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Abstract

This paper follows the transition from ethnobotany to a deeper scientific understanding of the food and medicinal properties of African agroforestry tree products as inputs into the start of domestication activities. It progresses on to the integration of these indigenous trees as new crops within diversified farming systems for multiple social, economic and environmental benefits. From its advent in the 1990s, the domestication of indigenous food and non-food tree species has become a global programme with a strong African focus. This review of progress in the third decade is restricted to progress in Africa, where multi-disciplinary research on over 59 species has been reported in 759 research papers in 318 science publications by scientists from over 833 research teams in 70 countries around the world (532 in Africa). The review spans 23 research topics presenting the recent research literature for tree species of high priority across the continent, as well as that in each of the four main ecological regions: the humid zone of West and Central Africa; the Sahel and North Africa; the East African highlands and drylands; and the woody savannas of Southern Africa. The main areas of growth have been the nutritional/medicinal value of non-timber forest products; the evaluation of the state of natural resources and their importance to local people; and the characterization of useful traits. However, the testing of putative cultivars; the implementation of participatory principles; the protection of traditional knowledge and intellectual property rights; and the selection of elite trees and ideotypes remain under-researched. To the probable detriment of the upscaling and impact in tropical agriculture, there has been, at the international level, a move away from decentralized, community-based tree domestication towards a laboratory-based, centralized approach. However, the rapid uptake of research by university departments and national agricultural research centres in Africa indicates a recognition of the importance of the indigenous crops for both the livelihoods of rural communities and the revitalization and enhanced outputs from agriculture in Africa, especially in West Africa. Thus, on a continental scale, there has been an uptake of research with policy relevance for the integration of indigenous trees in agroecosystems and their importance for the attainment of the UN Sustainable Development Goals. To progress this in the fourth decade, there will need to be a dedicated Centre in Africa to test and develop cultivars of indigenous crops. Finally, this review underpins a holistic approach to mitigating climate change, as well as other big global issues such as hunger, poverty and loss of wildlife habitat by reaping the benefits, or ‘profits’, from investment in the five forms of Capital, described as ‘land maxing’. However, policy and decision makers are not yet recognizing the potential for holistic and transformational adoption of these new indigenous food crop opportunities for African agriculture. Is ‘political will’ the missing sixth capital for sustainable development?
Sustainability2022,14,2355.https://doi.org/10.3390/su14042355www.mdpi.com/journal/sustainability
Article
TheFutureofFood:DomesticationandCommercialization
ofIndigenousFoodCropsinAfricaovertheThird
Decade(2012–2021)
RogerR.B.Leakey
1,
*,MarieLouiseTientcheuAvana
2
,NyongPrincelyAwazi
3
,AchilleE.Assogbadjo
4
,
TafadzwanasheMabhaudhi
5,6
,PrasadS.Hendre
7
,AnnDegrande
8
,SithabileHlahla
5
andLeonardManda
4
1
InternationalTreeFoundation,106108CowleyRoad,OxfordOX41JE,UK
2
AfricanForestForum,WorldAgroforestryCentre,UnitedNationsAvenue,
NairobiP.O.Box3067700100,Kenya;avanatie@yahoo.fr
3
DepartmentofForestryandWildlifeTechnology,CollegeofTechnology,TheUniversityofBamenda,
BambiliP.O.Box39,Cameroon;nyongprincely@gmail.com
4
LaboratoryofAppliedEcology,FacultyofAgronomicSciences,UniversityofAbomeyCalavi,
Cotonou05BP1752,Benin;assogbadjo@yahoo.fr(A.E.A.);lmanda8@gmail.com(L.M.)
5
CropScience,SchoolofAgricultural,EarthandEnvironmentalSciences,UniversityofKwaZuluNatal,
Pietermaritzburg3201,SouthAfrica;mabhaudhi@ukzn.ac.za(T.M.);hlahlas@ukzn.ac.za(S.H.)
6
InternationalWaterManagementInstitute(IWMIGH),WestAfricaOffice,
PMBCT112Cantonments,AccraGA015,Ghana
7
WorldAgroforestryCentre,NairobiP.O.Box3067700100,Kenya;p.hendre@cgiar.org
8
WorldAgroforestryCentre,YaoundéP.O.Box16317,Cameroon;a.degrande@cgiar.org
*Correspondence:rogeleakey@btinternet.com
Abstract:Thispaperfollowsthetransitionfromethnobotanytoadeeperscientificunderstanding
ofthefoodandmedicinalpropertiesofAfricanagroforestrytreeproductsasinputsintothestartof
domesticationactivities.Itprogressesontotheintegrationoftheseindigenoustreesasnewcrops
withindiversifiedfarmingsystemsformultiplesocial,economicandenvironmentalbenefits.From
itsadventinthe1990s,thedomesticationofindigenousfoodandnonfoodtreespecieshasbecome
aglobalprogrammewithastrongAfricanfocus.Thisreviewofprogressinthethirddecadeis
restrictedtoprogressinAfrica,wheremultidisciplinaryresearchonover59specieshasbeenre
portedin759researchpapersin318sciencepublicationsbyscientistsfromover833researchteams
in70countriesaroundtheworld(532inAfrica).Thereviewspans23researchtopicspresentingthe
recentresearchliteraturefortreespeciesofhighpriorityacrossthecontinent,aswellasthatineach
ofthefourmainecologicalregions:thehumidzoneofWestandCentralAfrica;theSahelandNorth
Africa;theEastAfricanhighlandsanddrylands;andthewoodysavannasofSouthernAfrica.The
mainareasofgrowthhavebeenthenutritional/medicinalvalueofnontimberforestproducts;the
evaluationofthestateofnaturalresourcesandtheirimportancetolocalpeople;andthecharacter
izationofusefultraits.However,thetestingofputativecultivars;theimplementationofparticipa
toryprinciples;theprotectionoftraditionalknowledgeandintellectualpropertyrights;andthe
selectionofelitetreesandideotypesremainunderresearched.Totheprobabledetrimentofthe
upscalingandimpactintropicalagriculture,therehasbeen,attheinternationallevel,amoveaway
fromdecentralized,communitybasedtreedomesticationtowardsalaboratorybased,centralized
approach.However,therapiduptakeofresearchbyuniversitydepartmentsandnationalagricul
turalresearchcentresinAfricaindicatesarecognitionoftheimportanceoftheindigenouscropsfor
boththelivelihoodsofruralcommunitiesandtherevitalizationandenhancedoutputsfromagri
cultureinAfrica,especiallyinWestAfrica.Thus,onacontinentalscale,therehasbeenanuptake
ofresearchwithpolicyrelevancefortheintegrationofindigenoustreesinagroecosystemsandtheir
importancefortheattainmentoftheUNSustainableDevelopmentGoals.Toprogressthisinthe
fourthdecade,therewillneedtobeadedicatedCentreinAfricatotestanddevelopcultivarsof
indigenouscrops.Finally,thisreviewunderpinsaholisticapproachtomitigatingclimatechange,
aswellasotherbigglobalissuessuchashunger,povertyandlossofwildlifehabitatbyreapingthe
Citation:Leakey,R.B.R.;Tientcheu
Avana,M.L.;Awazi,N.P.;
Assogbadjo,A.E.;Mabhaudhi,T.;
Hendre,P.S.;Degrande,A.;Hlahla,
S.;Manda,L.TheFutureofFood:
DomesticationandCommercializa
tionofIndigenousFoodCrops
inAfricaovertheThirdDecade
(2012–2021).Sustainability2022,14,
2355.https://doi.org/10.3390/
su14042355
AcademicEditor:MichaelS.Carolan
Received:11January2022
Accepted:9February2022
Published:18February2022
Publisher’sNote:MDPIstaysneu
tralwithregardtojurisdictional
claimsinpublishedmapsandinstitu
tionalaffiliations.
Copyright:©2022bytheauthors.Li
censeeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon
ditionsoftheCreativeCommonsAt
tribution(CCBY)license(https://cre
ativecommons.org/licenses/by/4.0/).
Sustainability2022,14,23552of77
benefits,or‘profits’,frominvestmentinthefiveformsofCapital,describedas‘landmaxing’.How
ever,policyanddecisionmakersarenotyetrecognizingthepotentialforholisticandtransforma
tionaladoptionofthesenewindigenousfoodcropopportunitiesforAfricanagriculture.Is‘political
will’themissingsixthcapitalforsustainabledevelopment?
Keywords:agroforestry;foodsecurity;geneticimprovement;indigenousfruitsandnuttrees;
livelihoods;nutrition;policy;povertyalleviation;ruraldevelopment;treecrops
1.Introduction
Theideaofdomesticatingtraditionallyimportantindigenoustreesasnewcropsin
thetropicsfortheirusefulandmarketablefoodandnonfoodproductsoriginatedinthe
1980s[1].Thisidea,whichincludesbothcultivationandgeneticimprovement,wastaken
upbytheWorldAgroforestryCentre(ICRAF)in1993,followingaconferenceinEdin
burghin1992aimedat‘RebuildingTropicalForestResources’andtheadventofWoody
PlantorReallyGreenRevolution[2].Sincethen,ithasbecomeapantropicalprogramme
expandingacrossthefirsttwodecadesinboththenumberofcandidatespeciesfordo
mesticationandtherangeofresearchtopicsinvolvedinitsmultidisciplinaryapproach
[3–10]targeting14ofthe17UNSustainableDevelopmentGoals[11,12]. Nontimberfor
estproductsarecurrentlybelievedtobeusedbyabout44%oftheglobalpopulation[13].
Tensofthousandsoftreespeciesaroundthetropicsandsubtropicsproduceboth
edibleandmedicalproductswhichhavebeenimportantinthedaytodaylivesoflocal
people.Withtheexpansionofagriculturetofeedagrowinghumanpopulation,manyof
thesespecieswereclearedfromthelandtomakewayforasmallnumberofstaplefood
crops,oftenwithlittleregardeitherfortheirtraditionalandculturalsignificanceortheir
diverseandhighlynutritiousfruits,nutsandleaves.Overthefirsttwodecadesoftheir
domestication(1992–2002;2002–2012),researchsoughttodeveloptechniques,skillsand
strategiescapturingthemanypotentialsocialandeconomicbenefitsthatthecultivation
oftheseproductscoulddeliverthroughcommunityagroforestryprojectsandexpanded
marketingandtrade.Inaddition,researchanddevelopmentprojectshavealsobeenini
tiatedtoprocessandaddvaluetotheproductsbothlocallyandinternationally.Trees,
especiallyindigenoustrees,playanimportantroleinthecaptureofenvironmental,social
andeconomicbenefits/profitsfromagriculture,describedas‘LandMaxing’[14],byin
vestinginthefiveformsofcapital(natural,human,social,physicalandfinancial)andso
convertingthesocalled‘inevitabletradeoffs’tosustainablepolicy‘tradeons’[15].
Thehighlightsofthefirsttwodecades[16]wereasfollows:
Greaterunderstandingofethnobotany,traditionalknowledgeofindigenousspecies
andtheirdeliberatecultivationbyfarmers;
Theapplicationofvegetativepropagationtechniquesincommunitybasedvillage
nurseriesusingmarcotting/graftingtocapturematuretraitsofindividualtreesand
stemcuttingsforthemultiplicationofelitetreesasputativecultivars;
Thedevelopmentofappropriatenurserytechniquesforthesexualandasexualprop
agationofcandidatespeciesinremotevillages;
Thedevelopmentofparticipatoryprioritysettingfortheselectionofcandidatespe
ciesfordomesticationandintegrationintoagroforestrysystems.Some50localspe
cieswereidentifiedforfurtherresearchanddevelopmentaroundthetropicsand
subtropics;
TheestablishmentofRuralResourceCentrestoprovide:(i)knowledgeandskillsfor
theparticipatorydomesticationoflocalspeciesusingappropriate,lowtechnology
techniquesadaptedforuseinremotelocationswithminimalinfrastructureinways
thatallowlocalcommunitymemberstobenefitfromtheirowninitiativesand(ii)the
Sustainability2022,14,23553of77
capacitytodevelopcommunityengagementandinfrastructuretomaximizeenviron
mental,socialandeconomicbenefits;
Thequantificationatthevillagelevelof3‐to10foldcontinuoustreetotreeintraspe
cificvariationincommunityfarmlandtounderstandtherangeofgeneticvariation
availabletolocalcommunitiesanditsaccordancewithlocalknowledge.Subse
quently,thisknowledgewasformulatedtoidentifymarketspecificideotypes.Pri
marily,itwasmorphologicalvariationthatwasinvestigated,butthiswaslaterex
pandedintonutritionalandotherbiochemicaltraits;
Thestartoftheevaluationofgeneticvariationusingmoleculartechnologies;
Theexaminationofcommunal,socioeconomic,legalandpoliticalissuesaffecting
boththeadoptionandimpactofdomesticatingindigenousfoodandnonfoodtree
speciesandthemarketingoftheirAgroforestryTreeProducts(AFTPs).
Attheendoftheseconddecade[16],itwasenvisionedthatthedevelopmentsinthe
thirddecadewouldincludethefollowing:
Improvedcaptureofontogeneticallymaturephenotypesbybetterunderstandingof
thephysiologyofgraftingandmarcotting;
Analysisofvariationsinnutraceutical,pharmaceuticalandotheringredientsof
AFTPstomeettheneedsofnewmarketopportunitiesandtheidentificationofmar
ketorientedideotypes;
Greaterinvestmentinpostharvestprocessingtoexpandlocal,regionalandglobal
tradeopportunities;
Investigationofreproductivebiologyforincreaseduseofcontrolledbreedingincen
tralizedtreedomesticationresearch;
Upscaledtreedomestication,especiallyinAfrica,focusingonspecieswithimpacton
incomegenerationandnutrition;
Impactanalysisbasedonwelldefinedcriteriaandindicators;
Abetterunderstandingoftheroleofdomesticatedagroforestrytreesintheachieve
mentofsustainablemultifunctionalfarmingsystemsandwiderlocalandregional
marketing;
ExpandedopportunitiesforsuccessfulmarketingofAFTPs,includingpublic–private
partnershipsincommercialmarkets;
Recognitionofintellectualpropertyrightstoprotectpoorfarmersandlocalcommu
nitiesfromunscrupulousentrepreneurs;
Enhancepolicysupportfortheupscalingofintensifiedagroforestrysystems.
ThisreviewexaminesprogressmadeinAfricaoverthethirddecade(2012–2021),as
reportedinthescientificliterature.Thus,itpresentsanupdateonreportsofthework
doneinthefirsttwodecades,ratherthanastateoftheartreviewacrossthethreedec
ades.
ProgressintheThirdDecade
Weevaluatedthepublishedliterature,bothbyregion/agroecologicalzoneandby
researchtopic/disciplineandfromapanAfricanpolicyperspective,usingInternetdata
basessuchasGoogleScholarandResearchGate,aswellasourpersonalknowledgeand
networks.Asimilarapproachinthefirstandseconddecades[16]examined16research
topicsandover50speciesfromaroundtheworld.Inadditiontoasetofmoregeneral
panAfricanresearchpapers,thepresentreviewisfocussedonfourAfricanregions(Hu
midWestandCentralAfrica,SahelandNorthAfrica,theHighlandsandDrylandsofEast
AfricaandtheSavannaandMiomboWoodlandsofSouthernAfrica).Thesepaperswere
thenpartitionedintoasetof23researchtopics(Table1).

Sustainability2022,14,23554of77
Table1.Thelistof23researchtopicsusedtocharacterizethescientificliteraturerelatingtothe
domesticationofindigenousAfricanfoodandnonfoodtreespecies.
NumberResearchTopic
1Thestateofnaturalresourcesandtheirgeneticdiversity,governanceandman
agement—understandingtheissues/constraints.
2Ethnobotanyoffoodandmedicinalspeciescandidatesforcultivation—includ
ingtheirroleindiets,healthandfoodsecurity.
3Commercialvalueofnontimberforestproducts(NTFPs)inlocalmarkets.
4Nutritional/medicinalvalueofwildNTFPs.
5Prioritysettingfordomestication/commercialization—speciespreferences.
6Characterizationofusefultraits:morphologicalandphysical.
7Assessmentsofgeneticvariationanditscharacterization.
8Germplasm,germinationandreproductivebiology.
9Vegetativepropagationofcandidatespecies.
10Selectionofelitetreesandideotypes.
11Productcommercialization—evaluation/marketabilityandprocessing.
12Domesticationconceptandstrategy.
13Testingofputativecultivars.
14Farmercapacitybuilding—roleofRuralResourceCentres,etc.
15Implementationofparticipatoryprinciples.
16Newtechniques,skillsandstrategies(includingsupportforgenderissues).
17Nurserydevelopmentsandimprovedplantingstock.
18Postharvestissues.
19ProtectionofTraditionalKnowledgeandIntellectualPropertyRights.
20Tradeandthelaw.
21LocalimpactsofcultivationandmarketingofAgroforestryTreeProducts
(AFTPs).
22Potentialregionalandinternationalimpactsofcultivationanddiversificationof
farmingsystemsandagroecology.
23
Policyrelevancetoupscalingrural/urbandevelopmentandtoglobalsustaina
b
ility(hunger,malnutrition,poverty,wildlifehabitat,livelihoodsandsocialeq
uity,socialjustice,economicdevelopment,etc.).
2.OverviewoftheLiterature
2.1.Regional
Wereviewedatotalof759articlespublishedbetween2012and2021from4Regions
ofAfrica,recognizingsomeoverlapbetweensociopoliticalboundariesandbroadecolog
icalzones.Outofthesepublications,39%originatedfromHumidWestandCentralAfrica
(HWCA),20%fromSahelianWestandNorthAfrica(SWNA),9%fromHighlandandDry
EastAfrica(HDEA)and15%fromtheSavannahandMiomboWoodlandsofSouthern
Africa(SMWSA).Atotalof17%wereofpanAfrican(PA)importance.Intotal,thisis
morethandouble(125%)thetotalnumberofAfricanpaperspublishedoverthefirsttwo
decadescombined(Figure1).
Sustainability2022,14,23555of77
Figure1.Thetotalnumberofresearchpublicationsrelatingtothedomesticationofindigenoustrees
inAfrica,bydecadeandregion.
2.2.ResearchTopics
Whenthe759publicationsweredividedbyresearchtopic,thosefrommosttopics
hadgreatlyincreasedinnumber,withsomenewtopicsalsobeingverywellrepresented,
especially:(i)thenutritional/medicinalvalueofNTFPs(see:[17]);(ii)theevaluationofthe
stateofnaturalresourcesandtheirgeneticdiversity,governanceandmanagement;(iii)
theethnobotanyofcandidatespecies;and(iv)thecharacterizationofusefultraits(Figure
2).However,topicsthatwerenotrepresentedmorefrequentlywere:(i)thetestingofpu
tativecultivars,(ii)theimplementationofparticipatoryprinciples,(iii)protectionoftra
ditionalknowledgeandintellectualpropertyrightsand(iv)theselectionofelitetreesand
ideotypes(Figure2).AtthepanAfricanlevel,threetopicswerewellrepresented.These
were(i)thedomesticationconceptandstrategy,(ii)thepotentialimpactofcultivationand
(iii)thepolicyrelevanceandimplicationsforSustainableDevelopmentGoals(Figure2).
Figure2.Thetotalnumberofresearchpublicationsrelatingtothedomesticationofindigenoustrees
inAfricabyresearchtopic(seeTable1)anddecade.
Sustainability2022,14,23556of77
Impressively,inadditiontothisgrowthinthenumberofpublicationsrelatingtotree
domestication,therewasahugeincreaseinboththenumberofresearchteamsinuniver
sitiesandresearchinstitutesconductingthiswork(833intotalfrom70countries[532from
34countriesinAfrica;182asresearchpartnersfrom17countriesinEurope;37from11
countriesinAsia;66fromfivecountriesintheAmericas;and16from2countriesinOce
ania];seeSupplementaryMaterialsTableS1).Interestingly,thisgrowthwasalsoassoci
atedwithmultidisciplinarypartnershipsandagreatlyincreasedrangeandnumberof
sciencejournalspublishingthesepapers(290journalsand28books—seeSupplementary
MaterialsTableS2).
2.3.Species
Intheliteraturefortreedomesticationduringthethirddecade,59Africanspecies
wereidentified(Table2),generallywithdifferentspeciesineachRegion,althoughafew
dryzonespecieswerereportedfromotherregions,notablybaobab(Adansoniadigitata)
beingimportantinthreeregions.Awiderrangeofethnobotanicallyimportantspecies
werealsodiscussed,indicatingthepotentialforfurthergrowthinthefuture.
Sustainability2022,14,23557of77
Table2.Africantreeandotherwoodyspeciesproducingedibleproductsthathavebeenidentifiedasprimecandidatesfordomesticationintheliteratureover
thethirddecade(speciesinboldhavepanAfricanpriorityintheregion).
HumidLowlandsofWestandCentralAfricaSahelianDrylandsofWestandNorthAfricaHighlandsandDrylandofEastAfrica SavannahandMiomboWoodlandsofSouthernAfrica
A
llanblackiaspp.Vitellariaparadoxa
A
dansoniadigitataSclerocaryabirrea
Prunusafricana
A
dansoniadigitata
A
llanblackiastuhlmannii
A
dansoniadigitata
Irvingiaspp.SclerocaryabirreaZizyphusmauritiana
A
zanzagarckeana
DacryodesedulisTamarindusindicaVitellariaparadoxaUapacakirkiana
GarciniakolaZizyphusmauritianaFicusnatalensisStrychnosspinosa
Colaspp.
A
zanzagarckeanaBalanitesaegyptiacaStrychnoscocculoides
Gnetumspp.
M
oringaoleifera
lbiziacoriariaDovyaliscaffra
RicinodendronheudelotiiBorassusaethiopum
M
arkhamialuteaVangueriainfausta
ChrysoplyllumalbidumCarapaproceraTamarindusindicaParinariacuratellifolia
TetrapleuratetrapteraParkiabiglobosaVitexdonianaSizygiumcordatum
CanariumschweinfurthiiVitexdonianaVitexmombassaeVitexspp.
PentacletheramacrophyllaDaliumguineenseSclerocaryabirreaFlacourtiaindica
BaillonellatoxispermaGarciniakolaZizyphusmauritiana
TrichoscyphaaciminataXylopiaaethiopica
A
frostyraxlepidophyllusDetariummicrocarpum 
GnidiaglaucaBalanitesaegyptiaca
PausinystaliajohimbeProsopisafricana
Tetracarpidiumconophorum
A
rganiaspinosa
GarcinialucidaIrvingiagabonensis
Gambeyaafricanum
Sustainability2022,14,23558of77
3.AdvancesinDomesticationTechnologiesandTheirApplication
Thefirstandseconddecadesofagroforestrytreedomesticationwerestronglybased
onthedecentralizedconceptofparticipatorydomestication[18]andabottomupphilos
ophyforveryrapidprogressarisingfromthevegetativepropagationofelitetreesselected
atthevillagelevel.Thefoundationofthiscommunitybasedapproachwasthedevelop
mentofRuralResourceCentresbyICRAF[18]toassistparticipatingcommunitiesacquire
theskills,knowledgeandunderstandingoflongknown,simplehorticulturaltechniques
suchasgrafting,airlayering(marcotting)andtherootingofcuttings,whichcanbeim
plementedbylocalpeopleinremotevillageswithoutaccesstofinancialcapital,andutility
servicessuchaselectricityandpipedwater.Itis,however,importanttorecognizehere
thatthereisaninadequateresourceofpeoplenationally,regionallyandinternationally
withtheappropriateskillsinvegetativepropagationtomeetthescaleoftheneedforthese
techniquesifparticipatorydomesticationistobescaledupacrossAfrica.
Participatorydomesticationprovidesthebasisforthedevelopmentofaselfhelp
strategyforcommunityengagement[19,20],leadingtothedevelopmentof‘sociallymod
ifiedcrops’[14,15].Itisfocussedontheintensificationofagroforestryformultipleenvi
ronmental,socialandeconomicbenefitsaspartofthethreestepapproachtorapidlyre
ducinghungerbyclosingtheYieldGapsinstaplefoodcropswhilealsoaddressingmal
nutrition,povertyandsocialinjustice[15,21,22].Thishasimportantpolicyimplications
forAfricanpeopleandfortheplanet(seeSection10below).
Inthethirddecade,therehasbeenreducedresearchactivityinParticipatoryDomes
ticationledbytheWorldAgroforestryCentre(ICRAF)dueprimarilytoalossofdonor
funds.Thisislessalossofsupportfortheconceptandmoreaneedtomakefundsavail
abletoothercauses.Meanwhileanewsourceoffundingbecameavailableforamore
centralizedtopdownbiotechnologicalprogrammefocussedonalaboratoryapproachto
cropimprovementbygeneticcharacterizationandtreebreeding[23].Itisbasedonthe
acquisitionofknowledgeofthemechanismsofgeneticvariationinmodernsciencelabor
atories,againledbytheICRAF[24].Interestingly,inparallelwiththischangeindirection
bytheICRAF,therehasbeenasubstantialincreaseindomesticationrelatedresearchin
Africanuniversitiesandresearchinstitutes(SupplementaryMaterialsTablesS1andS2),
mostofitinsupportoflocallivelihoodsandlanduses.Itmustbehopedthatdecentralized
treedomesticationhasnotsuccumbedtotheprevailingviewthatacademicadvancement
ismoreimportantthanreallifeimpact[25].Indeed.Itisimportanttorecognizethata
participatoryapproachtodomesticationwithbenefitsflowingtolocalcommunitiesises
sentialwhenthetraitsbeingselectedarerootedinTraditionalKnowledge[18].
Whilethecentralisedapproachisrelativelyeasytocoordinate,itsresultsdonotal
waysfilterdowntosmallscalefarmers,whoencounterhightransactioncostswhenre
ceivingexternalfarminputs,suchastreeplantingmaterialandinformationontheman
agementofcultivars[23].Thus,thecentralisedapproachdoesnotpromotelocaldevelop
mentbasedoncommunitydecisionmakingaboutwhichspeciestoprioritiseforcultiva
tion;thedevelopmentofefficientfarmmanagementmethodsinasmallholdercontext;or
theresolutionofsocial,economicandpoliticalbarriersfacedbysmallholderfarmers[23].
Thus,perhapstothedetrimentofshorttermupscalingandimpact,treedomestication
studiesinthethirddecadehavemovedtowardsamuchlongertermstrategythanthe
rapidandhighlysuccessfuldecentralizedcommunityapproach.Thisshiftinstrategywill
requireameanstodisseminateimprovedgermplasmtofarmerslackingthefinancialre
sourcestopurchasetheplantingmaterial[26].Inadditiontosomeindigenousfruitand
nuttreespecies,thiscentralizeddomesticationprogrammeadditionallyincludesover50
annualorphancrops(cereals,legumes,etc.)asimportantunderstoreyadditionstoagro
forestrysystems.Thisisaimedatfurtheringdiversificationoffarmingsystemsanddiets
[27];however,theirsuccessfulintegrationintocomplexagroecosystemsmayrequirese
lectionforhigheryieldsinshadyenvironments.
Sustainability2022,14,23559of77
Itisimportanttoemphasiseherethatthesetwodifferentstrategiesarenotmutually
exclusive.Historically,treecropshavebeendomesticatedfromthewildovermillennia
usingdecentralisedhorticulturalapproachesbasedontechniquesofvegetativepropaga
tiontocreatecultivarsthatcapturetheuniquecharacteristicsofnaturallyoccurringelite
individuals.Overrecentdecades,centralisedtreebreedingprogrammesbetweenthese
cultivarsandwildrelativeshasledtonewgenerationsofselectedvarieties.
Itisclearfromtheabovesummariesofthesetwostrategiesthattheylieatopposite
endsofadomesticationstrategyspectrum(Figure3)andthattheyhavedifferentmerits
relevanttothedomesticationofindigenoustreesandtoneedsoftropicalfarmersinthe
short‐andlongterm.Anumberofreviewpapershavebeenwrittenaboutthemeritsof
bothapproachesandtheiruseinrecentyears(Table3),includingtheconceptofasystems
approach[28].So,wewillnotreplicatetheinformationhere.However,toassistdevelop
mentagenciesanddonorstounderstandtheirdifferencesandlikelyimpacts,adetailed
cost–benefitanalysiswouldbehelpful.
Figure3.ThespectrumofdomesticationstrategiesformingtheDomesticationSyndrome:Central
izedformajorfoodcropsanddecentralizedfornewtreecrops(Reprintedwithapermisionfrom
ref.[26].2019JohnWileyandSonswithunderLicenseNumber5250250091483).
Table3.Sourcesofinformationaboutdifferentdomesticationstrategiesandtheiruse.
DecentralizedParticipatoryApproachCentralizedLaboratoryApproach
Leakey[14,15,18,29]Dawsonetal.[26,30]
Takoutsingetal.[31]Hendreetal.[24]
Degrandeetal.[19,20]McMullinetal.[27]
Essougongetal.[32]Jamnadassetal.[33]
Wouapietal.[34]
Finally,therecentlyrepublishedtreedomesticationstrategyforindigenousfruit
trees[35]presentsoverarchingprinciplesfortheestablishmentofthreeinterlinkedtree
populations:generesourcepopulation,selectionresourcepopulationandproductionre
sourcepopulation.Thestrategyincludeswaysinwhichsciencebasedprinciplesforlow
technology,appropriatemethodsofvegetativepropagationcanbeusedinremotearea
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withoutaccesstoelectricityandpipedwaterandyetmaintainthegeneticdiversityofthe
preferredtraitsoftheelitemothertree[36,37].Thisisthebasisoftheconceptofdecen
tralizeddomestication.
4.PanAfricanPrioritySpecies
Inthissection,wereviewtheprogressmadetodomesticatethosespecieswhichfor
ecologicalreasonsarefoundnaturallyacrosstwoormoresociopoliticalregionsinthe
continentofAfrica.Itisimportanttorecognizeherethatthesespeciesaremuchmorethan
‘FamineFoods’[38,39]andarehighlyappreciatedbylocalpeopleformanydifferentrea
sons.
4.1.Adansoniadigitata(Baobab)
4.1.1.Ethnobotany,UsesandNaturalResources
Adansoniadigitataisalarge,deciduous,droughttoleranttreereaching18–25min
heightandwithatrunkupto10mindiameter[40,41].Itistheonlyspeciesofitsgenus
inAfricaandisdistributedacrossthedrylandsstretchingfromSenegaltoSudanandfrom
EthiopiatoNatal.Itprovidesawidevarietyofnontimberforestproducts(NTFPs),alt
houghtheseareleastusedinsouthernAfrica[40–42].
Inadditiontoitsimportantediblefruits,baobabhasbeendeclaredtobeoneofAf
rica’smostimportantleafyvegetables[43],asseenregardingthelivelihoodsoflocalcom
munitiesinMali[44].Consequently,ithasbeenidentifiedasahighpriorityspeciesfor
domesticationacrossAfrica.Baobabproducts—leaf,fruitpulpandseedkernels—are
widelymarketedandhavebeenapprovedfortradeas‘novelfood’intheEuropeanCom
mission.Baobabisadaptabletoadverseclimaticconditions,suggestingitspotentialto
contributetoclimateresilientstrategiesinAfrica.Everypartofthetreeisused:roots,
bark,wood,leaves,flowers,capsules,gum,seedsandfruitsforfood,fibreandmedicine.
Itsediblefruitsandleavesarerichinvitamins,makingthemimportantfornutrition.The
barkfibresarealsoimportantproductsformultipleeverydayuses.Acrossthecontinent,
over300usesofthetreehavebeenrecorded.Itsmedicinalproductstreatuptomorethan
20diseases.Theseareusedforsubsistenceand/orsoldbylocalcommunitiestogenerate
income,makingasignificantcontributiontowardspovertyalleviationinruralareas.
InSudan,anethnobotanicalsurveyfound25differentusesforbaobabproducts
[45,46]withpreferencesforsweetleavesandlargefruitswithahighyieldofwhitefruit
pulp,asopposedtoacidicand/orslimypulpandbitterleaves.Rashford[43]hasreviewed
leafpalatability,nutritionalcontentandseasonalconsumptioninSubSaharaAfrica,with
widevariationinconsumerspreferences[47].InBurkinaFaso,bothgenderrelatedand
villagerelateddifferencesinuseandmanagementhavebeenrecognized,indicatingthe
needtoseeklocallyappropriatemanagementsystems[45].Theidentifiedtraitswerealso
consideredtobeimportantindicatorsofindividualcandidatetreeswithpotentialfordo
mestication.InKenya,baobabisrecognisedasbeingimportantfornutritionaldiversity
inthestaplediet.Fruitpulpisalsoconsumedassnacks,primarilyfreshwithoutanypro
cessing[48].However,about60%ofrespondentsstoredfruitsforlaterconsumptionwhen
nutritiousfoodwasscarce.Householdsalsosellbaobabfruitstoaugmenttheirincome
andthustopurchaseotherfoods(45%)andtopayforhealthcare(13%)andeducation
(23%).InSudan,thesaleofbaobabproductscouldbebothnegativelyandpositivelyaf
fectedbydifferentinternalandexternalfactors,butthosethatlifthouseholdsoutofpov
ertyarebeneficialtoruraldevelopment[49,50].Thisrequiresappropriateinstitutional,
technicalandfinancialsupport.
InKenya,whenamultistakeholderapproachwastakentotheinitiationofacom
munitybaseddevelopmententerpriseforbothbaobabkerneloilandpowderedfruit
pulp,thetargetimpactsweretobenefitcommunitylivelihoodsandtheircapacitytoman
ageandutilizetheresource,aswellastoenhanceconsumptionandfoodsecurity[39].
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TopromotetheconservationanddomesticationofbaobabacrossAfrica,Gebaueret
al.[51]comprehensivelyrevieweditsecology,diversityandcurrentutilizationandpre
sentedideastoencouragefurtherstudiesandinvestmentinthespecies.Theseinclude
studiesofitsphenotypicandgeneticvariation,populationstatus/uses,socioeconomicsof
management/processingandtrade,nutritionalvalue,horticulturalproduction,ecophysi
ologyanditsrootsystem.Inaddition,domesticationrelatedstudieshaveconsideredthe
potentialtocultivateyoungbaobabtreesinaleafproductionsystem[43].
4.1.2.GeneticVariationandDomestication
PhenotypicCharacterizationandProvenanceVariation
Anumberofstudiesthisdecadehavesoughttogainadeeperunderstandingofthe
variationinbaobab.Thefirstmadeaproximateanalysisof178fruitsamplesfrom11Sa
helianWestAfricasitesinBurkinaFaso,MaliandNiger[52]andevaluatedthevariability
infruitmorphology,aswellassugarandvitaminCcontent.
Likemostsuchstudies,therewasahighlevelofintraspecificvariationbetweenin
dividualtrees,buttherewasalsoalinearrelationshipbetweenrainfallandvitaminC
contentacrossthesesites,withvitamincontentbeinghigherinsiteswithlowerprecipita
tion.Anegativerelationshipwasfoundbetweenfruitsizeandbothlatitudeandlongi
tude,whiletherewasapositivecorrelationbetweensugarcontentandlatitudeandlon
gitude.Theauthors,however,concludedthatthelevelsofvitaminCandsugarwithin
sitevariationmeritedamoredetailedstudytoidentifysuperiortrees.Asomewhatsimilar
studyof10provenancesinMali[53]confirmedsomeofthesecorrelations.Thestudyalso
foundthatpulpironcontentwaspositivelycorrelatedwithtopsoilsodicityandbasesat
uration,whilepulpvitaminCcontentcorrelatedpositivelywithtopsoilsandfraction,
waterandbasesaturation.Furthermore,significantnegativecorrelationswerefoundbe
tweenrainfallandpulpvitaminCcontentandbetweenmeanannualtemperatureand
fruitandpulpweightandpulpfraction,suggestingthatthesetraitsareinfluencedbythe
environment[53].
InapanAfricanstudyof17baobabprovenancesfromKenya,Tanzania,Zambia,
Zimbabwe,MalawiandMali,significantvariationwasfoundinthepulpmoisture,pro
tein,fibre,ashandelementalcontentamongprovenances[54].However,thereweredif
ferencesatthenationallevelinmosttraits,indicatingopportunitiesforselection,bothfor
germplasmconservationandfordomestication.TwofollowupstudiesinKenyafound
treetotreevariationinawiderangeofnutritionalparameters.Inthefirst,treesfrom
coastalpopulationswithlargerfruitsproducedhigherfruityieldsthanthoseinland[55].
Someelitetreeswereselectedwithhighfruitweight,highpulpproportionandinterme
diateorsweettastingfruitpulpinthetworegions.Thesecondstudyinsixpopulations
confirmedtheimportanceofintraspecificvariationbetweenindividualtreesatthepopu
lationlevel[56],especiallyiniron,zincandvitaminC.Despitethecorrelationsreported
abovebetweenfruittraitsandthelocalenvironment,theseauthorssuggestthatgenetics
playastrongerroleinnutritionalvariationthantheenvironment.Thus,theseresultsfur
therillustratetheimportanceoftreetotreevariationinthedomesticationandcultivation
ofthisspecies.
InGhana,itwasconcludedfromstudieson14morphologicaltraitsthatthediversity
ofgeneticvariationmakesitsdomesticationhighlyappropriate[57].Tofurtherdetermine
thepotentialtodomesticatebaobabforitsnutritionalqualities,astudyhasexaminedthe
fattyacidcompositionofseedoilinprovenancesfromacrosstheAfricancontinent[58].
Thisidentifiedsignificantlydifferentmeanprovenancevariation,withthefollowingover
allmeans:saturatedfattyacids(17–22%),monounsaturatedfattyacids(32–38%)andpol
yunsaturatedfattyacids(22–26%).Palmiticacid,oleicacidandlinoleicacidwerethemost
abundantofeachofthesefattyacids,respectively.Inparallelwiththis,thenutritional
contentofleavesshowedhighvariabilityacrossgenotypesfrom36populations,which
couldbeplacedin4varianceclusters[59].Theseauthorsrecommendedtreebreeding
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betweendivergentpopulationstodevelopgenotypeswithhighernutritionalvalues,as
wellasgreatertoleranceofbioticandabioticstress.
Together,theabovestudiesillustratethepotentialtoselecteliteindividualbaobab
treesfordomesticationandexpandedcultivationindrylandAfrica[51]andthatfurther
researchontreegeneticsisneededtocomplementethnobotanicalknowledge[46]toen
surethatthecultivatedtreesmeettheneedsofthepeople.
Toaidinthisresearch,germplasmconservationanddomesticationprogrammes,ba
obab‘descriptors’,havebeenpublished[60].Theseinclude‘passport’descriptorsdefining
theparametersthatshouldbeobservedwhentheaccessioniscollected;‘management’
descriptorsforaccessionsinthegenebank;‘environmentandsite’descriptors,whichare
sitespecificparametersforcharacterizationandevaluationtrials;and‘site’descriptors
forgermplasmcollecting.Studieshavefoundthatdescriptorstodiscriminatebetween
phenotypeswereequallyexpressedacrossallenvironments.
GeneticVariation
UnlikeotherspeciesinthegenusAdansonia,baobab(A.digitata)istetraploid.How
ever,astudyofthevariationinfloralandpollencharactersandchromosomenumberin
specimensfromAfricahasidentifiedanewdiploidspecies,whichhasbeenfoundtoco
existwithbaobabbelow800maltitudesinAfricabutisreportedtoextendupto1500m
[61].IthasbeencalledAdansoniakilimasp.nov.andissuperficiallysimilartoA.digitata,
butthespeciescanbedifferentiatedonthebasisoffloralmorphology,pollenandchro
mosomenumber.However,byusingacombinationofphylogeneticanalysesandstatis
ticalcomparisonsofvarioustraitssuchasflowers,stomataandchromosomecounts,Cron
etal.[62]foundthatA.kilimaisneithercytologicallynormorphologicallydistinct,andthe
authorshavethusreducedtheproposedspeciestosynonymywithA.digitata.
Sixstudieshavesoughttogainabetterunderstandingofthegeneticvariationun
derlyingthemorphologicalandnutritionalvariationofbaobab.Thefirstwasaseedling
provenancetrialtoassistabreedingprogrammeforleafproductioninanirrigatedhedge
systeminMali.Itinvolved14provenances—10fromMali,2fromNigerand2from
BurkinaFaso.Highlysignificantdifferencesinleafproductionwerefoundamongprove
nancesandfamilieswithinprovenances,withmoderatetolowheritabilityestimatesfor
leafproductivity[63].Strongcorrelationsbetweendiameterandleafproductionindicated
thepotentialtousediameterasapredictorforleafproductionvisàvisitsseasonality.
SomewhatsimilarprovenancetrialswithseedlingswerealsoconductedinMalawi[64–
66].Ingeneral,WestAfricanprovenancesgrewfasterthanthosefromEastAfricabutwith
weakcorrelationsbetweenleafmorphologicaltraitsandclimate,althoughtheleaflet
numberwassignificantlygreaterindrierareas.TheMalawitrialshowedthatthemain
landpopulationsweregeneticallydistantfromtheoneontheisolatedLikomaIsland.A
studyinMalawialsosoughttodeterminewhetherthemorphometrictraitsinfruitand
seedcharacteristicswithinandbetweenprovenancescoulddelineatepopulationsfrom
differentzonesintolandraces.Theresultsfoundthatsingleseedweightshowedpro
nouncedevidenceofdivergenceofpopulationsintoecotypes[67].
Theotherthreestudiesusedmicrosatellitestoassessgeneticdiversity,thefirstin
sevencoastalandinlandKenyanpopulations[68],thesecondinpopulationsoftheNuba
MountainsofSudan[69]andthethirdinMalawi[67].Allthesegeneticstudiesalsofound
highmorphologicaldiversityinfruitsand/orleaves.Interestingly,therewasevidencethat
somebaobabmorphotypesretainedleavesduringthedryseason[70].Withinthehigh
diversityinthefruitlength,widthandlength/widthratioalongaclimategradientfrom
theBlueNiletoNorthKordofaninSudan,clavateandcrescentshapeswerefoundonly
inNorthKordofan,whilearhomboidshapewasonlyfoundintheBlueNile[71].In
Kenya,thegeneticdiversitywashigh,withlittledifferencebetweencoastalandinland
sites,despitelimitedgeneflowbetweenpopulations.Likewise,therewaslittledifference
ingeneticdiversitybetweensitesinSudan,althoughtwodistinctgenepoolswereob
served,withtheoneclosetoamainroadbeingmorediverse,suggestinganinfluenceof
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humanintervention[72].InMalawi,moderategeneticdifferentiationwasobserved
amongpopulationsindicatingthepresenceofalargenumberofcommonalleles,resulting
inahomogenisationeffect.Theauthorsattributedthelowgeneticvariationfoundtomar
ginalizedpopulations,anthropogenicfactorsandfoundereffects.
Aphysiologicalstudytoexaminegeographicalvariationindroughttoleranceusing
plantsfromMaliandMalawifoundthatbaobabrespondedtowaterstressbyleafshed
dingandallocatingmorebiomasstotherootsystem,whileregulatingphotosynthesisand
transpirationbystomatalcontrol[73].Waterstoredinthetaprootwasusedtoprotectold
leavesandfortheformationofnewleavesandroots,withasignificantcorrelationbe
tweentaprootwatercontentstomatalclosure.Interestingly,thetwopopulationshaddif
ferentdroughtstrategies,suggestingthatselectionfordroughttolerancemaybepossible.
Domestication
InNiger,farmershavechosenabaobabbasedagroforestryprogrammethatsuits
theirlivelihoodneedsforitspotentialtoimprovetheirhouseholdfoodsecurityandin
come[74].AstudyofseedgerminationtoenhancetheperformanceofseedsofA.digitata
inNigeriafoundthatasoakin98%sulphuricacidforanhourpriortosowingimproved
establishment[75].However,insouthernAfrica,domesticationbeganwiththeselection
ofclonalpropagation,especiallyinZambiaandMalawi[65].Totakeadvantageofthe
highintraspecificphenotypicandgeneticvariationwithinandbetweenpopulations,veg
etativepropagationtechniqueshavemanyadvantages.Onestudyreviewingthelitera
tureontheuseofvegetativepropagationinbaobabreportedonly30%rootingsuccess
withIBAhormone[76]andrecommendedtheneedforfurtherstudies,especiallyforthe
rootingofcuttingsandmarcotting.Anjarwallaetal.[77]andJenyaetal.[40],however,
reviewed‘topcleft’and‘sideveneer’graftingtechniquesinKenyaandMalawi.Topcleft
graftingshowedslightlymoresurvivalsuccessthansideveneer(71%vs.55%).Using2
yearoldrootstockwasslightlymoresuccessfulthan1yearoldones.ThestudyinMalawi
alsofoundthattopcleftgraftshadhighersuccessratesthansideveneergrafts.Theresults
indicatethatbaobabiseasilyamenabletograftingwhendoneattherighttimewiththe
correctsizeofscions—thesameisprobablytruefortherootingofcuttings[37].Bothstud
iesrecommendedgraftingforuseindomesticationprogrammes.Nevertheless,whenus
inggrafting,attentionmustbepaidtotherisksofgraftincompatibilityandtothecommon
situationthatshootgrowthfromtherootstockcanreplacetheintendedgrowthfromthe
selectedscion.Marcottsandrootedcuttingsontheirownrootsavoidtheserisks[37].
4.1.3.CommercializationofProducts
Asdiscussedearlier,nutritiousbaobabproductsareveryimportantlocallyacross
dryAfricaandhavepotentialmarketsaslifestylefoodproductsinEuropeandintheUSA
[78].Indeed,thankstoprocessingandpackaging,theyhavealreadygainedaccesstoin
ternationalmarkets[43]withseveralhundredformulationsinEuropeanmarkets,follow
ingapprovalfortradebytheEuropeanCommission.InKenya,womenhavebeenthe
mainactorsinthebaobabvaluechainnetworks,principallytradingunprocessedpulp
knownlocallyas‘mabuyu’,madeoutofbaobabpulpcoveredseeds,sugarandfoodcol
our[79].Thiscandyincreasesinvaluefrom<1USD/kgpaidtofruitcollectorsuptoUSD
1.5paidbytheconsumer.Thisisanimportantsourceofextraincomeforruralhouse
holds,especiallyinthedryseason,suggestingthatthereisgreatpotentialtoincreasemar
ketincomethroughthedomesticationofbaobabtrees.
Toenterinternationalmarkets,foodproductshavetomeetfoodsafetystandards
associatedwithprocessingandproductquality.InMalawi,abaobabprocessingsector
hasemergedtosupplybothfoodandnonfoodproductstolocalconsumers,basedon
smallscale,informalenterprises.StudiesbyDarretal.[78]haveshowntheimportanceof
valueadditionthroughapprovedstandardsofprocessingandpackaging.Baobabfruits
mayhaveotherpotentialuses.Forexample,thetransesterificationofbaobaboilhasbeen
carriedouttoassessthepotentialofbaobabkerneloilforuseasbiodiesel[80].Theresults
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showthatimportantfuelpropertiesofbiodieselfrombaobaboilmetboththeEuropean
andAmerican(ENandASTM)standards.Baobabisoneoftwospeciesexaminedtoeval
uateconcernsaboutecologicalsustainabilityandinequalityarisingfromtheincreased
formalizationoftradeinnaturalproducts[81].Thestudyconcludesthatbetterinformed
andmorerespectfuloflocalknowledgeisneededtoensurerespectforcustomarylaws.
MarketingConcerns
ConcernshavebeenraisedthatdevelopinginternationalmarketsforAfricanindige
nousfoodproductscoulddamagelocalsupply,environmentaldegradationandlossof
localdietarydiversity.Theseconcernsforsubsistenceusersofbaobabweregreatestin
WestAfrica,wherethereisahighdependenceontheseproducts,unlikesouthernAfrica,
wherebaobabhasbeenreportedtobe‘underutilized’andwherecommercializationcould
helpreducepoverty[42].Thisconcernwasthesubjectofaconference“Bridgingthegap
betweenincreasingknowledgeanddecreasingresources”intheCzechRepublicin2014,
whichexaminedtheseissues[82].Themeetingconcludedthatresearchshould“address
theimprovementofthelongtermfoodsecurityandnutritionoflocalcommunitiesinthe
targetregionsby(i)ensuringtheavailabilityofandaccesstobaobabproductswithhigh
nutritionalvalue,(ii)increasingtheuseofbaobabproductsindailydiets,and(iii)raising
incomesfromsellingrawandprocessedbaobabproductsofhighnutritionalvalue”.
Theseobjectivesconcurstronglywiththeinitiativestodomesticateindigenousfoodtrees
asnewcropsforsubsistencefarmers[3,14].Inaddition,Chivandietal.[83]haveidentified
theneedforresearchtofocusonhowtotapintohealthbenefitsofindigenoustreeprod
uctsforpeopleandanimalsandtodevelopnewproductssuchasnaturalsweetenersand
pectinsforindustrialuse.Theyalsoemphasisetheenvironmentalandlivelihoodbenefits
arisingfromincreasedcommunitybasedsustainableutilisationandconservationofthese
treespecies.
Concernshavealsobeenraisedaboutincreasedgrazingpressureonnaturalregen
eration.Studieshaveascertainedthatbaobabpopulationsdodeclineunderheavylive
stockgrazing,butthattheyaresustainableundermoderategrazingpressures[41],asseed
productionissubstantial[84].However,thepredationoffruitsbybaboonsalsoleadsto
populationdecline,asdoesdroughtduetofailureofthe‘rains’.Theseauthorsdidcon
clude,however,thatplantingandprotectionfromlivestockisrequiredtoovercomere
cruitmentbottlenecks.
4.2.Sclerocaryabirrea(Marula)
4.2.1.Ethnobotany,UsesandNaturalResources
TherearethreesubspeciesofSclerocaryabirreainAfrica:subsp.birrea,subsp.caffra
andsubsp.multifoliolata.Theirnaturaldistributionspansdrylandsin29Africancountries,
buttheyprimarilygrowindifferentareasinAfrica.Subsp.birreaisfoundintheSaheland
northernAfrica,andsubsp.caffraisfoundintheMiomboWoodlandsofsouthernAfrica.
InEastAfrica,bothsubspeciesarefound,whileinTanzaniaalonesubsp.multifoliolatais
native,butitcanalsobefoundtogetherwiththeothertwosubspecies.Undergoodcon
ditions,thetreecangrowupto15–20m,withameandiameterof80–100cmatmaturity
[85].Generally,S.birreaoccursinareasreceiving200–1600mmrainfallperannumon
sandytoloamysoils.However,inMalawi,S.birreathrivesmainlyinhot,dryareasat
altitudesof500–1000mwithameanannualrainfallof900–1000mmandmeanannual
temperatureof22to23°C.Thespeciesalsothrivesonhydromorphicsoilswithlimited
drainage,whichareseasonallywaterlogged[85].
TounderstandthepopulationstabilityofmarulaintheBushbuckRidgeareaof
SouthAfrica,treedensityandsize–classdistributionprofileswereevaluatedinareaswith
differentlanduses.Treedensitywashighestinhomegardensduetodiscardedseedsfrom
householdbeermakingandlowestincropfields[86].Femaletreedensitieswererela
tivelyconstantacrossalllandusetypes,suggestingthatthecombinationofculturaland
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economicvaluescanconservespecies,althoughharvestingthemforfuelwoodinrange
landsmayposeathreat.Theauthorsconsiderthatsuchfindingsmayberelevanttoother
socialecologicalsituationswherenaturalresourcesareimportantforsustainingliveli
hoods.
AmaturetreeofS.birreacanproduceasmanyas91,000fruits.Thesecanbeeaten
freshorfermentedtomakebeer[85],whiletheediblekernelsarerichinahighlynutri
tiousoil.TheleavesandthebarkofS.birreaalsohavemedicinaluses,andthewoodcan
becarvedintoutilitarianitemssuchasspoonsandplatesordecorativeanimalfigures
[85].Inaddition,thespeciesisabrowseforlivestockandprovidesshadeforcerealcrops.
WithinmanyAfricancommunities,thefruitsandkernelsinparticular,havegreat
culturalandceremonialvalue[85].Nevertheless,thenaturalresourceisunderconsider
ablepressureduetotheextensiveclearingofwoodlandsforsettlementandagriculture,
whichthreatentheecosystemservicesandgeneticdiversityofthespecies[85].Despite
this,asurveyinsouthernAfricafoundthatlocalcommunitieshadpositiveattitudesto
wardstheconservationofmarulatreesandwerealsokeentoparticipateintheirdomes
tication[87].Itwasalsoclearthatthereisaneedformoreinformationaboutlocalman
agementstrategies[88],includingitsvulnerabilitytowildlifepressuresfromfruitand
seedpredation,especiallyfromelephants[89].AnethnobotanicalsurveyinNamibia
foundthat87%ofthepopulationhadindigenousknowledgepasseddownfromtheir
parents,withamajorityindicatingthatthespecieswasinadeclinenotbeingaddressed
byanymanagement[90].
InZimbabwe,astudycomparedthedistributionandstemdensitiesofmarulainnat
uralvegetationandonfarmsandfoundthattherewerenearlytwiceasmanytreesin
farms(7vs.4stems/ha),withfemaletreesoutnumberingmales(55vs.34%)[91].Theau
thorconcludedthatthisistheresultoffarmerprotectionoffruitproducingfemaletrees
andadvocatedtheneedforfurtherworktodomesticatethisspecies.Anotherstudyex
aminedtheharvestingandutilizationofthespeciesbysmallholdercommunities[92,93]
andfoundthat49%ofhouseholdsconsumedripefruitsraw,while76%alsomadeatra
ditionalfermentedwine/beerand54%consumedthekernels.Theutilizationofthese
productsisamajorsourceofincomeforlocalpeoplemeetingtheeconomicneedsofpoor
communitiesinruralareaswhilealsocontributingtofoodandnutritionalsecurity[85].
Indeed,thelargerfruitedS.birreasubspeciescaffraismorecommonlycultivatedashome
steadtreesinsouthernAfricathanissubspeciesbirreaintheSahel,duetoitstendencyto
havelargerfruits.
4.2.2.GeneticVariationandDomestication
MarulawasoneoftheprioritytreespeciesidentifiedfordomesticationbytheICRAF
andlocalfarmersinthe1990s.When21provenances(20ofS.birreassp.caffraand1ofS.
birreassp.birrea)fromMalawi,Zambia,Zimbabwe,Namibia,Botswana,Tanzania,
MozambiqueandMaliwereplantedinMangochi,Malawi,thereweresignificantdiffer
ences—uptothreefold—betweenthepopulationsformostvegetativegrowthtraits[94].
StudiesinNamibiaandSouthAfricafoundsomeevidencethatfarmershaveinitiated
domesticationbyselectingdesirabletreesinthelands[95].TheMarracuenepopulation
fromMozambiquewasrankedtopforheight,diameteratbreastheight,crownwidthand
crowndepth.After7years,only8populationshadfruits,ofwhichtheMarracuenepop
ulationhadthegreatestfruityieldwithconsiderabletreetotreevariationinfruitproduc
tionrangingfrom1to1228fruitspertree[94].Afollowupstudythenfoundsignificant
variationamongtheprovenancesinthenumberoffruits,fruitweight,pulpweight,seed
weight,fruitlengthanddiameter[85],withnocorrelationbetweenthenumberoffruits
andotherfruittraits.Theauthorsrecommendedthatstudiesshouldinvestigatefruittaste
andotherqualitytraits.Aweakrelationshipbetweenfruitingandvegetativegrowthsug
geststhatgrowthshouldnotbeusedasapredictoroffruiting[96].Likewise,onlymod
eratecorrelationshavebeenreportedbetweenfruityieldandcrowngrowth.Thus,more
workisneededtounderstandanypossiblerelationshipsbetweenvegetativegrowthand
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fruiting.Relevantheremaybetherecentlyreportedprovenancedifferencesintheirsus
ceptibilitytoleafpredationbyinsectpestsinMalawi[97].
Geneticvariation,especiallytreetotree,inmanyparametershasimportantimplica
tionsfordomesticationofthisspecies[36,95,98].Inafollowupprovenancestudyfive
yearslater[99],phenologicaltraitsdifferedbetweenprovenances,althoughwithsome
overlap.LikemanyindigenousfruitandnutspeciesinAfrica,marulaisadioeciousspe
cieswithmaleandfemaletrees.Theearliestfloweringwasintheeasternprovenances
(MozambiqueandSwaziland)andlaterinothers.Theearlyfloweringgenotypesfruited
andmaturedbetweenAugustandJanuary,whilethelaterprovenancesfruitedfromSep
tembertoMay,withmaletreesfloweringslightlyearlierthanfemales,butsometrees
classifiedasfemaleinoneyearwerefoundtohavesomemaleflowersinthenextseason.
Fruityieldsalsodifferedbetweenyears(2016/2017versus2017/2018),indicatingprobable
environmentalinfluencesonphenology.Theseresultssuggestaneedformoredetailed
phenologicalstudies.ThisisespeciallythecaseintheSahel,wherethefloweringseason
inSenegal(FebruarytoMarch)isonlyafewmonthsafterthatinthesouthernhemisphere,
withfruitingfromApriltoJuly[100].However,atthemostsoutherlysite(Amaly),there
wasasmallamountoffloweringfromOctobertoNovember.Nevertheless,markeddif
ferenceswereobservedbetweenandwithinsitesfortheleafingandfloweringphasesand
asignificantdifferencewithinand