MAtAMA A-lNA
farthermost zones from the bore-hol~s. They apparently were c'a~s~d mor~ by<;~' co'mbif\at'iion 'ofse'ver31 natut'lH faot'or~. (ed'~p'it! ·a~d'." ci.~mati~)"t-hanJbY;the' effects of ,;g.r,.az.ing and trampl i~g '. As 'one '~p\sooche11 to- t'he' Waiter ing ~lac.es .. S9iJ. SldT'f,c:el,!'s- submit te~ '~'tt raiffe'rent' "tiYPe5' 'o~' e-vnlu tioib j!C:lior- .',.1; • f'r _ .' " ,, J""UAaY 16-22, 1~83 di'ng t(jths; t'iehure' <~\ all.' saWy 'soilS' holltdgeneoU$ ,powderli4 ~~fHAw"", USA ' 'a lay~i limits ~1i~if;~rosioh ~;"iS very vulM'tab1e.lL.t;o.defla-.• tion anJI~and '
p~§I~., ..,I people is also require~, B~f;.V~.51,~~~~~. JY(),RY.C~'J:"
I NTRODUCTI ON
When Stebb ing (1937) iDinted ou t that dryer c 1ima i\~ns tih~ "S~WtR'llRLrd5!:,enc'rl) l; j 1 1 • ;:. I," argued that in most. cases soil degradation was caused by the in that area. The area provided a large scale experimental action of man. site for an interdisciplinary team composed of investigators of several International, Senegalese, and French Institutions 1 Owing to the protracted drought of the 1970s in the including sociologists, economists, physicians, nutritionists, Sahelian belt, this caution was recently brought into sharp veterinarians, animal breeders, botanists and soil scientists. focus and with increased anxiety. According to many scientists The purpose of this paper is to present major (Boudet, 1972. Delwaulle, 1973. Bougere, 1979. Zonneve~d, findings relevant to grazing and trampling around the water 1980), overgrazing and trampling, especially near the watering holes on soil deterioration, namely on sealing, soil compac- points, are the major factors of soil degradation, and conse tion, wind and water erosion, and to suggest measures to quently of desertification of this area. However, published combat it. material relating accelerated erosion to animal husbandry remains scarce. Additional information·is essential. PROCEDURES Scientific attention has focused on a large region of Northern Senegal which, as recently as thirty years ago, THE STUDYAREA was still called "the Ferlo Desert". At that time, despite the The study area (figure 1) covers 10,000 km2 and continuous presence of fodder, livestock had to be herded out sustains a human population estimated at 30,000 and a bovine of the area, as ponds dried up after the rainy season. The livestock of 100,000 (Meyer, 1980). The arid rangelands are Ferlo was thus deserted by nomadic herdsmen nine months a scattered, with 13 drilled holes, 25 km apart. Water is year. Once numerous deep wells were drilled in the 1950s, raised from the largest water-table in West Africa, the seasonal migrations of nomadic communities declined rapidly, Maestrichtian water-table, located 200 m deep. and this region· was subjected to permanent pastoralism. The climatic data .indicate a very hot dry season Such a radical change in pastoral pratices affected that lasts nine months a year with rainfall concentrated many aspects of the human and natural environments. In order mainly between July and September. One must keep in mind to document such mutation~,studies were recently carried out that, in that 'zone, data can be misleading owing to the wide .... variability of climatic events. By way of illustration, the Il) •" ~ average annual precipitation ranged from 300 mm in the ~orthern - +• M A- L ..~ part . (PODOR) to 500 mm in the Southern part (LINGUERE and .....-. " ...1....)&;/ MATAM) in the period 1920-1969 whereas a general and prolon • iV~ r' ged deficit of approximatively 40\ was recorded throughout I .. I • ,.'t the 1970s (de Wispelaere, 1980a) (figure 2). In 198t, rainfall : @§z::I:~ .. _ ."'~..~ ~ W \1 : ~ .~ 1980). Thorn shrubs and small trees (Commiphora africana, ==.x" over ~ =:I. t\1 110 Balanites aegyptiaca) are patchily distributed the t~ + CJ .. en landscape • .... " ~: I The cuirassed Ferlo : the South-Eastern part of the study area has a more marked relief. An iron cuirass, which has been partly destroyed by erosion, is underlain by Tertiary sandstones. Shallow and gravelly ferruginous soils occur on that iron pan. They include extensive areas with sealeq surfaces, so that herbaceous layer (Zornia glochidiata) is N '1 3 .::>0 0' ::>IJ.N'11J.'1 o Il) ~ - ~ General average: 291.1 :t 32.1 average liII 1969: 313.0:!: 35.3 very scanty and unproductive: the fodder production doesn't average 'rom 1970 101979 : 194.4 :!: 44.8 usually exceed 100 kg dry matter/ha (Boudet, 1980). Pterocar- PODOR I :~~ 600 16-38' N 14·S6'W . pus lucens forms monospecific communities in that arid sC 500 'e 400 woodland. 300 I 200 y.ars METHODOLOGY Three representative drilled-holes were selected General average: 476.7 :t 39.3 for this study, along a transect from North-West to South- i~e average liII 1969: 517.4 :!: 40.0 flOG lINGUERE East, in accordance with the climatic gradient. Two of them i~o IS·2J'N j~ IS·Ol'W are located on the Sandy Ferlo. The third belongs to the Cui- ~~ At::~:~~~":' ~~ rassed Ferlo environment. Topsoil textures, determined on c r.-;~ ~ 200 V" V vr natural sites, vary from sand in TESSEKRE, to loamy sand in TATKI and sandy lo~min REVANE (figure 3). Grazing and tram years pling are assumed to be related to the density of livestock General average: 481.6 :!: 44.3 near the water-holes. Two major concentric areas can be average lil1l969 : 519.5:!: 44.g average 'rom 1970 101979 : 304.4 :!: 63.5 delimitated in The affected zone, from the vicinity of the watering place to a distance ranging from 5 to 7 km, namely within the area where impact of cattle is the most effective. The undisturbed zone, beyond the previous circle, e.g. the years area where cattle rarely wander. Because of this very low General average use, these range lands were considered as natural references Average from the first data to 1969 Average from 1970 to 1979 for environmental features. be rela by analy in Long c~n the THE However possible drilled another soil possible sieve Moreover, season FROM the measured to affected : sites. most. top studied characters. processes and and TESSEKRE was ot~er Likewise, from ZONES the zone area the in soil on), investigate of place the trampling. so surface to drawn attempted study spots occurs to I1STANT revealed mechanical deposits) and one determined. several soil it deflation. line were termite-mounds order density outside for fo season), of watering MOST sand from RESULTS in were is sealing rills, the bulk indurated THE relate radial elsewhere it ruined rainy with a to approach that IN to 0), and widely matter and dug collected the of outlining of Km assessment investigating and drifting, bare by covered were were varies replications) lines after distance FEATURES organic points trampling ancient means sand Most Surprisingly, of a often the and to wtich POINTS main and three 1). (denoted samples as compaction to quantitative morphological SURFACE several Two RING magnitude (with (albeit (before soil (sealing, sis, soil amounts hole tion On The The (Table its trenches ascribed grazing SOIL WATE ~ ~ c:: .- ~ ~ 8 c:: -- ~ ~ ~ ~ >o~ ... ~ c:: ~ u: E 8 >of!? ... ::>- u: "ll .2 ::?: ~ ... ~ E Z E 4i :: :: , 00'0 o~'o SZ'O OS'O ,ZOO'O 00 I I 0 0 ~ -...... :,~:-r N I J C") ----r---- • aAuelnwno r I 0 0 0 0 It) I~o:--+---r N w Z IQ « 6 -"- --+~~ +, 6 ,... w a: ~ (I)~> w _ a6elua::uad ~""C")N .; ~fn~W ~ "CIt)IQr: '~~'~' .!: fn ~W~~ o(l)~~ 0 ~ \~\~- \\ ~ .~ \\ .\ \ \' .\ 6 ~ \\ WO r ~ ~~ o II ~ o patterns. It was often noticed in TATKI that a broken calca reous horizon forms a shallow dome just below the crusted areas. Likewise, bare spots in REVANEoccur mostly where the irregular boundary of the gravelly layer is the closest to the surface. Thus soil surface heterogeneity is often induced by a lateral variation of the soil underneath. Detailed studies of morphology, as well as sieve analysis, clearly demonstrate that sandy micro-mounds wich bo~ndthe bare spots are but aeolian deposits. Provided that TABLE ·1 rainfall is normal, as in 1981, runoff washes down sediments .' from these mounds with the result that crusted areas are DIS~ANCE TESSEKRE " . TATKI REVANE STUDYSITE ... TO THE DRILLED-HOLE covered again with sand. Consequently. an important areal ' ... - (km) 7 ,. S 8 reduction of bare surface is then observed. AREAL EXTENTOF BARE ANDCRUSTEDSURFACES (\) 8 49 S2 SOIL SURFACEALTERATION IN THE AFFECTED ZONES AVERAGEDIAMETEROF SANDY FERLO : BARE AND CRUSTED . , When one approaches to the drilled-holes, the SPOTS (m) Z 11 12 cattle tracks become more pronounced and numerous. Since the .., hooves of the herds pound micro-mounds and spread the sand over the crusted spots, the whole surface becomes completely blinded with a sandy layer. Within a ring located between Km 0.5 and Km 2.0 from the watering points, wind sorted sur faces are marked out by sand ripples. In addition to these features, evidence of an accelerated deflation was found through grain size analysis (figure 4). Bulk density of the layer which underlies the sandy cover convincingly increased o as "the distance to the wells decreased (figure 5). Already zo E c( z E ::;~ low, the amount of organic matter is further reduced within o a: ... ~ ~1Il to U a: grazed However, high from I .~ the ring. values, resulting cattle 1ft iu ~~ §1< casts, were found for samples collected nearby the watering- ::E~ throughs (figure 6). The devastation that results from grazing and trampling by late dry season is quite overwhelming, yet ._----~ E return of rain shows how misleading this appearance can be in "" terms of productivity (figures 7 and 8). j: ~ 0z it Cl CUIRASSED FERLO : III ... i z it . ... a: In contrast to the previous study area, the cuiras 0 ~ z . .,' -~ :: ~ c( , sed Ferlo suffers a severe accelerated water erosion. Plant ... 3: z .' ~~ it ~ communities are degenerated allowing no hope of rapid reco- >a: l- ... > "\'\ very (table 2). Moving nearer to the well of REVANE, marks of l: .~ e ~_:'.> ~ a desolating deterioration get more and more conspicuous. z ~ FUnoff on bare and crusted spots triggers sheet erosion, is rr---' displayed by microcliffs and other pedestal features. Another stage is reached when linear erosion occurs amidst the !:i E iii " , stripped surfaces, associating rills and small gullies to E 0 :g ~ sand drift (figure 9) emphasizing microrelief features. As d ~ I .. "",j:~", Wo U a result, irreclaimable havoc is left behind...... '. "'-./ - " .....,~ 1~llll=llllO:;~~~~~~~~~ ILYII (0< I /IIOLLY1IlWn:xlY110(0) I /IIOLLY1BCI: nu \7 I ., 1 1 j i 10 -- i _-- 9 .. I _.~~ i +_o-o __ I ' 678 ::-..s. I watering-place -=-T ------ i REVANE TESSEKRE TATKI 0 the (km) --:' - i to )( o + » 345 j 2' ~)(- i Distance ; I<.... ~~i~-~=-....!. I 0 0 I I I !J 10 I O· o 8 9 6 5 4' 2 3) 1 o 1 '- Q) '- ~ ... 0) ?f. i - 0 ' 10 ----, • • 89 -- , 7 ""'"' • watering-place • TATKI TESSEKRE ~x (km) the • x + to "T-" .--- +-- , Distance , 1 2 3 4 5 6 +, X \ x \ /x I, o I I .y\ \ .. 30 ) 1. 1,80 1,70~ 1,40 1,50 1,60 ::::s > c Q) en (J E ... en "C - 'e;; ...lII: "- M - I I I I "I .. . . ,..., '.- .,., ,. .. a .'11 .. ;- ~ ~...., .... ~._ ~~"I': I .... io,...... ' ...... ,~ t ~ ...... ~. .-- -...... -,.;,..... " '..--~~3-...... ~. -. . .. ,; .C~r .,.... , \ >::·~~~~t·.t.;~)+.~r.~~,: ~-. -•• , "'. t ~~.r.. ·:.'-~~~.,,'~·3':I~·:-.7:"~!:·~~:;.··'''~~'7·''~.''~''~' '.-~ ..~-.~.. ~ ·.';.~·i . : , .' ','\:::;:.:f'.' ·.'.···; .. :·.li..·: .. '.1 ,I, .. ,. , " . .. .,'...... , w-....,.., , ' . ,: , .,: .', " \ " '" " '. , " , " I • • " ',' • '.'·.~~:·I' .... :. , '. \"0' ' '. 1 , ''-.:...... --.. " . ~' .- , " ~.]J ,- - . , • "V ' •• ..>I., .. -.:...... ,....' -' .- ...... " •- ... or • '-'f ..-..;",.,..... ~.... ';, :....- " -, - ' .. '". ,( .-:-..,.. "~ ...... -...... ~. . ' ... _...... 11.0. .. . ' . l,' ~ -....-".~ , DISCUSSION On the other hand, comparisons of aerial photographs DETERIORATIONOF THE VEGETATIVECOVER of 1954 and 1978, and remote sensing analysis revealed that ruined areas of the cuirassed Ferlo were associated with human Arguments involving the actual effect of animal (de Wispelaere, 1980b). Various evidence husbandry on vegetative degeneration have been presented in a settlements lines of suggest the impact of drought has been aggra variety of studies carried out in the Ferlo. Within a dry that seriously by damage done by browsing and period of 5 years, vegetation losses rose to 50\ of trees in vated the severe wood-cutting. Besides, in respect to its livestock-carrying density, the a fenced-in reserve located 25 km from TATKI, without grasslands REVANEcan a maximal bovine population reference to any effect of browsing (Poupon, 1980). Such of sustain of 1,200, yet the actual cattle exceed 1,500 (Boudet, 1980. observations do not weight in favour of the spontaneous rege Meyer, 1980). Thus, contrary to the sandy Ferlo, that region neration of protected gr~sslandsas it is often mentioned experiences serious overgrazing problem. (Delwaulle, 1973. Adu, 1982). Even more amazing is the signi ficant increase of fodder productivity combined with the SOIL COMPACTION improvement of quality, in terms of palatibility, which have As reported by Thompson (1968), Beckman and Smith been clearly pointed out in the neighbourhood of the watering (1973), and by Lagocki (1978), trampling induces compac points of the sandy Ferlo (Valenza, 1970. Boudet, 1972. soil tion. This study illustrates such effects in terms of bulk Bille, 1976). Two major causes can be assumed: first, as density in the sandy Ferlo. mentioned earlier in this paper, features of trampled mate rials are very similar to those of a ploughed layer the mulch SOIL CRUSTING effect of which allows complete infiltration and limits evapo In constrast to the observations of Mott, ~ridge ration. As a result, such a layer is a more suitable breeding and Arndt (1979) in Australia, and of Boudet (1977), Breman ground for grasses than the undisturbed soil. Secondly, cattle are still herded, during the rainy season, to the & al. (1980) in Mali, soil crusting cannot be attributed in the sandy Ferlo to grazing and trampling, since occurs natural ponds so that deferred grazing is permitted, allowing it vegetation near the drilled-holes to recuperate (Barral. mainly in the farthermost zone from the watering points and 1982). even in a study reserve, protected since 1969. It must be considered to be the result of a complex web of natural ". factors involved in a cyclic pattern (figure 10) which can be roughly outlined as follows : During periods of below average rainfall (the last TOTAL VEGETATI\.'~COVER drought for instance), plant resilience depends upon the thickness of the epipedon. For spots located directly above domed calcareous layer or above shallow gravelly materials, shortage of moisture is assumed tp NATURAL NON AFFECTED cause the collapse of plant communities. Moreover, the CYCLE ZONE meagre amount of organic matter makes these soils very UST1 ISANDMOVEMENT MIC"O·MOUIIIO / prone t~crusting, even under low rainfall (Valentin, DRY PERIOD" .,.. iITr4 ..~ 1981). Sealing is combined with a granulometric segre "'~j gation so that sand is easily removed then trapped by ~ remainding an of grasses, resulting in increase soil __ ...... I ------.i __ ...... I~ULI"'NAND WATER EROSiON I surface heterogeneity. Thus, climatic, plant, and soil ~ MIC" instance), the micro-mounds are eroded by the first ~ rains, covering bare spots with sand. As a result, I I ~ I SEVERE EROSION] seedling emergence is promoted and previously crusted ~ and barren areas are gradually recolonized. Such a Ill... 0,' ~'~ phenomenon was also described by Leprun (1979) in; ~ Malinese grasslands. SANDY FDUD CUIRASSlm FERLO lmL lm , " WIND EROSION effective rainfall, leading ~o a further plant degene In its early stages, wind erosion is not easy to ration. identify morphologically. Its insidious effects therefore must Animal husbandry: overgrazing due to errant cattle be detected by grain size analysis. As often recounted occurs rapidly since the injuried plant communities (Aubert & Maignien, 1948. Delwaulle, 1973. Bougere, 1979), permit only a very low carrying capacity. In addition trampling adversely affects soils of the Sahel belt. Yet this trampling by livestock converts their trails to chan study shows that fine sand movements do not occur most where nels for rill erosion. trampling is, the most intense, e.g. in the very vicinity of the watering places, but where still numerous cattle tracks Thus, within three decades, land abuse, coupled are coupled with an,extrel1\ely low values of organic matter with a frail environment has ruined the land inheritance for content, namely within the ring Km1-Km2. As a matter of fact, the foreseeable future. these results illustrate the value of cattle manure applica tion in light textured soils, as recently mentioned by Chandra and De (1982). CONCLUSIONS Heavy trampling and intense grazing around the WATEREROSION drilled-holes do cause land degradation in the Sahel belt. Yet Water erosion that occurs in the cuirassed Ferlo the magnitude of deterioration depends essentially upon the can be attributed to a complex interaction of several fac ecological surroundings. On one hand, the observations on the tors sandy Ferlo show that plant degeneration is not invariably Environmental vulnerable shallow data: soil texture, induced by animal husbandry and that regeneration can occur impervious long scanty soils, hillslopes, vegetation. surprisingly fast provided that rainfall returns to normal. Drought : decay of vegetative cover is triggered by In these sandy regions, land 'damage appears in the prevailing deplete water stores. Moreover, this process is encou form of wind erosion. Any change which disturbs the frail raged by a widespread surface sealing that reduces the balance of the cuirassed Ferlo (drought, enhancement of pasto ral pratices), leads, on the other hand, to much more J extensive havoc, left by accelerated water erosion. It must be conceded, however, that some processes, such as soil crusting, result much more from the combination of several natural factors than from any impact of animal husbandry, which is too often unjustly blamed. Moreover, some TABLE 2 pastural pratices which are still going on, such as limited CLASSES OF HEIGHT (m) TOTAL .....l .....l nomadism during the rainy season, must be considerered as a ~ '0.0-0.5 0.5-1.5 1.5-2.5 2.5 and above very interesting spontaneous answer to the problem of ~ Km8 128 127 160 140 555 overgrazing around the wells, and thus as a proper land-use. g Km6 133 67 67 73 340 El ~ Km1.5 83 53 7· .- ~ 7 150 Cl) RECOMMANDATIONS .... Cl Adequate measures to prevent-and to combat land degradation in the Southern fringe of the Sahara have been proposed repeatedly by scientists (Guilloteau, 1956. Adu, 1982). Some of these propositions cannot be over-emphasized and should be urged : 1. Asses the carrying-capacity of the range lands before any project including an enhancement of livestock population. Endeavc~r·to develop a region mustn't be attempted regar dless of the consequences on its environment. 2. Gain willing participation of the pastoral communities in projects. In contrast to a widespread prejudice, enmity towards progress is not anchored in nomadic tradition. 3. Establish tree-planted shelter belts around the drilled- . D· (;.', I{· holes, within the ring located between 1 and 2 kilometers REFERENCESCITED from the watering places, namely within the zones most 1. AUBERT, G. and MAIGNIEN ,R.. 1948. subjected to deflation. Various experiments in the Sahe L'erosion eolienne dans le Nord du Senegal et du lian countries have yielded evidence that afforestation Soudan fran~ais.Communication nO 103. 2eme sect. campaigns can be taken up by local populations. They have Groupe 1. Conf. Afric. des Sols. Goma : pp. 1309 to cope with the ever-increasing demand for firewood and 1316, 6 tabl., 9 ref. for building material. 4. Restore the land, if necessary, with the help of an expe 2. ADU, S.V .• 1982. rienced staff. Unfortunately, good will is not sufficient Desertification in West Africa. in : Desertifica and can even be dangerous (deep ploughing down-hill can tion and Soils Policy. Proc. of the 12th Intern. promote rill erosion for instance). Consequently training Congress of Soil Science. New Delhi : pp 26-43, should be implemented in order to spread the simple and 5 fig., 31 re£. efficient techniques of soil restoration beyond the limit of a small group of specialists. 3. BARRAL, H.. 1982. Le Ferlo des forages. Gestion ancienne et actuelle de l'espace pastoral. DGRST/ORSTOM, report, 85 p., 10 maps, 20 re£. 4. BECKMANN,G.G. and SMITH, K.J .. 1973. Micromorphological changes in surface soils follo wing wetting, drying and trampling. in : Soil microscopy. Proc. of the 4th Intern. Working Meeting . of Soil Microscopy. Kingston. pp. 832-845, 1 tabl., 4 fig., 12 ref. ).' 5. BILLE, J .C •. 1977. 10. BREMAN,H., CISSE, A.M., DJITEYE, M.A. and W.T.H. Etude de la production primaire nette d'un ~cosys ELBERSE. 1980. teme sah~lien.Trav. et Doe. ORSTOM, nO 65, 82 p., Pasture dynamics and forage availability in the 29 fig., 54 rH. Sahel. IsraiH Journ. of Botany, 28 : 227 -251, 2 tabl., 6 fig., 25 ref. 6. BOUDET, G.. 1972. Desertification de l'Afrique tropicale s~che. 11. CHANDRAS,S. and S.K. DE. 1982. Adansonia, s~r.2, 12 (4) : 505-524, 3 tabI., 8 fig., Effect of cattle manure on soil erosion by water. 16 rH. Soil Sci. 133 (4) : 228-231, 4 tabl., 13 ref. 7. BOUDET, G.. 1977. Desertificationou remont~ebiologique du Sahel. 12. GUILLOTEAU, J •. 1956. Cah. ORSTOM, s~r.BioI., 12 (4) .: 293-300, 1 tabI., Certain tendencies in soil, water and pasture con 5 fig., 6 rH. servation and in rural economy in Africa South of 8. BOUDET, G.. 1980. the Sahara. African Soils, 4 (1) : 30-64. Syst~mesde production d'~levageau S~n~gal: etude du couvert herbac~.l~recampagne. I.E.M.V.T. 13. DELWAULLE,J.C .. 1973. Maisons Alfort. 48 p., 19 tabl., 2 fig., 18 r~f. Desertification au Sud du Sahara. Bois et Forets 9. BOUGERE, J •• 1979. des Tropiques, 149 : 3-20, 12 plates, 8 ref. L'~tatde degradation des formations sableuses du Sahel voltaique ou l'urgence d'une intervention. 14. LAGOCKI, H.F.R .. 1978. Tr~·de l'lnstitut de G~ographiede Reims, 39-40 Surface soil stability as defined and controlled 91-101, 3 tabl., 4 fig., 5 plates, 4 r~f. by a drainage ~riterioh.in : Modification of Soi~ Structure. W.W. EMERSON, R.D. BON[and A.R. DEXTER, edit .• John WILEY & Sons. pp. 234-237, 2 tabl., 6 ref. 19 . POIJPON, H.. 1980 . 15. LEPRUN, J.C •• 1979. Structure et dynamique de la strate ligneuse d'une Etude de l'evolution d'un syst~mede proJuction steppe sahelienne au Nord du Senegal. Tr. Doe. sahelien au Mali. Volet pedologie. Rapport de cam et ORSTOM, nO 115, 351 p., 91 tabl., 46 218 ref. pagne 1979. ORSTOM, 20 p., 1 tabl., 7 fig., 18 pla- fig., tes, 9 ref. 20. STEBBING, E.P .. 1937. The forests of West Afr.ica and the Sahara : a study 16. MEYER, J.F .• 1980. of modern conditions. Chambers. London. Cited by Etude des syst~mesde production d'elevage au Sene- ADU (1982). gal. Volet zoo-economie. l~reannee. 28 p., 23 'i tabl., 1 fig. 21. THOMPSON,J. R.. 1968. Effect of grazing on infiltration in a Western 17. MOTT, J., BRIDGE, B.J. and W. ARNDT.. 1979. Soil seals in Tropical tall grass pastures of watershed. J. Soil and Water Cons., 23 : 63-65, Northern Australia. Austr. J. Soil Res. 30 : 483 1 tabl., 4 fig,., 5 re£.. 494,6 tabl., 3 fig., 42 ref. 22. VALENTIN, C.. 1981. Organisations pelliculaires superficielles de quel 18. PATTERSON, J.R., COLLIER, F.S., DUNDAS, J., BRYNMOR,J., ques 5015 de regions subdesertiques. Iynamique de AUBREVILLE, A., MATHEY, J., and R. BACHELIER. 1937. formation et consequences sur l'economie de l'eau. Rapport de mission foresti~reanglo-fran~~ise These 3~me cycle. Universite de Paris VII. 229 p., Nigeria-Niger (decembre, 1936 - £evrier 1937). in 22 tabl., 43 fig., 67 plates, 473 ref. Bois et For~tsdes Tropiques 148 : 3-Z6, 1 map, 12 plates. 23. VALENZA, J .. 1970. Etude dynamique de differents types de piturages naturels en Republique du Senegal. in : Proc. of. the 11th Intern. Congr. Grasslands, Surfers Paradise, Australia, p. 218. ':1 J 24. WISPELAEREde, G.. 1980a. List of tables, numbers and captions Les photographies aeriennes t6moins de la degrada tion du couvert ligneux dans un ecosysteme sahelien TABLE 1 Areal extent and average diameter of bare and senegalais. Influence de la proximite d'un forage. crusted spots in the zones undisturbed by trampling. in : L'arbre en Afrique tropicale:la fonction et le Comparison of the three study sites. signe. Cah. ORSTOM, ser. Sci. Hum., 17 (3-4) : 155 166, 2 tabl., 4 f1g., 8 plates, 12 ref. TABLE 2 Number of trees per hectare by late dry season measured at three different distances to the 25. WISPELAEREde, G.. 1980b. drilled-hole of REVANE. Systemes de production d'elevage au Senegal: Etude et cartographie de l'evolution de la vegetation par teledetection. Rapport de premiere annee. I.E.M.V.T. List of figures, ~umbersand captions Maisons-Alfort. 162 p., 23 fig., 19 ref. FIGURE 1 : General location of the study area. 26. ZONNEVELD,I.S .. 1980. FIGURE 2 : Rainfall and its variability,. in three towns Some consequences of the mutual relationship bordering the study area (after de Wispelaere, between climate and vegetation in the Sahel and 1980a). Sudan. I.T.C. Journal, 1980-2 : 255-296, 13 fig., 32 ref .. FIGURE 3 : Grain size distribution curves and sorting indexes of samples, (0-5 cm), from the zones undisturbed by trampling. FIGURE 4 Effect of trampling, denoted by the distance to the watering-place, on the rate A of deflation or accu FIGURE 9 Ruined grassland, km 2 from the water-hole of mulation of three granulometric fractions. REVANE. A (\) • Ca - Cb Ca • sandy FIGURE 10 cb with content in the Schematic natural cycle and soil surface featu top layer or micro-mound (\). res evolution related to the distance from the sandy and Cb • content in the 5 cm water-holes in the Ferlo in the cuiras directly underneath (\). sed Ferlo. When deflation occurs, A is <0 When accumulation occurs, A is >0. , FIGURE 5 Effect of trampli~g,denoted ~y the distance to the watering-place, on bulk density of the layer loca ted directly under the trampled and sandy layer. FIGURE 6 Effect of grazing, and of manure application around three drilled-holes on the organic matter content. FIGURE 7 Late dry season (May). Bare ~andysoil surface, km 2 from the water-hole of TATKI. ~.' ) , I I ; l: : "1 .; FIGURE 8 After the rainy season (November). Same place covered with palatable grasses. List of footnotes I.S.F~A. Institut Sen~galaisde Recherches Agricoles. O.R.A.N.A. Organisme de Recherches sur l'Alimenta tion et la Nutrition Africaines. O.C.C.G.E. Organisation de Coordination et de Coop~rationpour la lutte contre les Grandes Endcmies. G.E.R.D.A.T. Groupement d'Etudes et de Recherches pour le Developpement de l'Agriculture Tropicale. O.R.S.T.O.H. Office de la Recherche Scientifique et Technique Outre-Mer.