Desertification in reverse? Observations from northern Burkina Faso

Global Environmental Change 11 (2001) 271–282

Desertification in reverse? Observations from northern Burkina Faso

Kjeld Rasmussena,*, Bjarne Foga, Jens E. Madsenb

a Institute of Geography, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmarkb Institute of Biology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark

Received 11 February 1999

Abstract

The idea of degradation of arid and semi-arid lands, often termed desertification in its irreversible form, due to human impactand/or climatic change has been much debated since the mid-1970s. From the time of the United Nation’s Conference OnDesertification in Nairobi, 1976, certain areas of northern Burkina Faso have been pointed out as examples of severe desertification.

Several studies demonstrated that revitalization of a series of E–W oriented fossille dunes in the Oudalan province was ongoing. Thepresent study includes an analysis of the trends of vegetation development in the region, covering the period 1955 to 1994, withemphasis on the fossile dunes. It is demonstrated that desertification and revitalization of dunes were phenomena associated with the

period between the early 1970s and the mid-1980s, and that the decline in vegetation cover on the dunes seems to have been reversedin recent years. The analysis is based upon time series of aerial photos and satellite images, field studies of vegetation, interviews withlocal people and review of relevant literature. The findings are discussed with reference to the debate concerning desertification and

land degradation, as well as to the current revisions of the ‘range management paradigm’. The observations indicate that theenvironmental history of the region is complex and cannot be boiled down to ‘human-induced irreversible degradation’. Rather theysupport the idea of semi-arid cultural landscapes undergoing constant change in response to both human impact and climatic trendsand fluctuations. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Desertification; Land degradation; Vegetation change; Remote sensing; Burkina Faso

1. Background and introduction

Semi-arid Subsaharan Africa}and in particular theSahel-Sudan zone}is often described as undergoingenvironmental degradation (Ibrahim, 1984). The degra-dation processes in play may be soil erosion, soilnutrient depletion, deforestation, disappearance ofuseful species, etc. Desertification is a term often usedas a heading under which all these processes areorganized. It has been noted by many (Olsson, 1993;Helld!een, 1991; Swift, 1996) that the empirical basis forthe idea that desertification is taking place at the scalesand with the speed, which have been assumed, is weak.This paper aims at contributing to the establishment of amore solid empirical basis for these ongoing discussions,which have great implications in terms of environmentalpolicy.

Whereas regional or continental scale studies will berequired in order to improve on the estimates of theextent and rates of various types of environmentalchange, local scale studies are required in order tounderstand the processes and causes involved. Thisstudy will focus on a small region, a few hundreds ofkm2. The study area is probably among those in theSahel-Sudan best known in terms of their environmentalhistory of the last decades. Thus it provides a ‘testingground’ for the various ideas, concepts and methodsassociated with desertification. It is not claimed that thestudy region, part of the Oudalan of northern BurkinaFaso, is in any way representative for larger areas, andconclusions from this study should not be extrapolatedto the rest of the Sahel-Sudan zone. The role of a case-study, such as the present, is that results, contradictinggenerally held beliefs, may point to the need forreassessing the validity of these beliefs.The areas studied are two large, E–W oriented,

longitudinal, fossille dunes, which have by severalauthors (Krings, 1980; Chamar and Courel, 1979;Lindquist and Tengberg, 1993) been pointed out as

*Corresponding author. Tel.: +45-35-32-2563; fax: +45-35-32-

2501.

E-mail address: [email protected] (K. Rasmussen).

0959-3780/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 5 9 - 3 7 8 0 ( 0 1 ) 0 0 0 0 5 - X

areas of intense desertification. The area is}in theUNEP map of desertification risk (UNEP, 1992)}identified as a zone of high risk of (further)desertification. Thus it may be seen as an extreme case.The following questions are adressed in this paper:What changes in land cover have occurred on these

fossile dunes? Are these changes always (for the periodstudied) towards ‘degradation’? Are changes spatiallyuniform, and if not are spatial variations related to theintensity of the human utilization?

2. The concepts and paradigms of desertification and land

degradation

The following definition of desertification wasadopted by the United Nation’s Conference on Envir-onment and Development (UN, 1992)

Desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting fromvarious factors, including climatic variations andhuman activities.

In contrast to earlier attempts to define desertifica-tion, this definition does not explicitly specify theprocesses involved nor its causes. An earlier definitionby UNEP (UNEP, 1984) limited desertification to ‘‘landdegradation . . . resulting from adverse human impact’’.In some cases, even specific causes, such as overgrazing,overcultivation and fuel wood collection, have beenrefered to in definitions. In addition, the processesinvolved were sometimes described in more specificterms, soil erosion, loss of biological productivity and/orecosystem resilience being among those often men-tioned. Finally, irreversibility}or only very slowreversibility}has sometimes been considered a criter-ium for labeling an observed change ‘desertification’.The UNCED definition, cited above, is virtually

devoid of meaning and content, unless ‘landdegradation’ is properly defined. Williams and Balling(1995) define land degradation (in drylands) as:

reduction of biological productivity of drylandecosystems, including rangeland pastures andrainfed and irrigated croplands, as a result of anacceleration of certain natural physical, chemicaland hydrological processes, including erosion anddeposition by wind and water, salt accumulationin soils, groundwater and surface runoff, areduction in the amount or diversity of naturalvegetation, and a decline in the ability of soils totransmit and store water for plant growth.

Many other definitions are available, yet the one citedcatches the main contents of most of them. The questionof irreversibility is not directly refered to, however, eventhough much discussion has touched upon it. Most

authors implicitly or explicitly exclude processes, whichare characterized by being reversible within a time-spanof few years, from their definitions of desertification andland degradation.Whereas Williams and Ballings’ definition of land

degradation encompasses a wide range of processes of aphysical and biological nature, many studies use theterm in a narrower sense, often influenced by theauthor’s discipline. Thus, geomorphologists may focuson erosion processes, soil scientists on physical andchemical soil properties, ecologists on productivity ofnatural vegetation and botanists on changes in speciescomposition and loss of biodiversity. This implies thatland degradation may be seen to take place in the viewof a geomorphologist, yet not with the eyes of abotanist, or vice versa.Warren (1998) has further pointed out the need to

view land degradation in its economic, social andcultural contexts. Erosion may not}in certain caseswhere soils are deep}have significant economic impactsin the short to medium term, and concepts of degrada-tion and fertility are socially and culturally defined.As mentioned above, several authors have questioned

the concept, the various definitions and the empiricalevidence of desertification (Olsson, 1993; Helld!een, 1991;Swift, 1996). Yet the concept has persisted, even thoughthe definition has changed, as mentioned above.The literature on desertification and land degradation

may be said to define a ‘paradigm’. Recently, conflictingparadigms concerning the related issue of dryland rangeecology have been discussed Behnke and Scoones, 1993;(Warren, 1995). Representatives of the new paradigmquestion the validity of the traditional perception ofdryland ecosystems as equilibrium systems, allowingassessment of livestock carrying capacity and thusovergrazing. They claim that such systems should ratherbe seen as ‘event driven’ non-equilibrium systems(Behnke and Scoones, 1993). The desertification or landdegradation paradigm, if this may be said to exist,conforms well with the traditional equilibrium paradigmof dryland range ecology, whereas some of the critiqueof the desertification concept emphasizes the inherentvariability of drylands, and thus seems to be in line withthe new ideas of such systems being event-driven.

3. Study region

3.1. Geomorphology, soils and terrain

The location of the study region is outlined in Fig. 1.The landscape may be described as a pediplane on whichat least two series of longitudinal dunes have beensuperimposed, the oldest around 40.000 years old, theyounger around 16–20.000 years old (Krings, 1980). Theancient drainage pattern, consisting of broad valleys,

K. Rasmussen et al. / Global Environmental Change 11 (2001) 271–282272

has been disrupted by the dunes, giving rise to series oftemporary lakes along their margins. Some of theselakes may hold water throughout the dry season, andthose that do have great impact on the localization oflivestock towards the end of the dry season. The presentstudy concentrates on the younger dunes, in particulartwo of these, hereafter termed the ‘Oursi-’ or northernand the ‘Bidi-’ or southern dunes.These large longitudinal dunes, a few kilometers wide

and several tens of kilometers long, have been fixed byvegetation, and the aeolean surface morphology hasbeen reduced to a gently undulating landscape, in areasnot affected by revitalization of dunes during recentdecades. The dunes must have been formed underclimatic conditions drier than those of today. The coreof the dunes probably consists of relatively coarse sandfractions. The topsoil has been enriched by finer

material, deposited by duststorms. Krogh (2001) hasdescribed these dune soils in the case of the Bidi-dune.

3.2. The natural vegetation

It is not an easy task to define the ‘natural’ vegetationof Sahelian dunes. Prior to the period in which Manbegan utilising this environment, the climate changed}according to Mattson and Rapp (1991) from unu-sually wet (9.500–4.500 BP) to more arid (4.500–3.000BP) conditions. Following the idea of Sprugel (1991), itmay thus be concluded, that as a consequence of anever-changing climate ‘‘every point in time is special’’,and different plant communities are ‘natural’ at differenttimes, even in the abscence of Man.Today, the Sahelian dune flora is remarkable depau-

perate in species and most plants are non-demandingpaleotropical weeds and pioneers. The majority ofgenera mentioned by Kumar and Bhandari (1993) forsandy dunes of India are, for example, also found onSahelian dunes. It is conceivable, of course, that theprehistoric flora was more diverse and included a varietyof floristic elements from the Sahara.Woody vegetation is only sparsely and heteroge-

neously distributed on the dunes of northern BurkinaFaso, and the density of trees is often lower than on thesurrounding, xeric pediplain. A remarkable low numberof seedlings and saplings is observed in most species, andthis seems to be correlated with a high browsing activity.The largest tree found, Faidherbia albida, is a well-known fertilizer in traditional agriculture, and alsoCombretum glutinosum and the living fence postEuphorbia balsaminifera are indicators of cultivation.Two other dune species, Acacia tortilis and Balanitesaegyptiaca, thrives well with intensive livestock brows-ing (Toutain and De Wispelaere, 1977). Finally,Leptadenia pyrotechnica, is an aggressive pioneer thatcolonize disturbed dunes which have been provisionallystabilized by herbaceous plants. All these trees andshrubs are, at least to some extent, best regarded asanthrophytes, e.g. plants who owe their present dis-tribution to the presence of Man.Herbaceous plants only cover the dunes with a green

layer for a few weeks each year. The duration of theherbaceous phase, its density and its floristic composi-tion reflects local variations in precipitation and land usepatterns and varies from year to year. Open wind blowndepressions may be almost naked apart from a fewannual invaders and Cyperus conglomeratus, a sedgewhich stabilizes the sand due to its persistent tufts.Areas where the sand is gradually becoming fixed aredominated by grass species such as Aristida mutabilisand Cenchrus biflorus (cram cram). Tall, perennialgrasses of the genus Andropogon are only observedoccasionally and mostly on cultivated dunes which areperiodically protected from livestock grassing.

Fig. 1. The study area. The background is constituted by a Landsat

TM image from October 1992. The colour composite is created with

RGB=channel 3, 4, 5. Ponds (‘mare’s’) and intensively cultivated

areas are highligted. The villages Oursi, Yomboli, Bidi, M!een!eegou and

D!eeberelink, mentioned in the text, and the main town of Gorom

Gorom are shown. Corridors for North-South livestock migration as

well as the transects studied are shown.

K. Rasmussen et al. / Global Environmental Change 11 (2001) 271–282 273

3.3. The climate

The rainfall regime is characterized by great varia-bility in both time and space. Even within shortdistances, rainfall may be very different. The spatialand temporal variations have both regular and stochas-tic components. The regular component comprises thenormal annual variation, characterized by a short rainyseason, largely limited to the months of July, Augustand September, as well as the North–South gradient ofthe mean annual rainfall, which amounts to approxi-mately 2mm/(year km).Possible long term trends or cycles in Sahelian rainfall

have been discussed since the severe drought of the earlyseventies (Mattson and Rapp, 1991). Generally speak-ing, the mean annual rainfall of the region has beensubstantially lower in the period since 1970 than in thepreceeding period, (1954–1969 for Gorom Gorom) asillustrated in Fig. 2. In recent years rainfall may be saidto have recovered, yet the period is too short andthe stochastic component too big to warrant firmconclusions.The potential evapotranspiration amounts to 6mm

per day on an average, implying that rainfall seldomexceeds potential evapotranspiration on a monthlybasis.

3.4. Human utilization

The human utilization of the dunes may be dividedinto (1) crop production, (2) grazing by livestockbelonging to local farmers, (3) grazing by livestockbelonging to or herded by nomadic or transhumantpastoralists from outside the region, and (4) collectionof natural vegetation, in particular fuel wood.Since early times the dunes have been attractive seen

from a cultivation point of view, because soils were easy

to till, and because of good soil water availability causedby higher infiltration rates than on the pediplane. Thesouthern dune was entirely cultivated on long stretchesbefore the drought in the early seventies. In some areas,e.g. at Tassarmakat, just West of the area studied here, itstill is. Dune margins are mostly cultivated within thestudy area. The northern dune has not traditionally beencultivated intensively during the last decades, eventhough fields and remnants of old fields can be foundin the central parts. A population of large Faidherbiaalbida indicates former agricultural activity in the area(Toutain and De Wispelaere, 1977) and pollen analysishas shown that cultivation has been practised in theregion for approximately 3.000 years (Andres et al.,1996). Today, cultivation mainly takes place (within thestudied area) along the southern margin between Oursito the West and Darkoye to the East. The presentlycultivated parts of the two dunes are shown in Fig. 1.Fields are permanently cultivated, manure is applied(though often in small amounts), and no regularfallowing system may be observed. Sometimes fieldsare temporarily abandoned, due to low productivity orunavailability of household labour (Rasmussen andReenberg, 1992).The dunes are extensively used for grazing, both by

sedentary and nomadic livestock. The grazing pressureexerted by the sedentary livestock tends to dependstrongly on distance to the village and water sources,resulting in areas of depleted grazing resources alreadyearly in the dry season. The grazing pressure exerted bynomadic and transhumant livestock will be determinedby (a) the amount and quality of fodder resources,(b) proximity to water, (c) proximity to salt licks and (d)the location of livestock corridors through cultivatedareas, such as dune margins. With respect to thesouthern dune and the southern margin of the northerndune, relatively few such corridors exist, see Fig. 1, andthis may have considerable impact on the distribution ofgrazing pressure on the dune. As the dry seasonprogresses, fewer water sources will be available, andproximity to the few remaining ‘mare’s’ and lakes willbecome the most important determinant. Shallowhanddug wells may provide water for smaller herds,especially those belonging to sedentary livestock owners.The grazing pressure exerted by nomadic livestock willbe strongly reduced as the dry season progresses, sincethe the nomadic or transhumant Peul will move south-wards.The gathering of fuel wood has probably limited

effects on the woody dune vegetation at present, becausepediplains and river valleys are far more used for thisactivity. The situation may, of course, have beendifferent in earlier periods, but it should be notedthat cutting down of live trees for fuel wood is nowillegal and a matter of public concern, also at villagelevel.

Fig. 2. Annual and 5-year running mean of rainfall in Gorom Gorom

in the period 1958–1996.


4. Earlier studies of desertification in the region

According to UNEP (1992) parts of the West-AfricanSahel are particularly prone to desertification. Concern-ing the study region in question, several studies haveconcluded that this area is particularly strongly affected(Krings, 1980; Chamar and Courel, 1979 Lindquist andTengberg, 1993). These studies argue that desertificationis progressing, and in particular the dunes are pointedout as strongly affected.IEMVT (1978) has published a series of maps of

range types and condition, describing the state ofpastures in 1974 and the change in state from 1955 to1974, based on visual interpretation of aerial photos incombination with field work. The northern dune showsa mixed picture, ranging from a strong degradationaround Oursi in the West to no degradation in the Westof the study area. The southern dune is descibed asalmost entirely cultivated, and thus unclassified, apartfrom an area between Menegou and Deberelink, whichis in a slightly degraded stage in 1974 and in a good statein 1955.ORSTOM has operated a research station within the

area, and several vegetation studies have been carriedout, documenting species composition in the region(Claude et al., 1991).More recently, Lindquist and Tengberg (1993) studied

desertification, both on the dunes and on the pediplane,and calculated rates of environmental change fordifferent geomorphological units. The conclusions arethat dune areas are still undergoing rapid desertification.More detailed studies at village level (Chamar and

Courel, 1979) have been carried out in relation todevelopment assistance activities, which have encom-passed several anti-desertification activities, e.g. in thevillages of Oursi and Menegou.

5. Data and information available

The study has drawn upon various sources ofinformation, including topographical and thematicmaps, some of them generated by the abovementionedresearch projects, aerial photos and satellite images. Inaddition, limited field collection of data has been carriedout. The most important data sources are briefly listedbelow.Maps:

* Topographic maps in scale 1 : 200.000 producedby IGN in 1960 on the basis of aerial photos from1955/56.

* Thematic map of pasture state by IEMVT (1978).

Aerial photos from 1955/1956, 1974, 1976, 1981 and1995.

Satellite images: SPOT HRX X-mode images from1986, 1988, 1989, 1991 and 1996.Information collected in the field: Land cover, floristic

composition of the natural vegetation, land use Aeoleansurface features, signs of wind erosion, actual and pastInterviews with village leaders and farmers

6. Methods

6.1. Interpretation of aerial photos

Differences in the time of year of aerial photocoverage, as well as in quality and processing, makequantitative analysis of changes in greytones betweenthe three set of photos difficult. However, certain objectsand features may be identified visually on the basis ofpattern and shape. These include:

* Individual trees over a certain size.* Certain vegetation types and formations, character-ized by a certain image texture, e.g. dense stands ofLeptadenia pyrotechnica.

* Active aeolean surface morphology.* Human habitations.* Fields and fallows (to the extent that these arecharacterized by reduced vegetation cover relative tothe area around them and have well-defined linearboundaries).

* Livestock tracks and corridors.

6.2. Analysis of SPOT images

Satellite images from just after the rainy season, whenthe herbaceous cover is still visible, have been used tomonitor the temporal changes in herbaceous cover.Quantitative use of this data requires that data arecalibrated and corrected for view angle and atmosphericeffects. Due to the unavailability of the atmosphericdata required to carry out a full atmospheric correction,supposedly constant reference surfaces have been usedto correct}or rather normalize}the images radiome-trically relative to each other. The details of the satelliteimage processing methodology has been reported else-where (Fog and Rasmussen, 1997).Only relatively large objects may be identified on

satellite images with spatial resolutions in the order of20m, yet the time of acquisition (mostly October) andthe spectral information make them well suited for thefollowing purposes:

* Classification of the dune areas into a number ofclasses, defined by their present land cover. Visualinterpretation is used, since spatial patterns of pixelvalues are used as a basis for classification. Fieldobservations have supported the classification.


* Quantitative assessment of the extent of vegetation-free zones on the dunes.

* Assessment of changes in vegetation cover, using thesurface albedo as an indicator.

6.3. Botanical and geomorphological ground truth studies

Field work activities were organized as studies of50� 50m plots and as transects through selected partsof the dunes. Both plots and transects were selected onthe basis of analysis of satellite images, in order to coverthe main land cover types. No attempt to carry out astatistically based sampling was made. A total of 18plots and approximately 18 km of transects werestudied. The following data were gathered for each plot:

* The UTM coordinates, using a Trimble ScoutmasterGPS, taking averages over 600–1.000 measuremnets.This is expected to give a precision of 12m.

* The herbaceous cover was estimated using theBrown-Planquet method.

* The three most abundant herbaceous species wereidentified.

* Densities of woody species were counted and crowncover estimated.

* Geomorphological features, such as indications ofwind erosion, presence of aeolian forms, etc., werenoted, and sketches of their character and distribu-tion made.

* Notes were made on presence of signs of heavygrazing pressure, livestock tracks, traces of earliercultivation, etc.

Along transects, GPS-positions were taken withintervals in the order of 1 km, and notes were made onland cover, species distribution, signs of wind erosionand aeolian forms, present and earlier cultivation, etc.The purpose of these field studies was to provide an

input to the interpretation of aerial photos and satelliteimages.

6.4. Interviews with village leaders and farmers

Structured interviews were carried out with villageleaders, in most cases backed by a group of farmers, infive villages: Deberelink, Menegou and Bidi on thesouthern dune and Oursi and Yomboli on the northerndune (see Fig. 1). The main purpose was to establish theenvironmental history of the dune from before thedrought of the early seventies till today. The interviewscovered the following:

* Name, age, village affiliation and ethnic backgroundof the (main) respondent.

* Changes in the human utilization of the dune forcultivation and grazing during the periods pre-1970,1970–1987, post-1987.

* Changes in the herbaceous cover in grazing areas onthe dune in the same periods.

* Changes in woody vegetation cover in grazing areasand cultivated fields (pre- and post-drought) in thesame periods.

* Changes in the size and distribution of vegetation freeactive dune areas in the same periods.

* Perceived causes of the changes identified.

In most cases the interview was supplemented byvisits to representative areas on the dune.

7. Results

7.1. Aerial photo analysis

A comparison of field observation results andinterpretation of aerial photos showed that only largetrees may be identified, and only if certain conditions arefulfilled. A study of changes in densities of trees}invarious landscape elements}over the period studied isunderway ([14] Madsen et al., 2001).As will be returned to later, the bush species

Leptadenia pyrotechnica has invaded dune areas duringthe last 15–25 years. Dense stands of these bushes arevisible on aerial photos due to a distinct texture. Anattempt has been made to assess the changes in extent ofthese dense stands over the period studied for an area onthe Bidi-dune. The result is shown in Fig. 3. It appearsthat they advanced rapidly in the first decade after thedrought of the early 1970s, but have been occupyingsmaller areas in recent years, probably due to a die-backphenomenon.

Fig. 3. The change in spatial distribution of dense stands of

Leptadenia pyrotechnica on a section of the southern dune between

1974 and 1995, as detected by use of aerial photos. The area covered by

dense Leptadenia stands has increased and been displaced southwards,

probably due to a die-back phenomenon affecting the areas first

established.


Areas devoid of all vegetation may be observed on theaerial photos, and were confirmed during field work, yetprecise delimitation is difficult since variations invegetation cover are gradual. In Fig. 4 an attempt hasbeen made, however, to illustrate the changes over timefor an area on the Bidi-dune. Note that already in 1955/1956 a large totally denuded area may be observed atMenegou. The sharp limits of the area indicate that it isagricultural fields. The changes observed support theidea that the extent of denuded area has been diminish-ing over the last ten years. For the Oursi dune in theNorth the raw aerial photos give a good illustration of

the process. Fig. 5 shows the variation of the size of thevegetation free revitalized dune area at Oursi. The rapidgrowth of the area of active dune sands in the period1974–1981, as well as the slow but steady reductionsince then, is clearly visible. In addition, a considerableincrease in the woody vegetation, both in the villagearea South of the dune and on the North side, isevident.

7.2. Results of the analysis of SPOT data

Since the satellite images used here are all from theend of the rainy season and beginning of the dry season,late September to late October, they provide essentialinformation on vegetation cover not visible in aerialphotos, which are generally taken in the dry season. Inaddition, the fact that millet is still green allows betteridentification of active fields. As mentioned above,extraction of absolute values of vegetation coverpercentage is difficult, and instead a normalizationprocedure has been used, which allows only assessmentof trends. The procedure is based on the criticalassumptions that (1) surfaces with unaltered spectralreflectance over the period studied can be identified, and(2) the atmospheric effects on the signal are constantover the area studied. With respect to the firstassumption, vegetation free dune sands and a majorinselberg with sparse vegetation are expected to con-stitute surfaces of constant spectral reflectance. Withrespect to the second assumption, it should be notedthat a relatively limited area will be dealt with here.The parameter, extracted from the satellite images,

Which may be expected to yield the best correlation withherbaceous cover is the surface brightness (or ‘albedo’).This is the case when (1) the vegetation is withered andgrey/yellow/brownish and (2) the soil backgroundcolour is homogeneous and bright. The main reasonthat vegetation is causing a darkening of the surface isthe shadowing of the bright soil. This is supported bythe findings of Jacobberger and Jellison (1994). In Fig. 6the relationship between total vegetation cover (the sumof herbaceous, bush and tree cover), as determined inthe field (see Table 1), and the ‘brightness’ (the sum ofthe three normalized SPOT bands) is shown. A SPOTimage acquired on October 23 1996 during the period offield work has been used. This image covers, however,only the southern dune. A quite high correlation-coeffecient; r=�0.89, is found, yet the number ofobservations is low, and the result should be interpretedcautiously.In Fig. 7 the normalized brightness of the three

SPOT-scenes and the changes between images areillustrated for the Bidi-dune. It is obvious that adecrease in brightness, and thus probably an increasein vegetation cover, can be observed.

Fig. 4. Changes in size and location of vegetation free areas on a

section of the southern dune between 1955 and 1995, as detected by use

of aerial photos. A clear decrease may be observed in the 1981–1995

period.


7.3. Results of the field work

As mentioned earlier, the field work encompassed thefollowing elements

* Sampling of the vegetation in 18 50m� 50m plots.* Description of land use and land cover alongtransects, 18 km in all, shown in Fig. 1, mainly foruse as ground truth for aerial photo interpretationand satellite image analysis.

* Identification of vegetation free zones, active dunesands and areas with geomorphological featuresindicating past or present wind erosion.

* Collection of surface sediment samples.* Interviews with farmers and village leaders.

All information gathered was georeferenced by use ofGPS.

As mentioned 18 plots on the Bidi- and Oursi-duneswere studied. Table 1 summarizes the results of theestimation of ground cover, and Table 2 summarizes theresults of the survey of woody and herbaceous specieson the dunes, including both on the 18 plots and thetransectsThe information gathered has mainly been utilized as

an input to the aerial photo interpretation and satelliteimage analysis.

7.4. Results of interviews

Six interviews with local farmers and pastoralists werecarried out, four relating to the southern dune, two tothe northern. Interviews were semi-structured, based ona questionnaire. The respondents were either groups ofpeople or individuals, most of them 30–60 years old,

Fig. 5. Changes in the vegetation cover of the Oursi-dune between 1955 and 1995. The ‘Mare d’Oursi’ may be seen in the bottom of the photos, and

the village of Oursi is situated between the dune and the ‘Mare’. The aerial photos have been scanned, corrected geometrically and radiometrically

normalized. The invasion of both bushes and herbs/grasses between 1981 and 1995 is clearly visible. The heavy local grazing and trampling pressure,

which is the most probable cause of the revitalisation of the dune in the mid-seventies, may be infered from the many livestock tracks radiating from

the Mare and the village.


capable of reconstructing the environmental history ofthe area, as the villagers perceive it.Questions concerned the changes in the dominance

and composition of the herbaceous and woody dunevegetation, in the extent of the vegetation-free surfaces,location of cultivated areas and livestock numbers. Forall these variables questions concerned the status in fourperiods, pre-1972, 1972–1984, 1985–1990 and 1991 todate. The choice of 1972 and 1984 as limitations of theperiods is determined by the fact that these two yearsrepresent drought years remembered by most peopleover 30 years of age.

7.4.1. The northern duneInterviews with groups of farmers were carried out in

Oursi and in Yomboli. In Oursi the largest and bestknown active dune is found. The active part of the dunegrew from close to nothing to its maximum size in thedrought of the early 1970s. In the latest period (1991-)informants had observed regrowth of herbaceousvegetation in parts of the dune area, which have beenbarren since the seventies. A marked change in speciescomposition of the herbaceous vegetation has been

observed: Before 1972 tall perennial grasses, which maynow only be found in bas-fond areas, dominated,whereas Cenchrous biflorus (cram-cram) has now takenover dominance. Numerous large trees died during thedrought, and regrowth of woody vegetation, whichstarted shortly after the end of the drought years,primarily encompasses Leptadenia pyrotechnica.The informants themselves perceive the revitalization

of the dune in the seventies as caused by drought. Theyassess that following the initial reduction in animalnumbers caused by the drought, the livestock charge hasreturned to its pre-drought level or is even higher today.In Yomboli, the informants agreed that the drought

of the early and mid-seventies did not lead to generalreactivation of the dune, as it was the case in Oursi. Yetsince this time barren areas do exist, but cover only aminor fraction of the dune surface. They emphasizedthat the species composition of the herbaceous vegeta-tion has changed significantly, corresponding to thechanges observed at Oursi, and many large trees, e.g.Adansonia digitata (baobab), indicating earlier cultiva-tion, have died, and instead other smaller species haveincreased in numbers since the late 1970s.A detailed account of the changes in the agricultural

land use of Yomboli and their possible relations toclimate, geomorphology and other factors can be foundin (Reenberg et al., 1998), which also provides evidence

Table 1

Vegetation cover (herbaceous, bush and tree cover), ‘relative albedo’ and ‘normalized difference vegetation index’ (NDVI) for four plots on the

southern dune. Vegetation cover has been estimated in the filed, ‘albedo’ and NDVI has been determined from a SPOT satellite image.

Plot 1 Plot 2 Plot 3 Plot 4 Plot 5 Plot 6 Plot 7 Plot 8

Relative albedo 0.26 0.33 0.32 0.28 0.34 0.28 0.25 0.30

NDVI 0.095 0.047 0.039 0.047 0.016 0.055 0.039 0.047

Herbaceous cover 50% 5% 30% 35% 15% 60% 40% 25%

Bush cover 24.7% 9.8% 6% 3.3% 4.4% 7.7% 22.4% 12.4%

Tree cover 1.4% 1.4% 7.2% 14.4%

Total vegetation 74.7% 14.8% 36% 39.7% 20.8% 74.9% 76.8% 37.4%

Fig. 6. The relationship between ‘albedo’, as estimated from a SPOT

image, and vegetation cover, estimated on the ground, for eight plots

on the southern dune. The absolute values of the ‘albedo’ are

uncertain, but the variation is believed to be realistic.

Fig. 7. Changes in ‘albedo’}as estimated from SPOT HRV im-

ages}between 1986, 1988 and 1991 for the southern dune. Note that

the SPOT images have been acquired just after the rainy season, in

contrast to the aerial photos used in Figs. 3–5. Since vegetation cover

and ‘albedo’ are negatively correlated, the observed decrease in

‘albedo’ (dark tones) corresponds to increases in vegetation cover.


that livestock numbers in Yomboli are lower today thanbefore 1972.

7.4.2. The southern duneInterviews were carried out in Deberelink, Menegou,

Bidi and 2 km West of Bidi. The informant inDeberelink was a 45 year old Peul farmer. He told thatthe dune was to a great extent cultivated until towardsthe end of the seventies, where wind erosion removed thefiner soil particles. Until 1984 large dune areas werevegetation-free, yet since then regeneration of theherbaceous vegetation has taken place, and the barrenareas have become smaller. Many large trees died duringthe early drought. During the last 7 years Leptadeniapyrotechnica has spread in the area, but the respondentthinks that this is now on the retreat. Fields have largelybeen moved from the dune to the pediplane areas.Livestock numbers have recovered as the grazingresources have improved, yet have now been reduceddue to greater off-take.At Menegou, the informant, a 35 years old Peul

farmer and village chief, stated that the cultivation haslargely been restricted to the dune margins during thewhole period. The herbaceous vegetation was almostfully covering the dune in the pre-72 period, yetdisappeared almost entirely during the early drought.In the pre-72 period a few, minor active dune areasexisted, and these grew in the 1973–1987 period, kepttheir size until 1993 and have been reduced since thendue to reinvasion of herbaceous vegetation. Like on thenorthern dune, the species composition is quite differentnow from what it was before the drought. Tall grasseshave been replaced by low ones. Before 1972 numeroustrees, including Faidherbia albida, grew on the dune, yetthey died in the drought, and since then Leptadeniapyrotechnica has spread and obtained dominance. In

recent years other species have begun to appear.Livestock number were greatly reduced during thedrought, yet have now increased to above the 72-level.At Bidi (1) a group of elderly men were interviewed.

They were all Peul (Rimayb!ee) farmers. The farmingsystem and land use of Bidi have been described in detailby Reenberg and Fog (1995). Cultivation on the dunewas widespread before 1972 but was partly given upduring the drought. Fields were moved to the pediplanenorth of the dune. The herbaceous vegetation on thedune in the pre-72 period was much denser than today.It was greatly damaged during the early drought, with1974 as the worst year but has regenerated since then.No major changes in species composition were reported.Only few trees and bushes grew on the dune before 1972,and these disappeared during the drought. The last 10years Leptadenia pyrotechnica has increased greatly. Theinformants claimed that the livestock number is lowertoday than before 1972.Within the limit of Bidi, 2 km West of the village on

the northern edge of the dune, a 40 years old Peulpastoralist was interviewed. He reported that until 1972the dune in the vicinity was almost entirely cultivated.Wind erosion made this impossible during the drought,and little herbaceous vegetation remained. Since the lateeighties the herbaceous vegetation has regenerated. Hereports no changes in species composition. Before 1972there were many smaller trees in the fields on the dune.These died during the drought. Since the mid- to lateeighties Leptadenia pyrotechnica has been increasing innumbers. The livestock numbers have not yet recoveredfrom the reductions associated with the drought. Likemost other respondents he points to climatic changes asthe main cause of the observed changes.

7.5. Summary of results

The results obtained and observations made may besummarized as follows:

(1) The vegetation cover on the longitudinal dunesseems in general to recover, following the severereductions in the seventies (and in some cases 1980s)observed in earlier studies.

(2) Evidence supports the idea that the species compo-sition of the regenerating vegetation has changed,since the drought of the 1970s.

(3) The histories of various parts of the Bidi- and Oursi-dunes differ widely: The extent of dune revitalisa-tion was very variable, and so is the timing andintensity of the regeneration of the vegetation cover.

(4) The cultivation history of the dune seems to be quitecomplex and dynamic. This finding is in line withthe conclusion of Reenberg et al. (1998) for thenorthern dune at Yomboli. Cases of continuouscultivation on the dunes without significant land

Table 2

Selected plant species on dunes of N. Burkina Faso

Grasses and Sedges Vines and Lianas

Andropogon guayanus Ipomoea coptica

Aristida mutabilis Jacquemontia tamnifolia

Cenchrus biflorus Momordica charantia

Chloris prieurii Leptadenia hastata

Cyperus conglomeratus Bulbs

Dactyloctenium aegyptiacum Dipcadi spp.

Eragrostic ciliaris Shrubs and Trees

Herbs Acacia tortills var. raddiana

Alysicarpus spp. Balanites aegyptiaca

Borreria radiata Bauhinia rufescens

Cassia mimosoides Combretum glutinosum

Chrozophora brocchiana Euphorbia balsaminifera

Indigofera spp. Faidherbia albida

Limeum spp. Hyphaene thebaica

Zornia glochidiata Leptadenia pyrotechnica


degradation are found. In contrast, clear evidence ofcultivation causing local degradation on dunes areobserved, e.g. at Bidi.

(5) Land degradation effects of heavy grazing, browsingand trampling pressure are mostly found in theproximity of important water resources, such asMare d’Oursi, or villages, such as Menegou.

8. Discussion

It has been observed that the picture of a uniform andunidirectional process of desertification of the fossiledunes of the Oudalan is a simplification of morecomplex realities. A partial recovery of both herbaceousand woody vegetation has been documented in thepresent paper.Our results may be said to support the ‘new

paradigm’, introduced in Section 2, according to whicharid environments are seen as highly variable, ‘event-driven’ systems, rather than equilibrium systems beingmore or less disturbed or degraded by human impact.Sahelian dunes are cultural landscapes, and ‘events’ arenot just climatic phenomena. They include also humandecisions on production strategies and land use, whichhave visible effectsThe observations demonstrate that broad general-

isations on land degradation processes, based on local-scale studies, are risky. Significant variations exist at thelandscape level, and trends in ecosystem dynamics aresometimes totally reversed even within small regions asthe one studied here.Similarly, conclusions concerning ‘irreversible’

degradation, based on few years or decades of observa-tions may be premature. Time constants of climatic,vegetational and}even more so}soil change arein the order of decades and demand long observationseries.With respect to the long-standing discussion of

whether natural or human factors should be consideredthe most important causes of observed land degrada-tion, the present study does not point to simpleanswers. Whereas the drought in itself may have beenthe most important determinant of the history of thedunes over the last decades, the observed largelocal variations, associated with the human uses of thearea, imply that human impact is by no meansinsignificant.On the methodological side, the present study has

demonstrated that aerial photos and satellite images,used with caution, do provide unique information onenvironmental change on the time scale of decades.However, many misreadings of these datasources occur,and it is strongly suggested that extensive field work,including interviews with local people, is carried out inorder to allow extraction of credible information.

9. Perspectives and policy implications

The current study demonstrates that long term studiesare required to identify the environmental changes,caused by natural and human factors, associated withwhat is often termed ‘land degradation’ processes. Also,the spatial scale of the study must}on the one side}allow that environmental changes are related to thenatural resource management decisions taken by farm-ers and pastoralists, yet}on the other side}allow thatconclusions concerning general development trends, ifsuch trends exist, are drawn on a solid basis. It issuggested that the current study area is closely watchedand studied for a long period to come, both because itmay be seen as representative of the agriculturallymarginal South-Sahelian zone, so prone to productionfailure in drought years, because good base-lineinformation is available due to the past and presentactivities of several major research and developmentprojects, and because the area has been identified as anexample of intensive desertification as far back as theUNCOD in 1976. Thus the region provides a uniqueopportunity to study land degradation and recoveryprocesses with the required historical depth.In the wake of the UNCED-conference and the

‘Convention to Combat Desertification’ there is a greatneed to discuss how the conceptions of ‘desertification’,as formulated in the convention, may be translated intopolicy. The implications of the present study, howeverlimited in regional focus and overall scope it may be, isthat sweeping generalisations, resulting in implementa-tion of certain prototype activities at a large scale, mustbe avoided. Each small region has its own environ-mental history, and actions related to ‘combatdesertification’ must be adapted to local eco- andproduction systems and involve the local population.

Acknowledgements

This study has been carried out within the frameworkof the Danish SEREIN programme, funded by Danida,and the ENRECA collaboration project between Uni-versities of Copenhagen and Roskilde in Denmark andUniversity of Ouagadougou, Burkina Faso. ManyDanish colleagues and friends from Burkina Faso andDenmark provided advice and assistance. A specialthanks goes to Anette Reenberg, to our field assistant,Ousmane Takamba, and the local informants.

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Documents

Desertification in reverse? Observations from northern Burkina Faso