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The Influences of Ecological Factors on Tiger Bush and Dotted Bush Patterns Along a Gradient from Mali to Northern Burkina Faso

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The Influences of Ecological Factors on Tiger Bush and Dotted Bush Patterns Along a Gradient from Mali to Northern Burkina Faso CATENA ELSEVIER Catena 37 (J999) 25-44 The influences of ecological factors on tiger bush and dotted bush patterns along a gradient from Mali to northern Burkina Faso Jean Claude Lepron * ORSTOM. BP 5045. 34032 Monrpelher Cedex. France Recelved 5 June 1996; received ln revlsed fOTIn 18 December 1997; accepted 10 February 1998 Abstract Seven catenas across contracted vegetation patterns, including tiger bush and dotted bush. were studied in the Gourma region in Mali and in northern Burkina Faso (mean annual rainfall 200-550 mm). The tiger bush encompasses an alternate succession of bands oriented nearly perpendicularly to the wind direction NE-SW; sandy grassy microdune, sloped band of bare crusted ground and woody depressive band. A similar succession, without wind orientation. is found for the dotted bush. In this region, tiger bush develops under specifie conditions; (J) same orientation but opposite direction of wind and slope, (2) sedimentary and metamorphic rocks (sandstone and schist ironcrust), (3) shallow « 1 m) and Impervious soils. Under such circumstances, the dynamics of banded patterns result from alternate and successive action of water and wind erosion. Field evidences showed that vegetation bands migrate upwards with a velocity of 0.2-0.7 I m yr- . © 1999 Elsevier Science B.Y. Ail rights reserved. Keywords: Vegetation pattern; Catena; Mali; Burkina Faso 1. Introduction Clos-Arceduc CI 956) called 'brousse tigrée' (in English-tiger bush) the banded vegetation patterns he observed in Sahelian areas because the alternation of dark wooded bands and bare light bands was similar on aerial photographs to the fur of atiger. Since then, many other studies have been devoted to the tiger bush in AfTica, including Clos-Arceduc (1956), Audry and Rosseti (1962), Boaler and Hodge (1962), Gallais • Corresponding autbor. 0341-8162/99/$ - see front matter © 1999 ElseVIer SCIence B.V Ail rights reserved. PlI: S0341-8162(98)00054-X 26 J C. Leprllll / Catma 37 (1999) 25-44 (1963), Boulet et al. (1964), Heming (1970), White (1970), Wickens and Collier (1971), Janke (1976), Leprun (1978, 1979, 1983), Ambouta (1984). Updates of a former research (Leprun, 1992), as well as recent field studies in Mauritania (Gravier. 1993), in Burkina Faso (Serpantié et al., 1992; Couteron and Serpantié. 1995), in Niger (Galle and Peugeot, 1993; Seghieri et al., 1994) reinvigorated the debate on the origin of these patterns (Thiéry et al., 1995) and their main driving forces. In particular, most of these studies emphasised the role of overland flow in the development and maintenance of vegetated bands perpendicular to the slope. The objectives of this work were: (1) to estimate the respective impact of wind and topography upon the occurrence of tiger bush patterns, (2) to assess the upward migration of the vegetation bands. 4° 3° 2° 1° 00 50 mm 1° 50 mm 100 mm 300 mm 100 mm 16 ° 300 mm 500 mm 15 ° 750 mm --- 14 °500 mm ---- MOSSI -.... BASEMENT - ~ 13° 750mm 0 100 km 1G 2 ~ 3 ITIIJJ] 4D 5 L:'2J 6~ 7~ 8 0 90 10. Fig 1. Location of the ,tudled catenas and geological map. Precambrian: (j) basement; (2) Bourré granIte PaleozOlC: (3) Firgoun sandstone: (4) Labezanga series (Shlst); (5) Ydouban group (,.hist): (6) Hombon­ Douentza formatIon (sand,tone). (7) Bandtagara-Knutlala group bandstone) CenozOlc: (8) ContInental termInai (sandstone). Quaternary (9) flUVIal deposits. (J 0) TLger bush patterns. (J Il Hombon-Goundam thre,hold: (\2) lsohyet'. (l3) studied catenas (1-2) GOSII and GOS III: (3) ANS; (4) NDA: (5) KER: (6-7) ZAM land ZAM II: (8) TEN) Table 1 Physicochemlcal charactenstlcs of the surface layer of the soils of the studled catenas Clays Sllt Sand pH Organic N (%) P(%} S (m.e/lOO g) CEC (> 2 JLm) (2-50 JLm) (50-2000 JLm) matter (%) (m.e/lOO g) Microdune GOS 20 1.4 2.4 96.1 6.8 0.1 0.18 0.16 0.86 0.10 GOS 34 2.7 4.0 92.3 7.6 0.2 0.22 0.27 1.52 1.73 ANS4 5.1 3.6 90.7 7.8 1.0 0.33 0.20 2.59 9.70 NDA3 12.4 21.3 67.8 6.5 0.7 0.56 0.22 3.33 6.61 ~ KER 2 13.8 9.9 75.4 4.9 0.6 0.38 0.23 0.64 5.82 !"l TEN 3 6.1 lU 77.4 6.7 1.6 0.69 0.21 3.01 6.10 ~ ... § Bare ground band '-.. GOS 24 19.7 31.9 47.0 6.7 0.1 0.41 0.25 3.82 404 ~ rt GOS 32 21.4 38.7 38.3 7.2 0.6 0.58 0.51 5.85 4.98 ;:, :;, ANS 2 5.8 30.3 62.7 7.3 l.l 0.52 0.60 3.65 4.10 'l<..., NDA2 31.6 31.2 36.5 7.2 1.0 0.82 0.76 15.42 15.47 ~ '0...... KER 3 23.5 13.4 63.2 5.0 0.5 0.39 0.31 1.63 6.92 '0 ZAM2 9.9 15.9 69.4 5.2 0.5 0.26 0.37 0.83 1.03 ~ w TEN2 32.6 25.7 43.4 5.6 2.0 0.67 0.40 3.40 15.30 v, ~1 ~ Wooded band GOS 29 34.5 40.7 21.7 6.2 l.l 1.25 0.47 4.45 8.82 GOS 36 21.3 18.7 59.0 6.1 3.0 1.61 0.66 7.28 15.30 ANS3 8.6 20.1 71.9 7.6 1.5 0.60 0.51 4.75 7.60 NDA 1 23.3 29.2 46.4 7.6 2.9 1.88 0.60 14.93 1662 KER 4 35.2 13.2 51.0 5.0 0.8 0.63 0.49 2.50 1338 ZAM4 23.5 18.1 55.1 6.9 1.2 0.82 0.60 4.00 816 TEN 1 24.3 30.0 32.4 5.5 4.8 1.77 0.52 5.01 133 hl ..... 1" 00 Table 2 Main characteristic, of the selected catenas Name and location Lalltudejlongllude" MARb Bedrock DIrection' Helght (m) and Distancee Plt,f (mm) slope (%)Ù (m) GOS II 17 ,5 km from 15°49'00"N, 1°17' 10"W 200 Precambrian argI1laceous schist NE-SW 1.3 m, 2.1 '7e 60 16 Gossi to Gao of Ydouban (Reichelt, 1972) GOS III 2.6 km from 15°16'10"N.l o I0'40"W 200 Precambrian fine slhceous and NNE-SSW 4.05 m, 5.2% 78 10 GOSSI to Hombon, ferrugInous schi,t of Ydouban ANS 110 km from 15°53'30"N,1 0 35'15"W 225 lroncrust developed on Tertlary NNW-SSE 2.25 m, 1 3% 171 4 "'"0 Amsongo to Menaka argillaceous sandstone fonnations of ~ .., Continental tenninal sandstone § NDA 43 km from 15°34'40" N, 1°05' 00" W 300 Precambnan argillaceous schist ENE-WSW 1.5 m, 1.90/0 78 4 "- GOSSI. to N' Dakl. of Ydouban n KER near Kerano, 14°58'40"N,2°39'30"W 450 ironcrust on micaceous schlst NNE-SSW 1.65 m, 0.9'7c 185 4 "~ south of the Hombori- of Ydouban "'JlA; Douentza trail ~'- 'C ZAM 1 II km from 14°46'20"N,0030'10"W 450 ironcrust on TertIary argillaceous NNE-SSW 0.6 m, 0.8% 70 4 'C Oursi sandstone of Continental tenninal ~ N ZAM II II km from 14°46'20"N,0030'10"W 450 lroncrust on Tertiary argillaceous NNE-SSW 0.85 m, 0.6'70 140 5 v, 1 4:< OurSI sandstone of ContInental termInaI 4:< TEN 15°25' 30" N 2°36' 00" W 550 lron-crusted CT sandstone NNW-SSE 1.35 m, 1.3% 250 4 'Geographlc coordinates, latitude and longitude. b Mean annual raInfal1. c Dlfection of the catena. dHeight (m) and slope (%) between the highest and the lowest pomt of the catena. c Di,tance between two successive wooded bands. t Number of pedological pits. i.e. Leprun / Carena 37 (1999) 25-44 29 2. Material and methods 2.1. The study area The study area (Fig. l) ranges from lat 16°N in the Gourma region in Mali with 200 mm annual rainfal1, to Jat 14°30'N with 500 mm in the north of Burkina Faso. Scarce but heavy showers occur from July to October. The average temperature is 28°C in the north and 31°C in the south, and the average minima are 12°C in the north and l7°C in the south. The bedrock in Burkina Faso consists of the granito-gneissic Precambrian sh.ield in Burkina Faso, and shale and schist in the Gourma region. The shield is overlaid by tertiary argillaceaous sandstone formations referred to as Continental terminal and by a succession of sandy dunes, from 40000 years BP (Inchirian and Ogolian) to nowa­ days. 2.2. Field studies and laboratory analyses Eight catenas have been selected along the c1imatic gradient (Fig. l). Their study included topographie measurements using a topometer (aeeuraey 1 mm), vegetation A•, ABC DE F BAREGROUND WOODED ... MICRODUNE .. .. BAND .. .. BAND .... .. .. S.W. N.E. "'-WIND DIRECTION 10'm . 4 [ill]......~ .. Fig. 2. Cross section of tbe catena GOS II. (1) Sand, (2) sandy loam, (3) silty clay, (4) ferruginous nodules, (5) schist, (6) soil profile. 30 i.e. Leprun/ Carena 37 (1999) 25-44 (structure, species and sociability) and soil (description of profiles).
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