutmost
ROSELT’smethodologicalguide(LeFloc’hetal. importance to discuss first the perception levels
2007). and be aware that spatial heterogeneity is scale
The land cover maps of 1978 correspond to dependent(Wuetal.2002).
twotopographicsheets(ElKreiderandMecheria, Accordingtothepyramidofperceptions(Long
1:200,000 scale, North Sahara Datum). The firsts 1974;Godronetal.1968),ahomogeneousecolog-
vegetation maps were established in monochro- icalregioncontainsecologicalsectorsmadeupof
maticversionin1978andeditedbyourcolleagues ecologicalsystems.
infinalcoloredversionin1979and1981(Achoubi Inthesurvey’szone,wedistinguishthefollow-
et al. 1979; Aidoud et al. 1981). They were based ingperceptionlevels:
on the interpretation of panchromatic (black and
white) aerial photographs, (scale 1:40,000) cap- The ecological sector is related to physiog-
tured in 1975. The photographic correction and nomictypes(example:S.tenacissimasteppe),
restitution was carried out at the NCI (National to plant associations, or to vegetation se-
cartographyInstitute).Weusedthiseditedsheets quences(GodronandPoissonet1972).
EnvironMonitAssess
Fig.4 Spatiomap
representingthe
surveyarea
The ecological systems are delimited within tographsarecommonlymoreaccuratethansatel-
every ecological sector. They integrate the liteimageryandhavetheadvantageoflargescale
different land use models (grazing, agricul- (1:40,000), we were obliged to do regroupings in
ture), the state of vegetation degradation order to produce a 1:200,000 scale map. Further-
(vegetation cover) and the soil variables. At more, this scale was chosen because land cover
thislevel,anthropozoïcimpactsarediscerned mappingofAlgeriaisnotavailableatlargescales.
(Melzi1993). Despite the comparatively coarse spatial resolu-
The station level is the one that allows a pre- tionoftheTMimages,theirrestitution(regroup-
cisediagnosisoflandoccupation. ing) using the “sliding scale” mentioned above
allowedforbringingbothaerialphotographsand
The privileged perception level is tradition- satellite imagery together. Some authors suggest
allytheecologicalsystems(orecologicalstations) a“natural”scaleoftheLANDSATTMorSPOT
corresponding to a large scale, such as 1:25,000 imagescloseto1:250,000(WoodcockandStrahler
or 1:50,000 in temperate climatic zones. Long 1987;Light1990).Inordertoberigorous,itwould
(1974) considers that in arid zones, a medium havebeenidealtohavemorerecentaerialcover-
scale (1:100,000 or even 1:200,000) is adequate age.Theseareunfortunatelyunavailable.
to express the same level of perception (plant Anotherproblemwithmappinginaridzonesis
groupings) and proposes a “sliding scale” to map that the steppe vegetation, which lies low to the
and characterize arid regions. While aerial pho- ground, is not perceptible in aerial photographs.
EnvironMonitAssess
Forthisreason,theirbetterspatialresolutiondoes The classification key integrates the dynamic
notalwaysconstituteamajoradvantage.Thepoor filiations and on the major components of the
quality of photographs is another unfavorable el- landscape.
ement. Finally, similar to the Tunisian study (Hanafi
It is necessary to homogenize the work of the and Jauffret 2007), the problem of different data
earlier survey with our present work. For this sources (satellite image and aerial photographs)
purpose, we pursued an analogical classification was widely solved by systematically visiting the
(image-interpretation), developed from the im- whole area and using the same methodology
agery. The photo analysis allowed the delimita- whichwasusedtoproducethefirstmaps(1978).
tion of vegetations units. However, very sparse
vegetation cover does not allow direct observa-
tionofthevegetation.Tolimitthisconstraint,we Landcovermapsandlandscapeecology
relied instead upon detailed thematic knowledge
about links between soil and vegetation (Pouget Landscape ecology is a concept widely accepted
1980; Kadi Hanifi-Achour 2003) but it remains and used presently especially with the develop-
insufficient. Fieldwork was essential to get more ment of GIS software and computing capacities
informationtocompletetheinterpolation. (Heggem et al. 2000; Wang et al. 2001; Yang and
Thecomparisonofthetwomapsisbasedupon Lo 2003; Kilic et al. 2006). Landscape ecology is
physiognomic types, vegetation cover, and soil based largely on the notion that environmental
surface properties. The resulting database used patterns strongly influence ecological processes
the following parameters, which were then inte- (Turner 1989). We used the concept expressed
gratedintotheGIS: by Forman and Gordon (1986) who define land-
scapeas“aheterogeneouslandareacomprisinga
• Urbansites cluster of interacting and repeating ecosystems”.
• Physiognomictype In this sense, heterogeneity describes the diver-
• Species sity of landscape elements within the studied ar-
• Soil surface properties (sand, stone, gravel, easaccordingtofragmentationandneighborhood
rock,barrensoil,litter,andcovers) (Baudry and Burel 1999). It explains the flux
of matter and energy described above, between
different landscape elements, which can then be
Weobservedvegetationphysiognomyofprimary
evaluated using two complementary approaches:
and secondary dominant species as a summary
thefirst,theoldest,andmostintuitiveisstructural
expression of ecological features (Ionesco and
and aims to describe the composition of habitats
Sauvage1962).
by relating habitat to its constituent species. Its
• inherentinconvenienceresultsfromtheelaborate
The primary dominant species indicates the
nature of the indicators it produces (Turner and
major biologic characteristic. It defines the
Gardner1991).Thesecondapproachaimstogive
vegetationunitsoftencalledformation.
• a functional sense of heterogeneity according to
The second dominant species expresses the
the fragmentation and connectivity of its compo-
presentstateofvegetationrelatingtoitsecol-
nentlandscapes(Gauchereletal.2003).
ogy. The combination of the first and second
For these and other reasons, much emphasis
dominantspeciesdefinesasub-unitknownas
has been placed on developing methods to quan-
thefacies.
• tify landscape patterns as a prerequisite to un-
Global vegetation cover (GVC), valued from
derstanding pattern–process relationships (Baker
the vivacious species, is divided into three
1989; Turner 1989; Turner and Gardner 1991;
classes:
Johnson et al. 1992; McGarigal and Marks 1995).
ClassA:GVC≤25% Severalindicesdescribelandscapepatterns(Worms
ClassB:25<GVC≤50% etal.2004;Groometal.2005).Amongtheindices
ClassC:GVC>50% retainedforourstudyaretheShannonlandscape
EnvironMonitAssess
diversity index and the number of classes of the Largestpatchindex
largestpatchindex.
Landscapeecologyconsidersthelandscapeasa LPI equals the area (m2) of the largest patch
mosaic of land units. The elements that compose of the corresponding patch type divided by total
the mosaic comprise two orders: (1) class, in our landscapearea(m2),multipliedby100(toconvert
case classes of land cover, and (2) patch, which to a percentage). In other words, LPI equals the
are the elementary units. Finally, the landscape percentage of the landscape comprised by the
comprisesthreehierarchicallevels:thelandscape largestpatch.
(largest), the class, and the patch (smallest el-
(cid:3) (cid:4)
ement). The spatial indicators are measured by a
max a
ij
indicesthatdescribespatialorganization. LPI= j=1 (100)
The selected indices, as well as their denomi- A
nations,areborrowedfromMcGarigalandMarks
(1995),thesearegivenbelow. aij area(m2)ofpatchij.
A totallandscapearea(m2)
Percentageoflandscape(PLAND)
Shannon’sdiversityindex
It represents the part occupied by the class in
i
relationtoalltheclasses. Shannon’sdiversityindexisameasureofecolog-
ical diversity, particularly those expressing rare
(cid:2)n patchtypes.
a
ij
PLAND= Pi = j=1 (100) (cid:5)m
A SHDI=− (p ∗ln p)
i i
i=1
P proportion of the landscape occupied by
i
patchtype(class)i.
P proportion of the landscape occupied by
a area(m2)ofpatchij. i
ij patchtype(class)i.
A totallandscapearea(m2)
These analyses are often carried out with
It measures the percentage of the landscape
the Fragstat software established by McGarigal
comprised in a particular land cover class. It is
and Marks (1995). However, this software works
oneofthe mostused(FormanandGordon1986;
rather on raster images and could not be used.
Leitaoetal.2006;Esbahetal.2009).
We used the GIS software ArcGIS 9.2 that in-
cludesseveralembeddedfunctionssupportingthe
measurementsoflandscapemetrics.Moreover,in
Numberofpatches
accordance with Leitao and Ahern (2002) and
Leitao et al. (2006), these indices are appropri-
NP equals the number of patches of the corre-
ate to describe the fragmentation and shrinkage
spondingpatchtype(class).Itrepresentsthenum-
processes(Esbahetal.2008;Leitaoetal.2006).
berofpatchesofaparticularlandcoverclassand
translates the importance of fragmentation, con-
sidered as one of the principal threats to ecosys-
temintegrity(Forman1997;FormanandGordon Results
1986;Groometal.2005).
Evolutionofindices
NP ni
ni numberofpatchesinthelandscapeofpatch The following paragraphs describe the landscape
type(class)i. evolutionapplyingtotheabovedescribedindices.
EnvironMonitAssess
Table1 Synthetictable
Formation Area(ha) PLAND SHDI NP LPI
ofdifferentlandscape
1 Woodedsteppe 5,515 0.01 0.09 4 1.23
indicesusedinlandcover
mapof1978 2 StipatenacissimaSteppe 178,428 0.46 0.51 48 7.79
3 LygeumspartumSteppe 168,095 0.44 0.52 61 11.09
4 SeriphidiumherbaalbaSteppe 19,437 0.05 0.22 15 0.90
5 Salsolasieberivar.zygophyllaSteppe 2,251 0.01 0.04 1 0.59
6 Dune 4,345 0.01 0.07 1 1.13
7 Crops 2,380 0.01 0.05 3 0.43
8 Sebkha(Saltywetland) 4,179 0.01 0.07 5 0.57
Total 384,630 1.00 1.57 138
Percentageoflandscape formations.Amongthese,14arevegetationunits,
withtheremainderrepresentedbythedunesand
Table 1 lists the results extracted from the 1978 urbansites(Fig.6).
landcovermap(Fig.5).Itonlyshowsfiveforma- Extensive formations of S. tenacissima (alfa)
tions,(nineifweretainnon-vegetationunits)and andL.spartum(sennagh)occupythelandscapein
contrasts with the 2006 map, which considers 20 1978map.
Mecheria landcover map 1978 -Algeria-
0˚30'0"E 0˚0'0"E
Legend
Town
Road
Track
Formation
Steppe with tree
Alfaville Stipa tenacissima steppe
Lygeum spartum steppe
Artemisia herba alba steppe
Salsola sieberi steppe
Dune
El biod Salty wetland
Crops
Khebbaza
Tousmouline
Mecheria
N
0"
0' N
33˚3 30'0"
3˚
3
Touadjeur
E
0" 4
0'
3
0˚
Kilometers
0 10 20
0˚0'0"E
Projection Lambert Nord Algeria
Fig.5 Landcovermapof1978
EnvironMonitAssess
Mecheria landcover map 2006 -Algeria-
0˚30'0"E 0˚0'0"E
Legend
Town
Road
Track
Formation
Steppe with tree
Stipa tenacissima steppe
Alfaville Lygeum spartum steppe
Salsola vermiculata steppe
Atractylis serratuloides steppe
Anabasis oropediorum steppe
Noaea mucronata steppe
Aristida pungens steppe
El biod Thymelaea microphylla steppe
Retama retam steppe
Atriplex halimus steppe
Salsola sieberi steppe
Traganum nudatum steppe
Arthrocnemum glaucum steppe
Peganum harmala steppe
Khebbaza Dune
Salty wetland
Tousmouline Bare land
Reforestation
Crops
Urban
Mecheria
N
0"
0' N
33˚3 30'0"
3˚
3
Touadjeur
E
0" 4
0'
3
0˚
Kilometers
0 10 20
0˚0'0"E
Projection Lambert Nord Algeria
Fig.6 Landcovermapof2006
Indeed, the 1978 land cover map shows the dominant and has now been replaced by the
dominance of the S. tenacissima and L. spartum, L. spartum and other species indicating land de-
which occupied more than 90% of the study terioration, a trend observed by many authors
area.Twocolorsaredominantonthemap,green (Aidoud et al. 1983; Aidoud and Touffet 1996;
(charcoal-gray for S. tenacissima) and grey (clear Aidoud et al. 2006).The rhizomatous root sys-
grayforL.spartum;Fig.5). tem of L. spartumfacilitates this extension and
Formations of S. herba-alba, once covering seems to be an“opportunist” species, the devel-
19,437 ha, are present with lower surfaces and opment of which isfavored by drought and land
comeinthirdpositionwithonly5%ofthesurface. degradation.
The land cover map of 2006 (Fig. 6) differs Results show that the S. tenacissimasteppe
significantlyfromthatof1978. has experienced a formidable decline. It hasdra-
The main characteristics of the recent maps matically regressed from 178,429 ha in1978 to
(2006)aresummarizedbelowinTable2. 44,366 ha in 2006, which corresponds toa phe-
There is a higher heterogeneity of vegeta- nomenal 75% reduction. The term “Alfa sea”so
tion units (formations): 14 formations are now characteristic of the region in the 1960s, hold-
present, with 11 new units since 1978. However, sno significance at present. The same decrease
only eight of them have an area exceeding 2% has been observed in Tunisia, but was less pro-
of the study area. In 1978, S. tenacissima was nounced(HanafiandJauffret2007).