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GCTE -, 2t8i - ss formations de type "brousse tirrée" des régions arides et semi-arides. Sponsored by: ORSTOM I'lnstitut Fronyis & Recherche Scientifique pour le Développement en (oopérotion CNRS-Programme Environnement

Banded vegetation patterning in arid and semi-arid environment

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Text of Banded vegetation patterning in arid and semi-arid environment

  • GCTE -, 2t8i -

    ss formations de type "brousse tirre" des rgions arides et semi-arides.

    S p o n s o r e d b y :

    ORSTOM I'lnstitut Fronyis & Recherche Scientifique pour le Dveloppement en (ooprotion

    CNRS-Programme Environnement

  • Concepfin moquette, rahotion N. Zerbib - ORSTOM Bondy

  • Distribution, structure, origine et devenir des brousses dites tigres en Mauritanie sahlo-saharienne

    Rossetti Ch.

    2- Existing theones A geomorphological based "Tiger Pattern" generated by a former dune pattern in Sokoto Nothern Nigeria

    Zonneveld 1.

    c methods of study 1- Pattern at a regiorial scale

    Vegetation pattern recognition using SPOT images at regional scale in Niger

    Mougenot B., d'Herbs ]. M., Hamani S.

    2- Functioning at local scale Patterning and pioneer processes of the herbaceous front in a tiger bush

    Ehnnann M., alle S., Seghieri].

    sses and models 1- Runoff and erosion processes

    Surface crusts of the tiger bush Valentin C , Bellier ., Malam 1.. Rajot1.L.. Pichet].

    The importance of surface water redistribution for the water budget of banded vegetation structures (Spain)

    Bergkamp ., Cerda A., lmeson A., longejans].

    The soi1 surface features of the vegetation striped patterns in Mapimi reserve. North Mexico and their influence on the hydrodynamic

    ]aneau 1.1.. Mauchamp A., Tarin .

    Impact of water harvesting variations along a climatic transect in Niger upon produciivity and paems of tiger bush

    Valentin C, d'Herbs].M.

  • The distribution of soluble cations within a patterned ground gilgai complex. Western New South Wales, Australia

    MacDonald B.C.T, Melville M.D., White L.

    Erosion processes and polyphase mosaic components in a banded grassland. and Western New South Wales. Australia

    Dunkerley D., Brown K.

    Soil water balance Water balance in a banded vegetation pattern: the case of the tiger bush in Western Niger

    Galle S. Ehmann M., Peugeot C. -

    Soil and soi1 water balance in the watenhed of the riier Corouol - a case study in the Sahel of Burkina Faso

    Kappas M.

    Rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger

    Bromleyj., Edmunds WM.. Fellman E., Brouwer]., Gaze S.R., Sudlow j., Taupin j.D.

    Soil and hydrologic characteristics of a banded vegetation site in Niger and their effect on tree establishment

    Zanguina 1.. Thurow I L . , juo A.S.R., Manu A.

    Soil biotu Soil fauna and process regulation in patterned landscapes of serni-arid Australia

    Noble ]. C.

    Tenites-soil-vegetation interactions in a striped vegetation pattern (Burkina Faso)

    Ouedraogo P. iepage M.

    4- Populations dynamics Cornparison of spatial patterns of woody species in a spotted savanna and a tiger bush of Northern Yatenga (Burkina Faso, West Africa)

    Couteron P

  • Runoff contribution to a sahelian two-phase mosaic system: soi1 water regime and vegetation life cycles

    Seghieri 1.. alle S.

    5- Syntheiic models of fundioriiiig Stripes, strands or stipples: modelling the influence of three banding patterns on resource capture and productivity in semi-and woodlands, Australia

    Ludwig 1.. Marsden S.

    'Brousses tigres": an example of pattern formation in systems with non local interactions.

    Lejeune O., Lefeue, R.

    6 - Genesis of banding geneml niodeis Soil organic matter dynamic on Tiger Bush in Niger

    uillaume K.. Abbadie L.

    Using 137Cs to trace upslope migration of banded vegetation in South-west Niger.

    Chappe11 A., Valentin C, Warren A., Charlton M. d'Herbs ]. M.

    How do vegetated arcs move upward in Niger ? Mougenot, B., d'Herbs, ].M.. Ichaou, A.

    Banded vegetation patterning in a subantartic forest of Tierra del Fuego, as an outcome of the interaction between wind and tree growth.

    Puigdefabregas]., aliart E, Bianciotto O., Allogia M., Del Barrio .

    sequences for Management 1 - Productivity of patterned vegetation

    Does patchiness increase vegetation productivity, biodiversity and stability ? The case of the 'brousse tigre" in the Sahel

    Hiemaux FI

    2- Towards an iniproved land nianagenieni in the and and seni ind zones The evolution of soil-vegetation patterns following land abandonment and fire in Spain

    Carnmeraat L. H.. lmeson A.C.

  • Tiger bush that lost its stripes: the effect of grazing on the spatial pattern of herbage production in a banded semi-arid woodland in Eastern Australia

    Freundenberger D., Palmer R.

    Vegetation arcs and litter dams: similarities and differences Eddy]., Humphreys .S., Hart D.M., Mitchell PB., Fanning P C.

    Relationshps between rainfall. soil-water, and herbage production in three geomorphic zones in semi-arid wooded rangelands

    reene R.S.B., Noble 1. C.

  • des brousses dites tigres

    ette note n'a pas pour objectif de reproduire in extenso des observations fort longues effectues en juin-juillet 94 en cours d'exploitation. mais de fournir une orientation gnrale sur ces observions qui avec d'autres observations portant sur une trentaine d'annes doivent

    illustrer certains aspects quelque peu controverss sur l'volution de la vgtation en Afrique du NW suite aux grandes scheresses.

    *:* VUE GNRALI S U R LA QUESTION DU PCllNT DE V l l E DE LA PHYTOECOLOGIE

    J'ai eu connaissance de l'existence de cette structure remarquable de la vgtation par des collgues australiens en 1957 lors d'exploration dans la rgion d'Alice Springs en Territoire du Nord. L'quipe du CSIRO (Land Research Division) effectuait alors des relevs d'inventaires globaux par la mthode combinant une orientation morphologique combine des relevs de vgtation, partir de couvertures ariennes l'chelle du 150 000me. C'est sur la photographie arienne qu'a t, sauf mieux inform, dcouverte la structure particulire en bandes de physionomies mixtes. essentiellement arbustives en Australie (Acacia aneura. sauf erreur), puis mixtes en Afrique de l'Est. et enfin, par la cartographie au 1 :200 000me de I'IGN, en Afrique du NW.

    D'un double point de vue de la phytogographie, et une autre chelle de travail. de la phytocologie, cette disposition de la vgtation offre un exemple fort intressant comme indicateur du milieu. Sur le plan climatique certains types de climat favorisent-ils cette disposition? Sur

    * Ingnieur agronome EPF PART I

  • R BUSH AND RELATED STRUCTURES

    un plan cologique, quelles conditions de station permettent-elles cette constitution. Enfin se pose la question de savoir comment de telles dispositions peuvent-elles apparatre au moment de leur constitution, quelle est leur stabilit et quels facteurs peuvent modifier cette constitution. Accessoirement la phytocologie s'intressera la composition botanique de ces vgtations. mais ce n'est pas l. mon avis, un problme crucial, car en l'occurrence c'est la structure qui est cause d'interrogations. non la floristique, trop frquemment exhaustive d'tudes phytocologiques.

    *:* LA SITUATION EN MAURITANIE D U CENTRE NORD ET DU NE

    Notre proccupation en 1959/61. il y a plus de trente ans, n'tait nullement d'tudier en soi les BT. La prospection cologique engage par la FA0 avait pour objet de rechercher en zone saharienne et sahlo saharienne les structures de vgtation contrastes que l'on supposait alors favorables la localisation pour la reproduction des essaims du criquet plerin (phase grgaire). Depuis la dfinition de ces zones par les travaux de POPOV s'est considrablement affine. et le travail effectu sur les BT n'a. d'un point de vue acridien, qu'un intrt tout fait secondaire..Tel n'tait point alors mon avis lors de la prospection effectue avec P. AUDRY, pdologue de I'ORSTOM dtach au projet FA0 et avec qui un dbut d'investigation d'un chantillon de BT fut ralis sur le plateau de Nma en Mauritanie de l'est.

    Avant d'examiner la structure caractristique, car plus ou moins linaire. en bandes, ou en "arcs" selon la terminologie anglo-saxonne, tchons, tout au moins pour l'Afrique de NW de visualiser les zones o se produit ce phnomne.

    C'est vraisemblablement CLOS ARCEDUC, ingnieur gographe de I'ICN qui l'a, le premier, signal en zone sahlienne a tent de lui donner une interprtation. Dans les actes de la Confrence de Toulouse de 1964 (op. cit 1968) il rvle ce que l'accs libre la photothque (et du temps disponible !) lui permettaient de dceler : des images caractristiques en dessein de pelage de tigre (d'o la dnomination) au Mali (entre 7 et 5" W. 15 et 16"N), dans le sud du pays Dogon toujours au Mali (3"W et O" et en charpe entre les latitudes 14 et 16"N). enfin dans une zone d'extension considrable au Niger entre I " et 4" E et au del ; puis en charpe entre 12 et 16" de lat. N). II en avait fait mention prcdemment dans une brve note publie dans un bulletin de I'IFAN en 1956 que m'avait signale Th. MONOD. mais

  • TlGER BUSH AND RELATED STRUC

    dont je n'ai jamais dispos. C'est vraisemblablement cette occasion que cet observateur averti de la photographie arienne a propos le nom de brousse tigre dnomination qui fut reprise par les cartographes de I'lGN interprtant les photographies en termes d'images, non en termes cologiques. Je montrerai pourquoi je fais cette rserve..

    Nulle mention n'est faite de la prsence de BT en Mauritanie. En fait on les trouve en discontinuit sur des plateaux dans la rgion de Bomdeit entre II "55'et II "30'W et 17O35'N. ceci quelques minutes sexagsimales prs. parfaitement portes sur la carte au 1 :200 000me) comme sur les feuilles de Oualata et Nma. Sur la premire avec les extensions nord extrmes dans cette rgion 17" IO'par lambeaux entre 7" IO'et 7" W et sur celle de Nma .Le plateau de Nma prsente un cas intressant car les BT figurant sur la carte IGN et trs bien reprs (des prospections extebsives l'ont confirm se prsentent en blocs disjoints entre les lastitudes i 6O28' et 17" puis lgrement l'est de la longitude 7" ouest et 7'20' aproximativement. la distribution de ces blocs par rapport au support est dmontre dans la photographie arienne reproduite ici ( 1 :50 000me).

    Je reviendrai en dtail sur la distribution des BT sur le plateau de Nma dans le corps de cette note.

    *:* ~ ~ N S I D R A T I O N G E M E R A L E S SUR LES S T P U t T l I R E S GEOMETRIQUEMENT SIMPLES D E LA VEGETATION

    L'existence de structures* linaires pose un problme gnral d'interprtation qui se traduit dans la formulation d'une indication cologique de tels types de vgtation.

    II est quelque peu regrettable que la photographie arienne ait souvent servi, non comme instrument de diagnostic partir duquel une explication doit tre recherche sur le terrain. mais comme substitut l'enqute de terrain. C'est par ce court circuit que des raisonnement spcieux reposant sur I'arte fact de l'image peuvent tre faits, et c'est dans ce pige qu'est tomb CLOS ARCEDUC.

    Le cas le plus gnral de la linarit est la structure discontinue. en mosaques, par rapport la structure diffuse. Au niveau de la rpartition des espces ce phnomne a t, je crois, le mieux tudi par l'cole de

    *le rappelle que dans la description de la vgtation. le terme de structure indique la fois les caractristiques de PART I distribution horizontales comme verticales de celle-ci. hidemment les images photographiques ariennes petite ,-MW- chelle prises en considration dans les tudes de vgtations qui ne justifient pas des prises de vues spciales @@%@ des chelles plus grandes ne rvlent que la structure horizontale, c2st dire le mode de rpartition en surface. lg#%$, .

  • BUSH AND RELATED STRUCTURES

    Montpellier (CEPE), mais pour des vgtations relativement fermes (lments + - jointifs). Dans le cas des vgtations sub tropicales sches, ce caractre jointif est plutt rare ou alorsseulement saisonnie, comme pour les prairies phmres. Ds que la pluviosit devient le facteur cologique principal qui se traduit par la disette en eau et que la concurrence pour I'eau devient le facteur cologique dominant la discontinuit est de rgle, mais pas toujours, et de loin. la discontinuit de type linaire.

    MONOD a voulu distinguer rpartitions diffuse et contracte supposant que cette dernire tait caractristique des vgtations sahariennes. II n'en est rien. La galerie forestire, la lette derrire la dune de protection en Aquitaine sont des structures linaires. mais n'ont videmment rien de saharien. Le facteur qui commande la linarit au Sahel et au Sahara est celui de l'approvisionnement en eau : soit par le relief, soit par le type de texture superficielle du sol. Le ruissellement provoque la discontinuit. le couvert sableux le caractre diffus de la rpartition, avec nuances d'ailleurs comme le montre la colonisation d'espace sableux dans des stades initiaux de colonisation.. D'autres facteurs peuvent provoquer la discontinuit comme la prsence de termitires, la formation de crotes (la battance des sables argileux, ou bien .la concurrence pour I'eau et parfois des caractres de croissance comme les ocelles du Spinifex en Australie du Nord).. On ne peut se dispenser d'aller voir sur le terrain d'un il critique l'image produite par la photographie arienne dont l'avantage consiste ceopendant amplifier considrablement les caractres de rpartition et ainsi de les rendre plus visibles. CLOS ARCEDUC a mis en relation des brousses tigres au Niger sans rapport cologique les unes avec les autres dduisant tort qu'elles drivaient les unes des autres alors qu'elles traduisaient, dans les exemples choisis. des phnomnes stationnels diffrents. Cependant ce travail n'a maintenant plus qu'un intrt historique. et aucune porte pratique, notamment pas pour claircir certaines questions quant la dsertification dont j'aurai parler en fin de note.

    Pour ma part je suis arriv la conclusion gnrale que l'existence d'une gomtrie linaire. ou bien d'une distribution moins gomtrique comme la formation d'une mosaque mono ou polyspcifique. mais avec nette dominance ponctuelle d'une seule espce, indique avec. de fortes prsomptions. que la vgtation qui prsente ces types de structures horizontales est en passe de colonisation d'un espace prcdemment nu, que cette phase de colonisation peut se stabiliser temporairement mais durer un temps trs long (plusieurs dcennies), ceci en raison de phnomnes de concurrence interne aux groupements pionniers qui

    PART l "BE,; y++ ..yQ.;: $$&$ " ;;:$3&,i*3

  • TlGER BUSH AND RELATED STRU

    colonisent l'espace ou bien par la perptuation d'une instabilit des conditions stationnelles. Pour moi le phnomne intressant est celui de la dynamique stationnelle e t non une instabilit zonale qui de surcrot ne tient pas compte d'un autre groupe de facteurs : l'ensemble des perturbations biotiques. Les BT appartiennent ce type de vgtation instable.

    Ainsi l'insertion des BT dans le cadre gnral de la zonation nord sud de la vgtation en Afrique du N W se situe deuxniveaux : l'un bien sr dans le cadre de la disposition du gradient pluviomtirique (en quelque sorte la clef de la partition), mais, infiniment plus important, dans la ou les mlodies travers la diversit de conditions stationnelles dans un cadre cependant dfini. C'est par l'inversion du jeu de ces dterminantes que se situe l'erreur de CLOS ARCEDUC et d'ailleurs. dans une moindre mesure. de MONOD.

    l'ajouterai en guise de remarque gnrale. qui aurait pu constituer une conclusion mthodologique. ceci. L'cole australienne o j'ai reu ma formation majeure ne comment pas l'erreur, comme les phytogographes et l'ancienne cole de la phytosociologie europenne, de regarder la vgtation avec des yeux couverts de lunettes filtrantes dont le filtre est telle ou telle thorie. Elle la regarde comme un fait local qu'il convient de comprendre par l'tude des autres faits stationnels qui la conditionne. Elle fait ainsi progresser les tudes du particulier vers le gnral et non l'inverse, source de graves aberrations et de conclusions qui ne sont finalement que des vues de l'esprit. C'est pour cette raison qu' ma connaissance c'est l'cole australienne qui la premire a vu juste dans la reconnaissance du facteur responsable pour la formation des BT. AUDRY et moi-mme n'avons pas fait preuve d'imagination dbordante en reprenant notre compte leur hypothse. simplement l'affinant, l'illustrant et la mesurant dans une station donne sur le plateau de Nma dont je vais rappeler, parce qu'ils n'ont jamais t publis, les principales conclusions.

    Si l'on regarde les BT avec l'ide qu'il s'agit, non de groupements colonisateurs. mais de groupements climaciques. ou bien, exprim en d'autres termes. comme des lment du paysage au mme titre qu'un guelb. un reg ou mme un sol, on passe ct d'une ralit changeante. fonction de deux facteurs : majeurs :

    J le macro climat par son induction de conditions de morphologie superficielle stationnelle (dynamique du ruissellement et remaniements oliens, comme je le montrerai)

    PART l :v,?v*> vi#$d&$$E $9 gg& *xi ;&

  • BUSH AND RELATED STRUCTURES

    J.conditions de dynamique interne au groupement vgtal li principalement des phnomnes de concurrence entre dominantes ponctuelles, dfinition que je donnerai.

    +$ LA STATION D U PLATEAU DE NMA (TAT 1961)~

    Je disposais cette poque non seulement d'une facult de dcision quant au choix de stations tudier, mais galement de moyens exceptionnels sous forme notamment d'un dispositif de photographie arienne trs grande chelle**. de moyens aussi pour localiser les observations par point astronomique. Le rsultat en furent les documents reproduits ici aujourd'hui pour la premire fois publis et devenus introuvables.

    Les lgendes des diverses figures, mais surtout celles-ci illustrent mieux que ne pourrait le faire un commentaire les principales conclusions :

    J les bandes sont disposes 90" environ de l'orientation majeure du sens de ruissellement sur des pentes trs faibles semi horizontales et dans les ruptures de pente o il y a infiltration localise de l'eau de ruissellement

    J elles sont disposes en arcs, ce qui n'est pas expliqu et montrent une structuration horizontale caractristique. la fois en gomtrie et en rpartition d'espces localement dominantes et exclusives de tout autre appeles dominantes ponctuelles. La squence de dans la structure de la bande illustre la zonation climacique de la rpartition de ces espces dans le gradient gnral.

    J la distance interbandes est suppose non reflter. comme l'estime CLOS ARCEDUC une volution d'un systme de bandes serres des bandes plus espaces, fonction du vieillissement des systmes ou de leur situatiation dans la zonation climatique. mais un phnomne trs localis de gradient de pentes plus ou moins fort, ou bien des ruptures de pentes plus ou moins espaces. Ceci est une supposition logique dcoulant de l'observation sur une seule bande. car le prouver et demand un travail majeur de recherche qui n'tait pas l'objet de l'exploration d'alors.

    J les bandes montrent une structure verticale parfaitement illustrs par le schma ci-joint.

    PART , voir bibliographie 2qwg,, " dcrit dans la note prsenfie en bibliographie F$$f$?~>*;E .r..- *,

  • TlGER BUSH AND RELATED S'[RU

    *:* LA MME STATION TRENTE A N S D'CART

    L'orchestration gnralise autour de la pseudo dsertification partir de vues de l'esprit en l'absence d'observations srieuses de terrain m'a ce point choqu que j'ai dcid en 1988 de financer moi-mme une srie de trois voyages destins tablir la VRIT (que certains dsignent par le terme quelque peu ridicule de vrit au sol. le "ground truth" amricain comme si de vrits il y avait de deux catgories) par rapport l'tat en gnral fort bien document, et en tout cas parfaitement localis de stations photographies par ballon et au sol et quantifies. ce qui est primordial par rapport aux observations qualitatives inchiffrables et non comparables*.

    II n'est pas possible ici de prsenter I'ensemble des rsultats de deux de ces missions (l'une ayant t un chec personnel), mais il faut en limiter la porte pour une bonne raison : ce genre de travail quantifi requiert des moyens assez considrables qui ne sont pas la porte d'un priv. Et compte tenu de l'irresponsable gaspillage international et national de fonds opr par l'orchestration tonalit apocalyptique engendr par les Nations Unies dans le domaine de la pseudo dsrtifiction. concept flou. voire spcieux, il n'est plus aujourd'hui possible de justifier de telles tudes au niveau d'administrations constipes par l'excs antrieur de dpenses ensables dans des tudes sans objet prcis. sans rfrence aux observations srieuses passes et non archives, etc. etc. Je n'aurais pas de mots assez crus pour stigmatiser l'irresponsabilit d'une agence des Nations Unies dans ce domaine : I'UNEP en principe charge de coordonner l'ensemble des activits dans ce domaine, et ceci doit tre dit et connu car le gaspillag de fonds devenus rares se poursuit impunment, et non seulement I'UNEP, mais en France, par I'OSS.

    Donc en man avril 1988. dot de moyens qui n'taient autres que ma mmoire et mes pieds, je pus retourner sur le site de Nma. mais je ne pus retrouver le point prcis d'observation de i 961. Cependant ma grande surprise et en bordure du plateau, et non loin du site en question la brousse tigre avait disparu au profit d'une colonisation diffuse par du Commiphora africana sous fonne d'arbustes vigoureux et jeunes. L'tat des lieux qui avait chang de fond en comble tait la couverture par une mince pellicule de sable du reg gravillonnaire antrieur. Apparemment il ne subsistait pas grand chose des bandes antrieures.

    cette tendance de la phyto-cologie franaise se limiter aux obsenibations qualitatives a heureusement t PART , contre par l'cole de Montpellier notamment sous l'impulsion de LON qui a introduit les mthodes anglo [email protected]%$ saxonnes dans la description quantifie de la vgtation &$?* $:j ggF&g r.~..... 2~::. .?::

  • BUSH AND RELATED STRUCTURES

    Quelques photos furent prises. mais les observations considres insuffisamment documentes pour faire I'objet d'une note crite.

    Ce n'est qu'en juin juillet 1994 que je pus mettre en uvre des moyens corrects mais personnels, en matire de dplacement et de navigation pour retourner sur les lieux et constater sur le site ponctuel retrouv (grce au point astronomique antrieur relocalis par CPS) que je pus constater et documenter :

    J la bande avait effectivement disparu sous sa forme originale par la totale mortalit des espces annuelle et vivace du front de bande, et que la bande tait encore reprable par des vestiges de rewia bicolor et quelques Boscia senegalemis. les premiers sec (et peut-tre dfinitivement morts, car les pluies taient tombes et aucun reverdissement ne put tre constat), le second actif.

    J les interbandes en bordure du plateau et dans un moindre degr vers le nord est taient effectivement, mais partiellement colonises par la fois du C. africana, mais aussi par des plages denses dlEuphorbia balsamifera, lment prcdemment peu prsent,. distribues au hasard des ensablements superficiels.

    J je dcidai alors de consacrer du temps et des moyens investiguer le phnomne. Une no bande issue d'un ensablement fut tudie en dtail, photographie au sol et marque comme tmoin.

    J la conclusion principale est que les bandes se reconstituent, mais cette fois que le facteur dterminant est le vent porteur de sable qui cre des buttes d'ensemencement par les gramines. Celles-ci deviennent les germes en quelque sorte de nouvelles bandes. Ainsi je dispose actuellement d'un faisceau de prsomptions quant l'origine misemblable de ces bandes qui, dans un second temps, crent les diffrences de micro relief observes antrieurement ceci sur des surfaces sub horizontales.

    *:* CONCLUSIONS

    L'ensemble des observations quantifies recueillies en 1994 doivent faire l'objet d'une publication gnrale sur les transformations de la vgtation depuis le priode des scheresses de 73 et au del. Pour tre

    PART l

  • TlGER BUSH AND RELATED STRUCT

    pleinement convaincantes ces observations devraient tre confortes par des photographies ariennes diverses chelles.

    Ce que l'on peut dire c'est que globalement cet exemple de I'volution des BT montre que d'un point de vue de la perspective errone de la DSERTIFICATION, FAUX CONCEPT, il n'y a pas lieu de lancer des programmes gnraliss sur l'avance prtendue du phnomne sans se rfrer des tudes ponctuelles rfrences par des observations prcises sur l'tat pass, dans la mesure o celle-ci ne sont pas seulement des descriptions plus ou moins lyriques de cet tat. mais sont quantifies.

    L'ensablement loin d'tre un facteur ngatif peut trs bien remobiliser l'eau de surface pour permettre la colonisation spontane d'espaces prcdemment nus, et ainsi remodeler le payssage de faon positive, c'est dire dans le sens d'une plus gnralise colonisation par la vgtation. Ceci est un phnomnbe d'anti dsertification dont pesonne ne veut entndre parler. Et pour cause: la convention internationale de lutte contre la dsertification ne veuzt regarder, pour des raison politiques et non scientifiques, que les aspceets ngatifs.

    + B I B L I O G R A P H I E AUDRY P. ROSSETTI, Ch. (1962)- Observations sur les sols et la vgtation en Mauritanie du sud-est et sur la bordure adjacente du Mali (1959-1961) rono 267 pages FA0 Rome

    CLOS-ARCEDUC. A. (1956)- tude sur la photographies ariennes d'une formation vgtale sahlienne. Bull IFAN XVIII, srie A. no 3, 677-684

    CLOS-ARCEDUC. A. (1 968)- La gomtrie des associations vgtales en zone aride Exploration arienne et tudes intgres, Actes de la Conf. de Toulouse p 41 9-421

    ROSSETTI, Ch. (1996)- Rflexions sur l'utilisation des photographies ariennes pour l'tude du couvert vgtal. Actes du Ile Symposium international de photo-interprtation Paris 1966 IV p 63-67

    PART l

  • R BUSH AND RELATED STRUCTURES i;*--. ,-.Y,,%+ .>. .$ ,,,, ,

  • ased "Tiger pattern" generated m in Sokoto N o d t e m Njgeria

    *

    clear banded vegetation pattern can b aerial photogtaphs of the Northern part of Sokoto province in northern Nigeria. Becaus resemblance we called it after the th 1963164 'Tiger pattern" (FAO, 1966 1971, Sombroek G Zonneveld 1972)

    this pattern appean to be quite different from a observed in Rep Niger and described by White 1971 "Bmusse tigre". See also Thiery, d'Herbes and Vale summarize the genesis as moving vegetation arcs u sheet-run-off. We know banded patterns in wadi-fl

    the latter ones. Chatacteristic are moving vegetate on more silty subsoils.

    The banded pattern in Sokoto province however i caused by differences in surface hydrology due to variation in soi1 surface sealing that is connected with a by sheet erosion (pediplanisation) levelled former early Holocene dune landscape (Sangiwa covenand landscape). The difference in sealing is caused by a very small difference in si1 levelled former dunes and the filled in valleys.

    The lower silt content in the valley filling is due to the resedimentation where, by water (and wind) action , the last remnants of silt that were still present during the Sangiwa period are removed, a feature well known fmm elswhere in cover sand formation.

    ' LTC. Enschede, the Netherlands PART l

  • ING THEORIES

    The permeable soi1 of the former valleys bear a dense shrub. the stronger sealed remnants of the dunes show an open vegetation. Even relative big trees take part in, by the same process. filled in riven. with a vegetation that however is often disturbed by man and converted in grassland or temporary arable fields.

    Ploughing in the sealed parts has no results because immediately after a rain shower a crust is formed.

    The difference in genesis is clear from details of thi pattern. The banded pattern of the Sangiwa covenands shows up as a (pseudo-dune pattern with dunes and valleys perpendicular to the prevalling wind during the formation(about east-west).The so called "Brousse Tigre" in valley bottoms shows bands that are perpendicular to the run-off direction that depends on the general relief of the area. In modelling the fint mainly aeolic forces of the past should be simulated, modelling the last one is a matter of run-off1 run-on simulation.

    PART l

  • PATTERN AT A REGIONAL

    anding patterns and associated vegetation structures appear readily on aerial photographs. Satellite data provide new methods to detect and map vegetation patterns at local and regional scales. Vegetation patterns depend on ecological pararneters as topography. surface

    or soi1 features and rainfall. In western Niger, aerial photographs and SPOT satellite panchromatic and multispectral images (10 and 20 rn pixel size) have been used:

    J to identify and classify the vegetation patterns;

    J to extrapolate results at regional scale. Most patterns in the region cannot be discrirninated using Landsat TM imagery due to its too coarse resolution (30 rn).

    The selected classification systern is OASIS. Initial data are supervised or unsupervised classifications, NDVl or color index thresholding. In a window of approx. 10- 15 pixels, each vegetation structure is defined by a reference pattern.. The composition of this window centered successively on each pixel of the image, is calculated and classified according to its similarity with the reference windows. We obtained thus a new classified image.

    We propose a NDVl where red band is substituted by a combination of XS2 and panchromatic bands. This vegetation index has been validated along pattern transects in relationships with vegetation cover percentage. Other pararneters describing vegetation patterns include length and direction of bands or patches. Theses pararneters are extracted frorn image after binarization and reduction of vegetation

    PART II

  • structures to a single line. The digital nurnber of each pixel represents the structure length or the direction angle. A photointerpretation on aerial photographs is used as the reference rnap. However, sorne limits are very difficult to delineate. The combination of autornatic classification based on bands length and rnodified NDVl is very accurate cornpared to the reference map. The extrapolation of results requires to verify the stability of windows composition for each pattern along a large latitudinal clirnatic transect. Multispectral data without panchromatic band gives also good results consistent with regional scale studies.

    PART I I

  • processes of the herbaceous front

    ylvie GALLE**, ristian VALENTIN**

    ensely vegetated arcs parallel to the contour alternating with zones of bare soi1 are widespread in South West Niger. Seen frorn the air, this typical pattern resernbles a tiger fur. hence its cornrnon narne of tiger bush. Due to the occurrence of irnpervious soi1 crusts, the bare interband acts as the source of run-on for the.

    downslope band. Therefore. the fringe between runoff and runon zones has been hypothetized as crucial for the dynarnics of the systern. This paper airns (i) to present a detailed characterisation of this peculiar ecotone. (ii) to infer sorne insights on the dynarnics of the tiger bush systern.

    The zone of study was located 70 km East of Niamey and consisted of a rectangular of 60 rn long and 30 rn wide, perpendicular to a typical wavelength. The survey included rnicrotopography using a laser toporneter, surface features (crusts. termite rnounds, etc.). herbaceous and woody vegetation. Soil rnoisture (neutron probe) was rnonitored over four yean along a transect and related to the dynarnics of the herbaceous layer.

    The pioneer zone is occupied by annual grasses (predorninantly Michrochla indica). and further down the slope by shrub (Cuiera Senegalensis) underlain by an herbaceous layer of Cyanotis lanata. The mean slope gradient (0.3[ across the bare interband) gradually decreases in the vegetated arc. However, the upslope herbaceous pioneer zone does not present a straight edge but local convexities (narned salients or capes) and concavities (pockets or bays). In the capes. the pioneer Cuiera senegalensis were rnost often associated

    PART II ORSTOM. Niamey

    ** ORSTDM. Montpellier

  • IONING AT LOCAL SCALE

    with micromounds which might be ascribed to increased soi1 porosity (root and faunal activity) and dust accumulation. In contrast, topography of the bay was flattened due to the accumulation of surface wash deposits as testified by the extension of sedimentation crusts. Furthermore, bays offered greater water resource availability than the capes. A minimum threshold water storage of 20 mm within the upper soi1 20 cm was determined to enable the colonisation by Michrocloa indica. However, the colonization the bare interband by this herbaceous layer was not constrained only by water availability because this threshold has been exceeded in the bare interband during most wet rainy seasons. Such limitation could be attributed rather to a low seed stock in the bare ground and to erosion and grave1 crusts which are much more resistant to seedling emergence than sedimentation crusts.

    These results suggest that the contrast between capes and bays induce feedback processes. The microtopography of the capes tend to deviate the overland flow to the bays favouring thus the gradua1 levelling of these pockets by surface wash deposits, the water supply of the central zone of the arc and the colonisation of the bays by herbaceous species. This herbaceous layer tends in turn to attract termites and to prepare better environmental conditions for the establishment of pioncer shrub. Whenever these processes lead a bay to catch up with the line of the capes. these latter are turned into bays and the cycle can start again. This strongly support the hypothesis of upslope migration of the arcs, as a result of a preferential colonization of bays.

    PART II

  • RUNOFF AND EROSION PROC

    Grard BELLIER*", Issa MALAM***,

    anded vegetation patterning occurs in many arid and semi-arid regions of the world. Although its driving force may be the wind action in some areas, most authors hypothesized the role of the bare interbands as a souKe of runon for the downslope vegetated bands.

    Such assumption is supported by the concentric distribution of arcs to the contour on the plateaux. Banded vegetation patterns occur on a very large variety of soils which have in common to develop impervious crusts when left exposed to direct impact of taindrops. Even though the role of these crusts has often been mentioned. no detailed study has been conducted on this issue. The objectives of this paper were to present: (i) the different types of crusts in this typical environment. (ii) their spatial distribution, (iii) some consequences its dynamic.

    The study area is located in the Niamey region in Niger on lateritic plateaux. The methods of chatacterization included detailed field surveys at various scales. Samples of the soi1 surface were collected for mechanical analysis. thin section preparation. optic and SEM observations.

    A wide variety of crusts were characterized. In the bare ground interbands, they included structural sieving. runoff. erosion. grave1 and sedimentation crusts. In the vegetated band. only a limited area was protected by a permanent litter cover and was therefore kept immune to surface crusting. In upslope and downslope edges of the vegetated band, structural and sedimentation crusts were common. A particular emphasis has been given to cryptogams which colonize surface crusts, more peculiarly erosion and sedimentation crusts, in the bare ground interband as well as under vegetation. Again a wide variety of

    ORSTOM. B.P. 1141 6. Niamey, Niger ** ORSTOM, Bondy. France PART l *** Universit d'Orlans

  • OFF AND EROSION PROCESSES

    cryptogarns were identified.Due to a longer evolution, the crusts in the tiger bush environrnent were more cornplex than in cultivated land where they are rejuvenated by tillage practices. However, they involved a very clear succession along the transect of slaking, sorting, erosion, sedirnentation and bioturbation stages. This spatial distribution suggests a temporal succession and therefore an upslope migration of the systern.

    PART III

  • RUNOFF AND EROSION PROC- $,< 8

    * 2 &4 < Qi

    ce water redistribution banded vegetation structures

    mi CERDA, Anton IMESON,

    i'";+**; ;

  • FF AND EROSION PROCESSES

    Data regarding rainfall intensity, Storm duration and recurrence intervals were analysed for al1 climatic regions.

    Based on this analysis a complex interaction was found water redistribution for the water budget of the banded vegetation structure at fine spatial scale. (c I m2). A quantification will be given for different climatic regions and soi1 properties. It is concluded that the importance of surface water redistribution for the water budget of banded vegetation patterns varies dramatically between the various semi-arid regions with differences in both rainfall regime and soi1 types. Often its significance for the water budget of the banded soit and vegetation pattern is rather limited, depending on both soi1 surface as well as rainfall properties.

    PART I I I

  • s of the vegetation striped patterns Mexico and the influence

    Andr MAUCHAMP**, Gera

    he dynarnics of vegetation striped p

    the whole systern.

    *$. KEY WORDS

    hydrodynarnics. interception, stemflow.

    ' Oatom, A.P. 17116596, quito. Equator " Lrtacion Cientifiea Charles Danuin. A.F? 17113891, quito, Equator. "' Cenid-Rarpa, A.P. omez Palacio, DO. Mexico

    PARI' III

  • ing variations along a climatic ity and patterns of tiger bush

    and Jean-Marc I'HERBES*

    transect

    RUNOFF AND EROSION PROC

    anded patterns of vegetation have been recognised in many arid and semi-arid regions of the world, notably in West Africa where they are known as "tiger bush". They consist of bands of trees alternating with bands of bare soil. These vegetation stripes run parallel to the

    contour on virtually flat to gently sloping surfaces, as the rnost effective intercepting arrangement of dispened runoff. The sequential pattern of runoff, interception, and run-on zones in this landscape constitutes a natural water harvesting systern. This pattern is expected to differ across space and tirne, as driven by global change forces like clirnate change and increased human pressure. The objective of this paper are (i) to analyse the variations of the tiger bush patterns as influenced by major factors. e.g. slope gradient and surface features along a clirnatic transect; (ii) to test a model of vegetation productivity based on surface water redistribution; (iii) to simulate the possible impact of global change in terrns of clirnate and land use.

    The transect has been selected on a series of Panchrornatic SPOT images covering a 60 km x 200 km band from the vicinity of the W Came Park in southem to the region of Ouallam, near the Mali boarder. The mean annual rainfall ranges from 300 mm in the North to 700 mm in the South. Fifteen typical sites of tiger bush were identified on the satellite images and surveyed in the field. In each site. one to three transects were sampled including three bands and interbands. Major variables were siope gradient, soi1 surface conditions. e.g. soi1 crust types, litter, faunal activity, vegetation type. structure and density. The hydrological model used to sirnulate runoff from the bare bands and infiltration in the vegetation bands was based on the typology of surface features of Casenave and Valentin ( 1 992).

    . . . . . . . . .. .. .

    * ORSTOM. B.P 11416, Niamey, Niger P A R III

  • OFF AND EROSION PROCESSES

    This study led to the identification of the combined climatic and topographic boundary conditions for tiger bush occurrence. For a given slope angle, dryer conditions result in narrowing of the vegetation band and a consecutive extension of the bare interband. The late stage sees the vegetation reduced to a series of aligned dashes increasingly surrounded by bare surfaces. Convenely, at the wet margin of the zone of occurrence, bare areas tend to be restricted to more or less circular crusted spots, often related to termite mounds. In a given location. patterns range from broad bands on nearly flat land (0.2%) to small "dashes" on steeper slopes (1.5%). Along the climatic transect crust types are also primarily controlled by slope gradient. Satisfactory relations were obtained between simulated infiltration in the vegetation band and assessed biomass, except where human disturbance was too severe. The difference between expected and assessed biomass indicated the importance of human use of the tiger bush. Simulations clearly showed that climatic change would have a minor impact on vegetation productivity compared to increased human pressure. Management strategies are proposed accounting for landscape hydrological processes as influenced by topographic. climatic and crust conditions.

    PART III

  • RUNOFF AND EROSION

    le cations within a patterned ground New South Wales, Aurtralia

    D*, M.D. MELVILLE *, L. WHITE**

    atterned ground in arid and semi arid Australia is landscape feature where spatial variation of vegetation and bare areas are rhythmically repeated. This patterning is exhibited in mulga tree communities. The study of the origins and characteristics of patterned ground in Australia has been generally confined to the

    relationship between the hydrological regime and its control on plant growth. Our study shows the characteristics and proposes the processes controlling the soi1 cation distribution in chenopod patterned ground. There are important interrelationships between soi1 cations, hydraulic properties and the plant spatial variation across the patterned ground landscape.

    The microtopography of patterned ground is generated by the "gilgai phenomena" as a result of the differential wetting and swelling of the clayey subsoil of the vegetated arcs. The distribution of soi1 cations within chenopod patterned ground is therefore not uniform across the landscape because the bare areas are "salt dumps". The bare area soils are dominated by sodium and it's absolute and relative concentrations decrease towards the centre of the vegetated arc. Magnesium and calcium have a similar elevated concentration under the bare ground but with greater concentrations relative to sodium under the vegetated arcs. Potassium is concentrated in the vegetated arcs and decreases under the bare soi1 surfaces.

    The pattern of salt distribution is caused by the interaction of the differential runoff from the bare areas and enhance infiltration of water into the vegetated arcs. the differential evapotranspiration between bare and vegetated surfaces. the gilgai complex and the vegetation-induced

    School of eography, University of New South Wales, Sydney, NSW 2052. ** CSIRO, Centre for Environmental Mechanics, PO Box 826. Canberra. ACT 2601

    PART III

  • OFF AND EROSION PROCESSES

    Salt turnover. These factors cause the lateral rnovernent of salts out of the vegetated arcs and their concentration in the bare ground. We propose that overtirne the patterned ground and gilgai cornplex gets stronger and the Sal t distribution also becomes stronger until a point is reached where the systern is in equalibrium with the clirnatic environment. The implications for a recent land management practice are briefly discussed.

    PART III

  • RUNOFF AND EROSION PROCE$& ,* &, 'F"

    Australia

    olyphase mosaic landscapes are widespread in the Australian arid and semi-arid zones. We describe here a mosaic developed in native grassland located south-east of Broken Hill in arid western NSW. The erosional processes active in the mosaic and the implications for broader hillslope erosion are the focus of this work.

    The site typifies a regional low-gradient landscape and has an overall slope of 0.004 (approx. 0.2"). The landscape contains subtle topography related to occasional ephemeral stream channels and other small drainage features. The climate is arid, with the mean annual precipitation at Broken Hill, about 40 km distant, being 250 mm. Annual potential evaporation is about 2.5 m. Rain is distributed sporadically and unreliably throughout the year.

    Vegetation at the study site is dominated by native grasses. This has a tussock growth habit, with 4-10 tussocks of varying size per square metre, and a tussock height of about 30 cm. The grass tussocks are concentrated into arcuate bands (mosaic components) that are separated by relatively unvegetated surfaces. Surface characteristics correspond closely to those described by Dunkerley and Brown (1 995) (J. Arid Env. 30: 41-55). The grassed bands display rough surfaces, pock-marked by crabhole collapse features. The unvegetated bands have a surface spread of small stones lying upon or embedded in the regolith surface. which often carries a patchy microphytic crust formed by cyanobacteria and lichens. No crabhole collapse features are dweloped within the unvegetated bands. The upslope margin of grassed bands is marked by a narrow zone of low forbs, while the downslope margin is sometimes associated with an increased abundance of chenopod

    * Dept of eography and Environmental Science Monash University, Clayton Victoria 3168 Australia PAFI' III

  • OFF AND EROSION PROCESSES

    shrubs. Average downslope widths of the mosaic components are as follows: unvegetated surfaces 9 m, zone of forbs 2.3 m, grassed bands 24 m. Widths of al1 components are quite variable. Grassed bands are associated with a fall of about 20 cm, while the unvegetated zones. being narrower and somewhat flatter, are associated with a lesser fall.

    lntricate microrelief is associated with the surface mosaic. The unvegetated bands in general have a lower gradient than the grassed bands. but the gradient is not constant, the grassed bands being concave-upward in profile. A low ridge of material including coarse sand, and 1-2 cm high. is associated with the zone of forbs; this must be created by splash. perhaps aided by trapping of sediment in ephemeral litter dams lodged against plants. Downslope of the forbs. gradient increases steeply onto the uppermost parts of the grassed bands, a configuration favouring runon.

    Hydrologic functioning of this landscape is akin to that described by Dunkerley and Brown (1995). The unvegetated surfaces shed water efficiently; wetting depths of only 2-3 cm were observed after a storm of moderate size. even after many hours of ponding. Water infiltrates more deeply in the zones of forbs and grasses, and was observed to reach 10 cm within the forbs and even more deeply within the grasses, from the same storm.

    Erosional behaviour of the site is less clear. Sediment transport appears likely to be strongly compartmentalised as for runoff and runon. However, we have no data bearing on the issue of whether the study slope ever becomes an integrated hydrologie entity. transmitting water and sediment to the slope foot. We hope to investigate this aspect of site behaviour in a series of trials with simulated rainfall. Some aspects of the site bearing on erosional behaviour will be described in the paper, and their significance reviewed. These include the following:

    O 5011 STRENGTH Unconfined compressive strength was determined with a penetrometer a t many sites in grassed and unvegetated zones. The mean compressive strength is at least 5 times higher in the unvegetated zones. Splash of regolith particles from these sites is thus more difficult than from the more friable surfaces of the grassed zone. However. given that splash is enhanced by the presence of a thin water film, it may be that splash from the unvegetated zones is enhanced relative to that within the grasses, where ponding or surface flow seem highly unlikely, under

    PART III

  • RUNOFF AND EROSION PROC

    appropriate conditions. Shearing of particles from the surface by water flow will be restricted within the unvegetated surface owing to its mechanical strength.

    0 S U R F A C E R O U G H N E S S

    Surface roughness contrasts among mosaic components have not been reported previously. However, this aspect seems of great importance within runoff-runon systems. Surface elevation data were collected on a 10 cm x 10 cm grid with a falling-pin elevation probe in both sections of the mosaic. Roughness, assessed as the standard deviation of the elevation measurements, is nearly 3 times higher within the grassed bands (7.7 mm vs. 2.7 mm). The roughness of the grassed zones must retard any surface runoff, and increase the opportunity time for infiltration. Thus. regolith erosion from such sites must be restricted. The relative smoothness of the unvegetated zones must aid runoff movement there.

    0 C A N O P Y C O V E R

    This is significant within the grassed bands. By line transect methods, we have estimated this to be about 24%. Thus, some precipitation is intercepted within the grassed bands and this restricts the likelihood of runoff generation within these zones. Less than 10% cover was found in the unvegetated zones, relatively increasing the likelihood of runoff production there. Both tendencies are reinforced by the distribution of crabhole collapse features already mentioned, which must act to trap water within the grasses. Splashed grains entering the grassed zones must be partly trapped beneath the canopy.

    0 S T O N E COVER

    Stone cover is significantly greater on the unvegetated bands. Additional surface roughness is thus generated, and a fraction of the regolith fines lying beneath stones is protected from splash or entrainment by surface flow. Owing to the surface roughness, flow depths in surface runoff will be increased, potentially further restricting the surface forces generated by raindrop impact through the water-layer cushioning effect. Overall, at any site the behaviour of runoff will reflect a balance between the relative smoothness of the regolith surface and the drag exerted by the surface stones. Stone size, cover

    PART III

  • OFF AND EROSION PROCESSES

    fraction, and geometric arrangement thus become important aspects of site behaviour.

    Overall then, the site conditions appear likely to restrict greatly integrated slope runoff and sediment entrainment. Degradation of the site characteristics by any disturbance seems likely to be associated with an increase in local erosion rates. However. much remains to be learned from mosaics in this region, which display some unusual features that will be described in the presented paper.

    PART III

  • SOI1 WATER BA

    ded vegetation pattern: h in Western Niger

    1 EHRMANN**, Christophe PEUGEOT*

    n intensive survey of a bare soillvegetation system was performed on a banded vegetation pattern (tiger bush) located on a lateritic plateau in western Niger from 1992 to 1994. The global functioning of the system is strongly related to the hydrological cycle, and therefore it is of concern to characterise and quantify its

    components. The objectives of this paper is to study the spatial distribution of soi1 water within the bare soillvegetation system. and to establish the water budget at both the event and the annual time step.

    Field measurements involved soi1 moisture (neutron probe) and local runoff (runoff plot) monitoring. They show that infiltration of water into the soi1 is strongly related to the location within the system: water stotage from the bare soi1 is low, much higher in the front bush, and very high in the centre bush. This spatial pattern is steady over yean. Runoff on the bare soi1 zone accounts for one half of the annual tainfall depth, and due the system geometry, the bare soi1 zone constitutes a natutal surface water feeding area for the bush that thus benefits of a water amount about 3 times higher than the annual tainfall depth. At the event time step, soi1 water stotage can exceed 12 times the tainfall depth in the centre bush. with an average value of 7 times. As shown on the water profiles. infiltration under the bush is tapid and deep (more than 6 m). probably due to a faunal macroporosity. Estimated to about one time the annual tainfall, deep infiltration is likely to contribute to aquifer recharge.

    The water feeding of the bush by the bare soi1 zone, associated with vegetation dynamics allow to explain vegetation banding. The hydrological functioning describe here (runoff, sediment yield)

    ORSTOM B.P 504534032 Montpellier Cedex 1 , France " ORSTOM, B.P 11416, Niamey, Niger

    PAR'I' III

  • WATER BALANCE

    associated with the issues of other recent studies. strengthens the hypothesis of upslope displacement of the vegetation bands over yean.

    PART III

  • nce in the watershed of the river orouof- f of Burkina h o

    he paper contains the exploration O water balance in semi-arid regi study site is located in the watershed of the river Corouol (northern Burkina Fas region comes up with 462 mm at 375 mm at Markoye (long ter

    rainfall can be diminished to 230 - 360 mm p pattern in that region is not developed in wel patterns. but is distributed in special landsca etc.). lnside these special landscape units contracted or diffuse. Soil properties are of natural vegetation and to agricultural crops properties are :

    J Properties which affect soi1 moisture, s characteristics of silt-and clay-size particles, arrangement in structural units.

    J Soil properties which affect the colloidal mineralogical character of the clay and its arran other particles.

    J Characteristics of the regional hydrological system which affect the water properties of the soil, such as regional water-table levels and their season of variation.

    J Soil properties. such as the presence of i layen, or profile heterogeneity, which affe root systems of the vegetation.

    * lnstitute of Physical eography, University of Mannheim, L9, 1-2. 68131 Mannheim. ermany PAR'I' III

  • WATER BALANCE

    With the use of the "Catena" - principle three landscape profiles (each 3 km long) in the region of Gorom-Gorom. Saouga and Mnegou are mapped. The mapped landscape segments cross the different landscape units and inside of each landscape unit soil profiles (up to I to 3 mm depth), soi1 samples and soi1 analysis were carried out. Concerning to the classification of soi1 cover complexes (designated as "tats de surface" by Casenave G Valentin, 1988) the soil moisture is measured by tensiometer in each landscape unit. The results of soi1 moisture measurements during two years (1 994 and 1995) are discussed. Additional to the mapping of soi1 types and soi1 moisture conditions the relief types and vegetation distribution in different landscape units were mapped. By the combination of relief, soi1 types and vegetation phytogeomorphologicaI units are derived. The phygeomorphological unites are transferred to patterns in aerial photos and spectral signatures of satellite images (e.g. Spot and Landsat TM).

    The deduction of special spectral response patterns for the derived phytogeomorphologicaI units will be the next aim of the research. first results of these approaches will be demonstrated.

    e3 REFEREMCE

    CASENAVE, A ; VALENTIN. C, 1998. Les tats de surface de la zone sahlienne - influence sur l'infiltration , ORSTOM. 1988.

    PART III

  • SOI1 WATER

    ect recharge to the deep unsaturated zone

    M. EDMUNDS**, E. FELLMAN**, S.R. GAZE*, J. SUDLOW*

    n estirnate of direct groundwater recharge below a region of natuml wood.land (tiger bush) bas been made in south West Niger using the solute profile technique. Data has been collected from a 77 m deep well dug within the study area covered by HAPEX-Sahel, an

    rge scale energy. water and carbon balance experiment carried out during the surnmer of 1992. So far as the authors are aware this is the deepest profile to have been obtained in the Sahel region and as such provides a uniquely long record of recharge in the historic past. 20

    The principle of the chloride profile technique is as follows. Chloride is introduced into the soi1 bath in rainfall and as dry deposition. Since chloride does not evaporate from the soi1 surface, and vegetation does not take up significant quantities, it becornes concentrated by evapotranspiration. Water in the soi1 can be broadly categorised into either upward or downward moving water, with the zero flux plane (ZFP), separating the two.'rhe upward water flux is driven by evaporation of water from the soi1 surface and transpiration through plants taking up water via their roots. Where there is sufficient infiltration for water to move below the root zone and the maximum depth of the ZFP, the water will continue to move down as deep drainage until it eventually reaches the water table.

    Under conditions of recharge the ZFP represents the point below which a net, steady state, moisture and solute transfer takes place toward the water table. The arnount of chloride crossing the ZFP varies in relation to antecedent rainfall over one or more seasons. thus causing oscillations in the chloride profile. Under steady state conditions the

    lmtitute of Hydrology. Wallingford. UK " British eological Sunrey. Wallingford. UK "' ICRIVn; B.P. 12404. Centre Sahlien, Niger "" ORSSDM, B.P. 1141 6. Niamey, Niger

  • R BALANCE

    - average chloride concentration of pore water in this profile, Cs , will be - - - - proportional to the concentration factor, PI(P- , (where: P =

    long term average precipitation and = long tenn average evapotranspiration). This assumes no loss or gain of chloride to or from minerals, and that the water and chloride are transported at the same rate. Where the surface run-off component is negligible. and assurning there are no reactions with rninerals the recharge is given by:

    Where:

    - Rd is the space and time averaged direct recharge flux.

    - - Fp and Fd are the average tainfall and net dry deposition fllmr (=input)

    - is the averaged concentration of chloride in the rainfall

    - Cs is the averaged interstitial water concentration

    It follows that direct recharge can be calculated if the following values are known; the volume averaged concentration of chloride in the -

    C - rainfall ( p ); the averaged interstitial water concentration (Cs ); the long term average rainfall (F) and, the net dry deposition chloride flux - - ( F d ). If Fd = O. then the fraction of rainfall contributing to direct -

    C C recharge is given by the ratio p I s .

    The solute profile well is positioned a t latitude 13" 15'44" N and longitude 2" 3' 31" E, a t an elevation of 262 m. It lies a t the edge of a large area of tiger bush close to the village of N'douroua on the western side of the Say plateau. Locally, the ground slopes gently toward the south west a t a gradient of I in 120. The site is underlain

    PART III

  • SOI1 WATER BA

    by the Continental Terminal formation, a 49.4 m thick sequence of terrestrial sediments, which are Miocene to Pliocene in age. These sediments rest on a pre- Cambrian basement of mainly granitic gneiss. Only the upper unit of the Continental Terminal, the "Cr 8As argileux du moyen-Niger" is present in this area. The water table lies within the basement at a depth of 74. i m. which is 24.7m below the base of the Continental Terminal.

    During well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from O - 10 m; every 50 cm from 10 - 62.5 m. then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analyzed for chloride; moisture contents of samples were obtained gravimetrically. This data has been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from 5 EPSAT rain gauges in 1992. A mean recharge rate of 13 mmlyr is estimated for the upper 70 m of the profile. with a total residence time of 790 years. 'This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas.

    PART III

  • veneta tion

    WATER

    site

    rainage patterns of a watershed form the framework of important energy flow and nutrient cycles that occur on the landscape. If planning does not occur at a watershed level of resolution. activities on a smaller planning unit will be susceptible to being undercut by

    events outside the project focus area. In southern Niger, banded vegetation patterning (known locally as brousse tigre) occurs on laterite plateaus which are primarily used as communal grazing land and as a source of firewood. At the study site located near Hamdallaye, Niger (13'34 N, 2'35'E) these communal lands are severely degraded. This degraded portion of the landscape has become the source of runoff that triggers serious gully erosion and disrupts crop production downslope. Originally, the vegetation bands were 20 to 40 m wide. separated by 40 to 60 m wide sparsely vegetated corridors. The vegetation bands were dominated by 2 to 5 m tall deciduous trees. the most common species being Combretum micranthum, Combretum nigicans and Acacia macrostachya: The edge of the tree bands were vegetated with herbaceous species such as Ctenium elegans and Pennisetum pedicellatum. Heavy firewood harvest and grazing over the last several decades has fragmented the banded vegetation allowing runoff to pass through the thicket gaps, wentually concentrating into torrents which cascade off the plateau.

    Due to the vast scope of the degradation, i t was not practical to devise technologies to try to stabilise the entire plateau area at once. Therefore, technologies were tested that take advantage of knowledge regarding deterioration patterns and nutrient flows on the plateau that regulate the rehabilitation process. The emphasis of this approach was to concentrate on using, rather than combating, natural processes. It

    Institut National de Recherche Agronomique du Niger. Niam~y. Niger " Texas A O hi University. College Station. Texas, U.S.A. "*Alabama A O M University, Normal. Alabama. U.3.A

    PAKI' III

  • WATER BALANCE

    was hypothesised that tree establishment and growth would be better in the gaps of degraded thicket bands than in the bare corridors between the bands. It was also hypothesised that if the gaps in the degraded thicket bands could be revegetated, the continuous thicket bands that grow on contour would once again act as natural barriers to overland flow and sediment transport and reduce downslope flooding and erosion. Further. it was hypothesised that runoff and eroded soi1 caught in the revegetated thicket gaps would accelerate the rejuvenation process.

    The surface horizon of soils inside the thicket bands is a dark brown fine sandy loam, laminated layer that has strong coarse subangular blocky structure that parts to a thin platy structure. Surface soils in the bare corridors ands in the degraded thicket bands are severely crusted. whereas the soi1 surface in the thickets is not crusted. The soilllaterite boundary is undulating and abrupt, varying as much as 50 cm vertically over a I m horizontal distance. These poorly drained soils are rich in bases and available P. soils under the thickets have a relatively higher fertility than the soi1 of the barren corridors. The degraded portions of the thicket bands exhibit the relic fertility features of relatively recently vegetated soil. Soil of the bands are significantly deeper than those in the open corridors. Moreover, soi1 profiles in the vegetated and recently degraded portions of thickets exhibit extensive biological activity that is characterised by numerous biochannels and many roots. There is little evidence of biological activity throughout the soi1 profiles of the barren corridors between bands. This may partially explain the spatial arrangement of corridors and thicket bands.

    Infiltration rates of the various components of the banded mosaic were determinated using a rainfall simulator to apply water at a rate of 150 mmlhr (equivalent to an intense rainstorm at the site) to I m2 plots. These tests were conducted before the onset of the rainy season (April- reflecting the hydrologic characteristics of the site at the onset of the rains) and midway through the rainy season uuly). Infiltration rates in the bare corridors and the degraded thicket gaps were low (32 mmlhr) and not significantly different between April and July. The infiltration rate within the thicket was significantly greater than in degraded areas, and was greater in April (77 mmlhr) than in July (41 mmlhr). Low infiltration rates in the bare corridors and the degraded thicket gaps were attributed to surface crusts and rapid sealing of the surface when the soils were wetted. Seasonal variations in infiltration rates within the thickets could be explained by changes in Iitter cover and plant biomass. During the dry season, deciduous trees shed their leaves resulting in a good surface mulch. The litter cover of the soi1 surface

    PART III

  • SOI1 WATER BA

    facilitates retention storage of runoff and protects the surface pore structure by dissipating the raindrop energy before reaching the soi1 surface. Litter biomes rapidly decomposed as the rainy season progressed due to enhanced termite and microbiological activities. Thus. the reduction in surface cover resulted in a decrease in infiltration rate.

    Seedlings of Acacia holosericea and Prosopis juliflora were planted in 2m2 microcathments that were installed in both the degraded thicket gaps and the in the barren corridors. These microcatchments stored overland flow and accumulated soi1 and litter deposited by runoff events. The accumulation of soi1 and litter in the microcatchment harboured a large termite population which attacked and killed the P. juliflora seedlings. The A. holosericea seedlings seemed to be resistant to termite attack. The survival of this species was not different between the barren corridors and the thicket gaps. After 13 months the A. holosericea seedlings (IS cm tall when planted) had a 96% su~ iva l rate and had grown to an average height of 224 cm.

    Flooding the gullies downslope has stopped as a result of tree establishment in the rnicrocathchments which re-established the continuity of the vegetation bands. Farmers downstream have recognised the practical value to them of restoring the banded vegetation on the plateau and have initiated additional collaboration with the Institut National de Recherche Agronomique du Niger to expand these reclamation activities to other degraded areas.

    PARI' III

  • regulation in pattemed landscapes

    $$ ky* a x,~; fi- $< ??- <

    hilst it has long been recognised that soi1 fauna can t s , A 4 $gg~~:+~ play significant roles in mediating many fundamental

    22 + ,m$< d- ecosystem processes such as decomposition of organic matter, nutrient cycling. seed dispersal and herbivory, comprehensive studies of such activities are comparatively rare in many arid ad semi-arid

    ecosystems. Ecological studies of soi1 fauna in semi-arid rangelands of Australia have been severely constrained by a basic lack of adequate inventory data on species distribution and abundance, especially for soi1 invertebrates. Furthermore. the rapid extinction of many of the smaller vertebrate fauna. some of which are now recognised as having had significant influences on some ecosystem processes prior to European pastoral settlement last century, has further hindered ecological insights into prehistoric landscape processes.

    Nonetheless. increasing awareness in recent years of the importance of soi1 ecology and soi1 biology has seen several research projects undertaken throughout Australian rangelands. This paper will briefly review the results obtained from some of these by focusing on studies undertaken in patterned landscapes of semi-arid eastern Australia since vegetation patterning reflects fundamental patterning of habitats for a *

    wide range of soi1 fauna. Particular emphasis will be placed on the ecology of different functional groups of soi1 fauna ranging from soi1 microarthropods (Acari) through termites, especially the widespread harvester termite Drepanotermes pemiger, to soil-inhabiting vertebrates. These will include such groups as the reptiles as well as marsupials like the now locally-extinct burrowing bettong (bettongia lesueur) together with exotic species such as the European rabbit (Oryctolagus cuniculus)

    * National Rangelands Pmgram, CSlRO Division of Wildlife and Ecology, PO Box 84, Lyneham. AC72602 Australia

    h -

  • The paper will refer to case studies undertaken in a patterned mulga (Acacia aneura) landscape near Louth in western New South Wales where initially, emphasis was directed towards examining, and quantifying, spatial patterns and abundances of surface-soi1 inhabiting microarthropods and surface features built by larger invertebrates, especially termites. Because these latter animals are regarded as key species mediating the decomposition of plant residues, both herbaceous and woody, field experiments have also examined the effects of termite removal on the rate of decomposition of senescent grass tussocks in two contrasting communities.

    Because historical evidence suggests that episodic fires have had major impacts on ecosystem processes in the past. prescribed fire is now regarded as an important management tool, especially for controlling shrub encroachment, to be used whenever sufficient fuel stocks are available. There is little, if any, knowledge however, as to how different fire regimes have interacted with different functional groups of soi1 fauna. Accordingly, the effect of prescribed fire on the relative abundance and composition of soi1 microarthropods in different soi1 microsites will be discussed in the context of a patterned semi-arid mallee (Eucalyptus spp.) community.

    Further discussion will centre on the hypothesis that some soil- inhabiting fauna such as the burrowing bettong required periodic fire to enable them to graze young foliage of shrubs resprouting at ground level, especially when there was little alternative green feed available. This fire x browsing interaction may in fact have constituted a prehistoric form of integrated shrub control which, in turn, reinforced spatial patterning of vegetation by impacting only on those patches capable of producing herbaceous fuel.

    While macroscale patterning on a landscape scale ( 1 - 100 km) is controlled mainly by terrain factors, mesoscale patterning of vegetation, that is. 0.5- 1 km, is controlled instead primarily by biotic processes under the control of a suite of soi1 fauna which may also interact with abiotic landscape processes. This paper will discuss this hypothesis using examples from Australian research outlined above.

    PART III

  • vegetation pattern

    his study was "ndertaken in northern Burkina Faso, Yatenga province, in the Bidi site, as part of the SALT (Savanna on the Long Term) program. Using 137G to trace soi1 erosion and potential upslope migration of bandedvegetation in south-west Niger.

    The climate is sudano-sahelian. The annual rainfall (400 to 600 mm) is concentrated over 3 months, from July to September. The study area is situated on the granitic substratum, with a very low slope (0.8%). Soils are tropical ferruginous, with a concretionary level close to the surface on the summit and lower slope. The vegehtion is distributed along the toposequence. from the hard-pan summit down to the bottom. Typical banded vegetation ("brousse tigre") is located on the upper slope. It is characterized by vegetated (ligneous) bands. more or less parallel to the slope direction. with a herbaceous stratum (pioneer zone) upside and a degraded zone downside.

    The aim of the study was to assess the distribution of termite nest populations (densities, surfaces and volumes involved) in this landscape and the effects on soi1 (texture. metabolism), on water fluxes (runoff, infiltration) as well as on vegetation (woody species, seedlings).

    All the parameters were collected within a transect 2900 m long and 50 m wide: mound species, stage (alive or dead), height and diameter together with woody species (height) growing on mounds and outside. Termite foraging activity was followed along line-transects starting from selected nests. A special experimental device was installed to protect a vegetated band from upside runoff and to compare the evolution of the vegetation and termite activities with untouched bands.

    'iaboratoire d'Ecologie, E.N.5.. 46 rue d'Ulm. 75230 Paris cedex 05 PART III

  • Termites are considered to be indicators of the evolution (up-slope migration hypothesis) of striped vegetation, as water fluxes influence the nest population structure over time. In turn. termites could exert a profound influence on the dynamics of the vegetation. through their actions on soils, water fluxes and nutriments.

    Three main termite trophic groups were encountered on the study site: the humivorous group (Cubitermes). the gras-foraging group (Trinervitermes) and the fungus-growing group (Macrotermes). A high proportion of Trinervitermes mounds (86.4%) were dead and the densities are low as compared with other biotopes (maximum 25 nests ha- 1 , 2 live nests located within the upslope herbaceous zone). Cubitermes mounds could be locally abundant (80 nests ha-1). mostly within the ligneous band, 30.7 live nests in average ha- 1 , 26.8 dead ones). Macrotermes subhyalinus is relatively abundant in the striped vegetation: 21.2 live nests ha- I in the wooded zone. together with 36.8 ha- 1 dead nests. This last species is considered to be the most important in this biotope, as far as soi1 volume rehandled and surfaces are concerned. Therefore. most of the studies concentrated on the Macrotermes nests.

    The present position of the eroded stages of Macrotermes nests (ranked from live nests to completely eroded). showed that none of the live nests are found outside the wooded area and that large numben of dead eroded mounds are found in the bare area. This observation could be interpreted by the shifting upslope movement of the wooded bands.

    The mound bases represented 185 m2 ha-1 and the mound soil volume reached 19 m3 ha- 1 . Mound soi1 is richer in finer particles, mostly clay. There is a texture gradient in space (according to the structure and erosion of the nest): the mound center contains more than 50% clay. the mound wall contains 40% clay and the erosion cone 20-30%, as compared with less than 10% clay in control soil. As the mound dies and is eroded. mound material is further expended and enriches the surface soi1 with finer particles. Under heavy tain ("splash effect") and extreme climatic conditions, it is well known that soils rich in clay promote surface crust formation and therefore increase runoff. Lower infiltration rates and higher runoff production is seen on eroded material from Macrotermes nests. Ongoing infiltration experiments will confirm this fact and produce significant figures.

    On the other hand. termite foraging activity, in mixing organic and mineral particles, enhanced the infiltration rates. In their foraging activities, located within more or less permanent territories, live

    PART III

  • 7- 7 9-7, < ~ige* ' y L,> . ':, %E% 4 ,,,, if*! g,g, 3 [email protected]*#?$

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    .;-,2;~ >NW,[email protected] gp$.' 4 (>q* g'

  • biotope (mainly in the wooded and herbaceous areas) influence the water and nutrient fluxes.

    In summary, we could distinguish a "positive" effect on vegetation, as live nests increase the infiltration rates along a gradient starting from the mound and a "negative" effect as eroded material from nests increase runoff production. Under this hypothesis, the effect of termite action will be to reinforce other environmental conditions, such as climatic changes (drier periods) and human impact (overgrazing, fire- wood sampling). Ongoing further experiments are necessary to precisely assess the respective distances, intensities and variations over time of these "positive" and "negative" actions of termite populations.

    PART III

  • POPULATIONS

    Yatenga

    n northern Yatenga banded vegetation is a rather scarce feature which appean closely linked to glacis (with a slope between 0.5 and 1.5%). Plains, with no consistent slope, bear a savanna-like vegetation with scattered trees and shrubs and a connected herbaceous

    savannas). With regards to plains as well as glacis. soils are shallow and loamy above a more or less fragmented ironpan.

    For both vegetation types. very few is known about their current dynamics and even life history traits of constitutive woody species are very poorly documented. The aim of this study is to draw information from an analysis of present spatial patterns of woody individuals. An emphasis is placed on the 2 dominant species of striped vegetation: Combretum micranthum C. Don, a shrub which averages 3 m in height at maturity and Pterocarpus lucens Lepr.. a small tree which can reach 8 m in height in favourable growing conditions.

    Woody individuals were mapped for one reference plot in each vegetation type: the plot was 10.2 ha (320 m x 320 m) in the spotted savanna and 14.4 ha (300 m x 480 m) in the tiger bush. Locations of al1 individuals the height of which exceeds i .5 m were recorded. whatever the species; for some species, individuals between 0.5 and 1.5 m were also mapped.

    Most data analysis was based upon the theory of point processes (as reviewed by Cressie, 1993); spatial patterns of woody individuals were studied using spatial pair correlation functions and ((Ripley's K- functions~, which allow testing of multiscale departures from complete

    Ecole Nationale du nie Rural, des Eaux et des Forth .F! 5093 - 34033 Montpellier - France PART III

  • spatial randomness of individuals (null hypothesis); alternative hypotheses are attraction (clustering) or repulsion (regularity) between individuals.

    The spotted savanna has a greater species richness and diversity than the tiger bush. As a consequence, the proportion of basal area accounted for by C. micranthum and P. lucens is far less (41 % as against 80%). Nevertheless, the latter species ranks fint in spotted savanna (29% of basal area) whilst the former is of less importance (12%). On the contraiy. C. micranthum is the dominant species in the tiger bush (46% of basal area). followed by P.lucens (34%). This last species is the only one that can be stated as playing a major role in both vegetation types.

    In spotted savanna. most species display clustered spatial patterns (the most clustered are Combretum nigricans, Cuiera senegalensis and C.micranthum). Nevertheless, mature Pterocarpus lucens (height > 1.5 m) do not depart significantly from complete spatial randomness. whereas ((juvenilesn (height between 0.5 and 1.5 m) are clumped. The pattern of the tallest individuals (height > 6.5m) shows a slight repulsion under 10 m. and intraspecific competition might occur.

    In tiger bush most species have highly clustered spatial patterns, except for mature P.lucens. Nevertheless, this species has stands slightly denser in vegetated bands than in interbands and is not completely independent of the overall structure. which is principally determined by C.micranthum. In some places. dense Pterocarpus stands are associated with poorly structured banding patterns. which might result from a taking over of Pterocarpus on C.micranthum.

    The overall vegetation structures (spotted vs. banded) appear influenced by life traits of woody species. Due to its capacity for vegetative reproduction. C.micranthum is prone to constituting dense thickets, which can benefit from run-off along slopes, giving rise to or perpetuating banded structures (this species is thought to be dominant in extensive areas of western african tiger bushes). This advantage dwindles in the absence of systematic run-off, or under sufficient rainfall. In those conditions taller species, with efficient seed production and dispenal, have a greater abundance, yielding spotted patterns or poorly structured banded ones. This is particularly true for P.lucens but to some extent also for Crewia bicolor, Commiphora africana or Dalbergia melanoxylon.

    PART III

  • POPULATIONS

    iger bush is a particular vegetation pattern with densely vegetated bands alternating with bare areas. I t has been reported in many parts of the and and semi-arid zones. In Niger. bare areas serve as an impluvium for the downslope vegetated bands. The fint objective of this paper is to quantify the relationships between the

    intraseasonal soi1 water flows and the seasonal vegetation dynamics (e.g. during a rainy season). We verified the need for the run-on from the bare zone to sustain the vegetated stripes. We focused on a zone where the most important gradient in soi1 water content and rapid changes in vegetation distribution are observed, i.e. the transition zone between the bare soi1 and the thicket.

    Two representative vegetation stripes have been monitored from 1992 to 1995. Data were collected along transects placed perpendicularly to the vegetated bands, from the bare zone to the center of the vegetated thicket. along the line of maximum slope. In this way, the succession of vegetation features have causal relationships and connectance due to the flow of waterand the associated soi1 surface features such as crusts. Vegetation data include phenology and demography of the two main species of the herbaceous stratum from 1992 to 1994. and evolution of leaf water potential and phenology for the two main species of the woody stratum during the rainy seasons 1994 and 1995. Data on the crust distribution were collected once in 1992 and again in 1994. Soi1 moisture profiles were measured at depths of between I m to 5.6 m using a neutrons humidimeter (Solo) from 1991 to 1995. Pipes for use of the neutron probe were set on the same types of crust occurring from upslope to downslope on each vegetated band. A wall was built during the dry season 1992-93. It was placed just at the herbaceous

    *Centre ORSTDM, B P 11416, Niamey, Niger **Centre ORSTOM. BP 5045 Montpelliez France PART III -f;i'$!g"

    2.*#

  • LATIONS DYNAMICS

    stratum upslope boundary to stop the run-on from the bare zone to the vegetated band.

    Data from 1992 allow us to give details of the processes involved and to verify the similarities in the functioning of the two stripes. The data from the following years allow us to compare the effects on the soi1 water and vegetation dynamics of deleting the run-on. Results show that for the herbaceous strata the distribution of Michrochloa indica (Linn.f.) P. Beauv., well adapted to an open environment, closely correlates to the distribution of sedimentation crusts. immediately upslope to the thicket cover. and Cyanot lanata Benth, a shade- tolerant species, closely correlates to the distribution of microphytic crusts inside the thicket. Infiltration occurs first and deepest throughout the rainy season in the centre of the thicket under microphytic crusts. Herbaceous stratum grows simultaneously under the woody thicket. Then from the end of August infiltration increases upslope of the thicket, under the sedimentation crusts which were slightly colonised until this time, while only Microchloa indica moves upslope until there are insufficient seeds. The wall has no significant effect on the dynamics of the two main species of the low strata. in relation to the low differences in the soi1 water flows down to 40 cm. There are no significant consequences on the Combretum micranthum C. Don phenology and evolution of the leaf water potential because of the deep and extensive infiltration of the rains inside the thicket where this species is dominant and because of the run-on from the pioneer zone mainly covered by sedimentation crusts during the first part of the rainy season. On the other hand. the period and the extent of the reproduction phases of uiera senegalemis J.F. Cmel are greatly reduced and the leaf water potential increases very quickly after the last rains in relation to a great decrease in the soi1 water stock of the layers colonised by woody roots located in areas where there are sedimentation crusts and where this species is most abundant. Results are discussed in relation to the existing functioning models.

    PART III

  • "@.$LQ . A

    > A

    SYNTHETIC MODELS OF FuNC'TIONEIG ,?y>;+"'

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    on resource he influence capture and productivity

    ;$#j,,y;" @fg&g$ >;::.,. :L>x* .:;$$&;&fi -- $.&+4;1;

    n rnany serni-arid environrnents around the world, -3:; , &;$$:y; ;JF&:[email protected]

    lirnited iainfall and runoff-runon processes lead to ,&j;.iz4g$ t"nw qqFmx; .-&: .&y ~%,;,u, vegetation patchiness. This phenornenon has been ,%$ ,,.$,,>[email protected]$gl!$: ;Sa,, % :+&:

    (Ludwig & Tongway 1995). This optimisation is based on the theory $g:%f!j$ that arid lands function as source-sink or runoff-runon systems (Noy- [email protected]@: Meir 1973). This theory predicts that in environrnents with lirnited [email protected] resources, plant productivity will be higher if these resources are g;zb;r3p2:, ~.a:p:+r;sa ., Id-Ym.z7 concentrated into patches and not uniformly dispened over the

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    landscape. The question addressed in this study is: how much does the type of patchiness influence loss of resources and hence productivity compared to a landscape with no patchiness? Does it really matter whether patches are small and scattered (stippled) over the landscape, or occur in short bands (stripes) or as long bands (strands)?

    A 'flow-filter' simulation rnodel was developed to quantify how runoff flows down, and possibly out of a landscape (Ludwig et al. i 994). If the amount and intensity of iainfall (R) exceeds the water infiltration rate (IR) or water storage capacity (SC) of the soi1 within the interpatch area then runoff (ROff) occun. This ROff can run out (ROut) of the landscape system or be captured by patches. If the IR and SC of the patch is exceeded then ROut occun frorn the patch. The total ROut frorn a landscape following a iainfall event a t time (t) is also a function of the slope (S), total area of patch (AP) and area of interpatch (Al):

    ROut t = f (R,IR.SC.S,AP,Al)t

    * CSlRO Division of Wildlife & Ecology, PO Box 84, Lyneham, ACT 2602, Australia PART III

  • ETIC MODELS OF FUNCTIONING

    Annual net plant production (NPP) for the landscape system was estimated by using a linked simulation model, SEESAW, which sirnulates the ecology and economics of serni-arid woodlands (Ludwig et al. 1992, 1994). A subrnodel within SEESAW computes annual net primary production (NPP) through time (t) as a function of plant available rnoisture (PAM) and available nutrients (AN) and temperature (TEMP): NPPt = f (PAM,AN,TEMP)t

    PAM is estimated by a submodel, called WATDYN, which cornputes soi1 water balance dynarnics (Walker and Langridge 1995). For the simulations we used a serni-arid landscape system of fixed size (I ha), slope (1 %) and shape (rectangular), but with different patch structures:

    J stippled - small, circular and dispened patches;

    J stripes - larger, elongated and clustered patches;

    J strands - long linear and basal patches; and J no patches. Patches occupied 25% of the area (or were absent).

    Pararneter values used for the simulated semi-arid landscape were based on actual field rneasurernents. The rainfall and temperature data used to "drive" the simulations was based on a 31.5 yr (mid- 1962- 1994) from Cobar, NSW