Geographical Distribution and Local Occurence of Dichapetalum
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GEOGRAPHICALDISTRIBUTION AND LOCAL OCCURRENCE OF DToHAzETALUMCYMOSUM (HOOK) ENGL. (GTFBLAAR) rN NAMIBIAAND SURROUNDING AREAS R.I. DE SOUSA CORREIAAND L. YAN RENSBURG ResearchInstitute for ReclamationEcology, Potchefstroom University for ChristianHigher Education P.O. Box 19752,Noordbrug,2522,RSA ABSTRACT INTRODUCTION This paper discussesDichapetalum cymosum's habitat, The family Dichapetaluminclude two genera: Dichapetalum coveringits distributionacross southern Angola, Namibia, and Tapura.Dichapetalum is representedby about200 spp., Botswanaand South Africa in orderto ascertainit's ecological trees,shrubs and rhizomatousshrublets: cosmopolitan in nicheas delimitedby someclimatological and edaphic factors. tropicalcoutries, rare in sub tropics.There are threespecies From this study the distributionarea of D. cymosumwould o'fDichapetalum in southernAfrica (Dyke 1975). seem to extendfrom the north-westernlimits of the loose Kalaharisands, in southernAngola (Cunene River basin - Dichapetalumcymosum, is a well-knownpoisonous mediumcourse), across north-easternNamibia (Okavango rhizomatousplant endemic to SouthernAfrica, and knownto River basin and the endorreicdry system of various occurin cedain areas of the dry savanna, mostly in the Kalahari omiramba),across Botswana(southwest of the Okavango Basin(Dreyer 1975, Smith 1966). lts occurrencehas also Deltaand of the MakgadikgadiPan and northof the Molopo beenreported in Angola(south), Zambia (south), Zimbabwe andacross the Limpopo River basins) into South Africa, across (west),Botswana (central), Namibia (north-east) and South the Marico/Crocodile,Limpopo, Nyl, Pienaarsand Elands/ Africa(north-west) (Vahrmeijer, 1981). D. cymosumcan be Olifantriver basins. typifiedas a dwarfshrublet with deep-seated,rhyzomatous roots.The leavesare allternately arranged and are generally The generalecological characteristics of the distributionarea soft and hairywhile the inflorescence,an axillarycyme, of D. cymosumseems to be a sandy,well drained and nutrient produceswhite flowers that developsinto drupaceous fruits. poor soil, low to moderate(400 - 600 mm) rainfalland The leavesare very poisonousto stock,though stock avoid temperaturesranging between a meanmaximum of 36'C and the plantswhen other green herbage is available.The toxicity a mean minimumof -4"C. For most of the area these of the leaveswas knownsince the earlysettlement days. ln "gifblaar" conditionsoccur along the ecotonal area separating formations SouthAfrica the commonname is derivedfrom the of Baikiaea/Pferocarpusfrom formationof Colophospermum toxic natureof the leaves. Amazingas it might seem, the and or Catophractes.This ecotoneis characterizedby the pulpof the fruitis not toxicand the Bushmeneat the pulpof presence of Burkea africana, Terminaliasericea, Acacia thefruits. The nameof the plantin the nativelanguage in the "Hainomata", "Don't erioloba,Combretum spp. and Ochnapulchra, and varies in surveyedarea of Angola is meaning bite width from very narrowat both ends to much wider in the the seed". middlesection. D. cymosumis absentwhen the combined thresholdof -4"C and 400 mm of rain, in sandy soils A second species of Dichapetalumoccurs in Okavango: (Rietfontein- Namibia)is reached,while Burkea africana Dichapetalumrhodesicum. lt is verysimilarto D. cymosumin extendsnorthwards outside the range of thesetwo parameters all morphologhicalaspects, but not much is knownabout its intoareas typified by an averagerainfall above 700 mm anda toxicityor lackof it. lt is a very rare speciesonly found in a rockysubstrate. The -4"Ctemperature distribution limitation, restrictedarea. however,still seems to hold. Fromthis it mightbe concluded that frost is possiblyone of the primarycontributing factors Knowingthat in Namibiathe occurrenceof the plantis haltingthe distributionof D" cymosum. Althoughgenerally restrictedto the north-easternpart of the country,which falls occuringin soilof a sandynature there is a smallrange of insidethe KalahariBasin, with soils consistingmostly of variationinside the Kalahariarea and a more important aeoliansands, a set of surveyswere conductedacross the variationinside the Bushveldcomolex in Transvaal.The territoriesof the Kavango,Bushmanland and Hereroland possiblerelation between the pH of the soilon the distribution (west,east and south)to determinethe differentveld types of the plantis stillunresolved. And on the otherside it seems and theirrelationship with the distributionand habitatof the thatbiotic factors can at leastlocallv influence the distribution species.For the benefitof thestock-farming sector, all known of the soecies. occurrenceof the plantare pinpointedand all areaswhere the plantwas not foundare indicated.A generalmap of the plant'sdistribution in Namibiaand some detailed regional maps are included,as aresome areas in south-westernAngola and areasof its distributionin Botswanaand SouthAfrica. AGRICOLA2OOO 1q sand MATERIALSAND METHODS undulatedbecause of the existenceof consolidated dunes,ancient dry river beds (knownlocally as omiramba) plateauis 1200 m, but at 1. Generalprocedure and oans. Mean altitudeof the placesit can reach 1300m up to as much as 200 m above and well Directstudy of some areasof SouthernAngola, Kaokoland, low-lyingomiramba and pans. The mostextensive whichdissects Ovambo,the Easternedge of the CentralPlateau and most definedomuramba is the Omuramba Omatako, endingin the of Kavango,Bushmanland and Hererolandtook placeusing the areafrom the south-westto the north-east, pans, Khabi,are all existingroads and tracks and sometimesusing straight OkavangoRiver. Larger Nye-Nyeand Thereis a marked transectsby compassacross the bush. The botanical situatedsouth of Tsunkwein Bushmanland' sandmantle from the compositionwas notedevery time that changes in vegetation decreasein the depthof the dunesand permanent drainagetakes soiland topography were visible, and this was checked against northto thesouth. The only surface water from the topographicmaps and the distanceswere registeredusing placevia the OkavangoRiver, but very little ends up in the the speedometer. Vegetationmaps of all the areas were dry omirambafrom the Namibianterritory compiledbut they will be the subjectof differentpublications. Okavango. Informationon thevegetation, soils, climate and physiography of neighbouringterritories not visitedwas studiedthrough 4. Glimate existingliterature in orderto derivea completepicture of the globalplant ecologY of the area. The area is generalyhot and dry, with an averagerainfall rangingbetween 350 and 550 mm and mean maximumand -6'C, In 2. The study area minimumtemperatures of 36 and respectively. Namibia,the climateof the surveyedarea is semi-aridand + mm in the north- The regionsurveyed in Namibiacovers an area of sub-tropical.Rainfall decreases from 600 + (Rietfontein- approximately450 km longand 250 km wideextending from east (Kavango)to 400 mm in the south-west gradient the mean annual the OkavangoRiver in the north(Lat. 18'S) to the Rietfontein Otjinene).There is almostno on (34.6'Cat Rundu Districtof the Hererolandterritory in the south(Lat. 22'S) and maximumtemperatures from north to south transitionis different fromthe Botswanaborder in theeast (Long. 21"E) to theedge and32.4"Cat Gobabis),but the seasonal whileat Gobabis of the centralplateau GrootfonteinMaterbergiSteinhausen in inthat in Runduthe hottestmonth is October (Weather 1986). However' the west (Long. 18"E). The total lengthof the distribution it is December Bureau,Pretoria, in winter is higher band. which runs in a north-wesUsouth-eastdirection from the mean annualminimum temperature (-4.5"C),which can be of Angolato SouthAfrica, is approximately3 000 km,with outliers (0.6"C)in Runduthan in Gobabis unusualat Rundu, aroundthe ChobeDepression, in northernKalahari. ecologicalimportance, because frost is occurringbetween the lastweek of Juneand the firstweek of from May to 3. GeomorphologYand drainage July only, while at Gobabis it is common September(Weather Bureau, Pretoria, 1986)' With the exceptionof the southernmostpart of the strip of distribution,in theTransvaal, South Africa, the restof thearea 5. Soils is a flat and sandyplateau at an altitudeof 1100to 1700m' (1941) classifiedthe with no mountainsor largerocky hills. Drainagetakes place ln SouthernAfrica, Van der Merwe groups, whichhe denominated via a few major rivers,e.g. the Cunene,Okavango, Zambezi differentsoils into many oneof into two sub-groups' and Limpopo,and is typicalof semi-aridareas, where some KalahariSands, further sub-divided on Limestone.The of the tributariesare usuallyintermittent rivers' A complex KalahariSands Proper and Kalahari Sands of the KalahariBasin endorreicsystem of ancientriver beds and largedepressions firstsub-group occurs in the central area peripheryof the first. and pansis prominentin Botswanaand Namibia.Slopes are and the secondin a narrowstrip on the and depth of the very gentle towards the rivers and depressions' However, The distinctionis basedon the nature or limestone someof the largerorniramba are deeplycarved in a V-shape underlyinghorizon, with or withouta calcrete paid the distributionof D. acrossthe sandyplains, e.g. Omatako, Daneib, and Eiseb,at layer.Although attention was to the vegetationand least in some sections. cymosumand its generalcorrelation with soils,no systematiccollecting of soilsamples took place. The area alongthe rightbank of the MaricoilimpopoRivers on the SouthAfrican side of the borderis similarto the area 6. Vegetation mentionedabove, but south-eastwardsthe area is rugged' gradientin