The Role of Trees in Agroecology and Sustainable Agriculture in the Tropics

25 Pages·2014·1.2 MB·English

PY52CH06-Leakey ARI 19July2014 13:35
The Role of Trees in
Agroecology and Sustainable
Agriculture in the Tropics
g
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w Roger R.B. Leakey
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alr DepartmentofMarineandTropicalBiology,JamesCookUniversity,Cairns,Australia,
nu QLD4870;email:[email protected]
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hol. 2014.52:113-130.242.36.112 on 08/ AFMinransytu9p.,uR2be0lvi1s.4hPehdyotonpliantehoals.a20R1e4v.ie5w2:1in13A–d3v3anceon Kamguerolytfiwofuronersctdtrisyo,nbailoadgirviecruslittuyr,eb,isoulostgaiicnaalbnleitrinotgeennsififixcaattiioonn,treedomestication,
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yt phyto.annualreviews.org Abstract
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v. Thisarticle’sdoi: Shiftingagricultureinthetropicshasbeenreplacedbysedentarysmallholder
Re 10.1146/annurev-phyto-102313-045838
u. farmingonafewhectaresofdegradedland.Toaddresslowyieldsandlow
nn Copyright(cid:2)c 2014byAnnualReviews. incomeboth,thesoilfertility,theagroecosystemfunctions,andthesourceof
A Allrightsreserved
incomecanberestoredbydiversificationwithnitrogen-fixingtreesandthe
cultivationofindigenoustreespeciesthatproducenutritiousandmarketable
products. Biodiversity conservation studies indicate that mature cash crop
systems,suchascacaoandcoffeewithshadetrees,providewildlifehabitat
thatsupportsnaturalpredators,which,inturn,reducethenumbersofher-
bivores andpathogens.This review offerssuggestionson howtoexamine
theseagroecologicalprocessesinmoredetailforthemosteffectiverehabil-
itationofdegradedland.Evidencefromagroforestryindicatesthatinthis
way,productiveandenvironmentallyfriendlyfarmingsystemsthatprovide
foodandnutritionalsecurity,aswellaspovertyalleviation,canbeachieved
inharmonywithwildlife.
113
PY52CH06-Leakey ARI 19July2014 13:35
INTRODUCTION
The future of agriculture is the focus of much international debate (25, 39, 42, 56). The main
areaofconsensusisthatitneedstobemoresustainableandmoreproductive,andthatbusinessas
usualisnotanoption(40,54,56,85,108).Theproblemsarenumerousandcomplex,especiallyin
thetropicsandsubtropics,whereinteractionsbetweenbothbiophysicalandsocioeconomicissues
areaffectingagriculturalproductivity,mainlythroughacombinationoflanddegradationdriven
bypopulationgrowthandadecliningresourceofproductivelandforagriculture.Thishasledto
farmersbecomingsedentarysmallholderswithoutthefinancialresourcestoreplenishsoilfertility
(71,72).
InhisbookTheComingFamine,JulianCribb(25)statesthat“...thechallengefacingtheworld’s
1.8billionmenandwomenwhogrowourfoodistodoubletheiroutputoffood—usingfarless
water,lessland,lessenergyandlessfertilizer.Theymustaccomplishthis...amidmoreredtape,
g economicdisincentives,andcorruptedmarkets,andintheteethofspreadingdrought.”Adding
or
s. moredetailtothisscenario,theInternationalFoodPolicyResearchInstituteinWashingtonhas
w
vie indicatedthattheworldpopulationwillrisefromthecurrent7billiontoapproximately9billion
e
alr by2050,andthattosupportallthesepeople,foodproductionwillhavetoincreaseby70%(57).
u
n However,itisnotclearwhetherthisprojectionisacceptingthestatusquointermsoftheliving
any.
w.nl standardsofthepoorinleastdevelopedcountriesoroftheeffectontheenvironment.Thereal
wo
we needis,ofcourse,tofeedmarginalizedpeopleandalsotoimprovetheirlives.
om al us These projections make pretty gloomy reading, but is it a lost cause? The context of the
d frson problems is important. We need to understand that hundreds of millions of poor farmers are
Downloade14. For per ttforeycrihensngtotltooogcsileuesap.rpTfoohrrtistthnheeewiprrsfaeacdmtiecinleiteaosrfyosnlhififeatsitsnymgleacliulslattirhveeaatrioeosfnu.llatAnosdfsewpdoitephnuotlauarttyioafncacrgemrsosewrtsot,htfheaerntydilhitzhaevereslataconksduorfovtnihveeewr,
3. 13/ aswellasfeedandprovideforallthedailyneedsoftheirfamiliesonapproximatelytwotofive
2014.52:113-132.36.112 on 08/ hgporeoceRtdsauewrgcertasoirvondiftgiyn.cgtlehsaroroleuudgtilhoannbsido,twmeicuthhcnhoouoltfoagthyseoaufnordcceguesonofefitndiccisomcmuosedsiiofiorncatihtsieopnloifloearlriitnzheerdooubfgeshotwcbieeaetlntseewrrsvhoiecitlehhseuirfsbtsoaonmimderptyhraoinnvdge
hol. 0.24 organicfarming.Athirdcommonlyadvocatedsolutionistopaygreaterattentiontoagroecology,
paty 8 whichiscertainlyrelevanttoaddressingthelossofbiodiversityandecosystemfunction.However,
ob
yt achievingasustainablefutureforagriculturewillinvolvemorethanjustimprovingagroecological
h
v. P functions.Instead,itisnecessarytotakeamoreholisticviewofaproblemthatencompassesthe
Re needforimprovedsoilfertilityandhealth;reducedriskofpests,diseases,andweedoutbreaks;
nu. improved crop yields; improved rural and urban livelihoods; and opportunities for economic
n
A growth.
UNDERSTANDINGTHEPROBLEMBEFORESEEKINGSOLUTIONS
Currentestimatesarethatlanddegradationaffectstwobillionhectares(38%ofworldcropping
area) (34), with soil erosion affecting 83% of the global degraded area (6). The problems are
especiallysevereinAfrica,wheremorethan80%ofcountriesarenitrogendeficient,andwhere
nutrient loss has been estimated at 9–58 kg/ha/year in the 28 worst affected countries and 61–
88kg/ha/yearin21others(18).Nitrogen(N),phosphorus(P),potassium(K)deficienciesoccur
on59%,85%,and90%ofharvestedlandarea,respectively(18).
Understandingthecomplexityofthenumerousandinteractingsocioeconomicandbiophysical
causalfactorsunderlyinglanddegradationandtheproblemsofmodernagricultureinthetropics
114 Leakey
PY52CH06-Leakey ARI 19July2014 13:35
andsubtropicsisdifficult.Inthepast,theyhavebeenlumpedtogetherasadownwardspiralin
whichpovertydriveslanddegradationandlanddegradationdrivesfurtherpoverty,butthisisa
grosssimplification(110).Inanattempttomaketheconceptmoremeaningful,Leakey(70)added
somestepstoaconceptualdiagramofthelanddegradationandsocialdeprivationcycle.Inthis
highly interactive cycle, a desire for security and wealth drives deforestation, overgrazing, and
unsustainableuseofsoilsandwater,allofwhichcauseagroecosystemdegradation.Infarmers’
fields,thisisseeninsoilerosion,breakdownofnutrientcycling,andthelossofsoilfertilityand
structure.Theconsequenceofthisdegradationisthelossofbiodiversityandthebreakdownof
ecosystemfunctionsthatregulatefoodchains,nutrientcycling,andtheincidenceandseverityof
pests,diseases,andweeds.Allofthesethingsresultinlowercropyields,whichinturnleadto
hunger,malnutrition,andincreasedhealthrisks,allofwhichmanifestasdeclininglivelihoodsand
soreturnthecycletoadesireforsecurityandwealth.Itisrecognizedthatatallofthestepswithin
thisconceptualdiagram,therearearangeofsocioeconomicandbiophysicalinfluencesthatwill
g
or determinethespeedofthedownwardprogressatanyparticularsite.Suchfactorsincludeaccessto
s.
ew markets,landtenure,andlocalgovernance;externalfactors,suchasnaturaldisastersandconflict
vi
e andwar;andeconomicdrivers,suchasinternationalpolicyandtradeagreements.
ualr Thenetresulthighlightedbytheaboveconceptualcycleoflanddegradationandsocialdepri-
n
any. vationisthatindevelopingcountrieswherefarmersarepoorandmustrelyonasmallpieceof
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wo landforalltheirhouseholdneeds,thebenefitsexpectedfromexistingagriculturaltechnologies,
we
om al us such as improved crop varieties and livestock breeds, are constrained by a need for income to
d frson purchasetheinputsessentialforfoodproduction.Asaconsequence,cropyieldsdeclineasthesoil
ownloade4. For per fb(e1er3ttw5il)ei.teynisthdeepylieetleddp.Totheinstlieaaldosftnoewwhmatoidsecranllevdartiheetieysiealdndgatpheinyfioeloddaccrtoupasll,ywahcihcihevisetdhbeydiafffearremnceer
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2014.52:113-132.36.112 on 08/ ACdeoDfincDvieeRnnctEiioeSns,SablIuNatguGrnicfSuolOrttuuIrnLeatwFeolEyuRilndTmaIdaLvnoyIcToaftYethtehlaetasintodregvaenloicpefderctoiluiznetrrsieasrwehuesreedthtiosnmeaekdeisugpretahteesset
pathol. y 80.24 tehxepefnarsmiveertshaarneitnooinpdouosrtrtiaolipzuedrchcoausenttrhieems..AEnveanltewrnhaetnivaevapirloabmleo,tethdebyyartehetyoprigcaalnlyicmagurcihcumltuorree
ob movementistheuseofmanure,compost,andmulching.Theproblemhereisthatorganicmanure
yt
Ph isseldomavailableinthequantities(10–40Mg/ha/year)neededforlargeareas(80).
ev. Anotheralternativeistoutilizebiologicalnitrogen-fixingtechnologies,suchastwo-yearim-
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u. provedfallows, relay cropping,or Evergreen Agriculture,by which nitrogen-deficientsoilsare
n
n enriched with fast-growing leguminous trees, shrubs, or vines. A study involving sixteen tree
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species found that the rate of nitrogen fixation was higher than the 23–176 kg N/ha/year for
foodorfodderlegumes(55),andinhighdensityplantings,suchasimprovedfallowsorfodder
banks,couldbeashighas300–650kgN/ha/year(91).Together,theseresearchfindingsoncrop
productionconformtothegeneralexperienceofadoptingfarmerswhoreportyieldincreasesof
two-tothreefold.Thecombinationofinorganicfertilizersandbiologicalnitrogencan,however,
besynergistic.
Agroforestryhasalongandwell-documentedhistoryintheuseoflow-cost,biologicalnitrogen
fixationbyleguminoustreesandshrubstorestorenitrogendeficienciesinthesoilsacrossAfrica
andotherareasofthetropicsandsubtropics(109,150).Arecentmeta-analysisof94studiesin
sub-SaharanAfricafoundthatthesetreetechnologiesincreasedmaizeyieldsby0.7–2.5Mg/ha
(or 89–318%) (119). Furthermore, these yield increases were more stable than monocultures
treatedwithrecommendedapplicationsofartificialfertilizer(118).Indirectly,leguminoustrees
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PY52CH06-Leakey ARI 19July2014 13:35
canalsorecycleothermacro-andmicronutrientsthroughuptakefromtheirmoreextensiveand
deeper root systems, which act as a safety net to absorb and prevent the loss of deep nutrients
andsubsequentleaflitterfallandfinerootturnoveratthesoilsurface(87).Bycomparisonwith
artificialfertilizers,nitrogenoftreeoriginislesslikelytobelostto,orpollute,groundwater.
Environmentally, biological nitrogen fixation by leguminous trees and shrubs is important
for soil health, as it increases the organic matter and carbon contents of soil, which improve
soilstructure(aggregatestability,porosity,andhydraulicconductivity),reducesoilerosion,and
promotegreaterwaterinfiltration.Soilorganicmattercontentisimportantinallagroecosystems,
but it is especially important in areas where rainfall can be a limiting factor. Ecologically, the
organicnitrogennottakenupbythecropsisstoredinthesoilorganicmatterpools(114)andsois
availabletootherorganisms,suchasmycorrhizalfungiandearthworms(120–122).Thisnitrogen
resource is less likely to be leached and to pollute groundwater than is the use of inorganic
fertilizers.Legumimoustreeshavealsobeenshowntoconsiderablyimproverain-useefficiency
g
or (theratioofabovegroundnetprimaryproductiontoannualrainfall)(118).Likewise,water-use
s.
ew efficiency was higher in maize intercropped with leguminous trees than in the sole maize (19).
vi
e Ingeneral,theuseofbiologicalnitrogenfixationtoimprovesoilfertilityalsoleadstocropsless
ualr susceptibletopestsanddiseases(113).Onthenegativeside,aswithinorganicfertilizers,there
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w.annly. canbeN2Oemissionsassociatedwithhigherlevelsofsoilnitrogen.
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om al us BIODIVERSITYANDAGROECOLOGICALFUNCTIONS
d frson
ownloade4. For per T(a1ph3pe0li)cr,aoatlnieodnoitfhseaimsconploootgroticacanclcueprrrfeoodrceothsnseeasfulaitnrugraeeg-oerifncgoululotgubhraalslcaagslureiscttuaoilnthuaarbveeiliiistmywpehalalcstreobcneoetgnhneisztgeuldodbi(e1ad3lp7for).orHbmloeawmneyovfeyrlea,anirtdss
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3. 13/ degradation.Thechallenge,therefore,istodemonstrateconclusivelyandtothesatisfactionof
2014.52:113-132.36.112 on 08/ pw(o3ofh9ltii)hlc.eeyCupmaslriaaennk,gteoernfdsac,tcoaurgurorrapislbesru,(ehs1sia1ons3uet)sro.sc,ebasenstduaskatceaanindnaeobmtlyitc.osTitnhhtairstodcdoiuvnceceresspeptmeicsiixedesedtshccrariotbpceopdhinaogsstssusyessrttaeioimnuassbcplaeensitbnsetaenpndrsoidfidicusaectatisiovenes
pathol. y 80.24 proAcebsosvoefwreebsuawildtihnagtisnotielgferarttiilnitgyleagnudmalisnooubsotorseteasgarnodecshorsyusbtsemincfuronpcptiionngs.syIsntermecsocgannitiinointiaotfetthhies
ob roleoftrees,Leakey(68,69)describedagroforestrypracticesas“phasesinthedevelopmentofa
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Ph productiveagroecosystem,akintothenormaldynamicsofnaturalecosystems...andthepassage
ev. towardsamatureagroforestofincreasingecologicalintegrity.Bythesametoken,withincreasing
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u. scale,theintegrationofvariousagroforestrypracticesintothelandscapeisliketheformationofa
nn complexmosaicofpatchesinanecosystem,eachofwhichiscomposedofmanyniches...occupied
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bydifferentorganisms,makingthesystemecologicallystableandbiologicallydiverse.”Overrecent
years,therehasbeenaconsiderableincreaseinthenumberofstudiesexaminingthebiological
diversityassociatedwithcomplexmultistrataagroforestrysystems,especiallycocoa(21)andcoffee
(124),andtheircomparisonwithotherfarmingsystems(23,112;Table1).Manyofthesestudies
find that agroforestry systems maintain a level of biodiversity that is considerably higher than
otheragriculturalsystemsbutgenerallyalittlelowerthanthatofnaturalforest.Again,atypical
findingisthatagroforestsprovidehabitatsuitableforforest-dependentspeciesandso,inmost
cases,areimportantforwildlifeconservation.
Thebelowgroundecosystemisoutofsightandsooftenoutofmind,butitisverycomplexand
cruciallyimportanttotheproperfunctioningofagroecosystems.Lavelle(67)identifiedfourcom-
plexassemblagesoforganismswithdifferentrolesthatdifferinthesizeandnumberofindividuals—
fromthesmallesttothelargest:themicroflora,themicropredators,thelittertransformers,andthe
116 Leakey
PY52CH06-Leakey ARI 19July2014 13:35
Table1 Publishedassessmentsofbiodiversityinagroforestrysystems(seealso84,97)
Organisms Country Crop Reference
Phytotelmataplantsandceratopogonidpollinators Brazil Cocoa 37
Batandbird Brazil Cocoa 36
Bat Brazil Cocoa 35
Ants Brazil Cocoa 30
Soilandlitterfauna Brazil Cocoa 26
Sloth Brazil Cocoa 15
Plants Cameroon Cocoa 125
Soilfauna Cameroon Cocoa 90
Mammals Cameroon Cocoa 3
Birds CostaRica Cocoa 103
org Dungbeetles,mammals(includingbats),birds CostaRica Cocoaand 51,52
ws. banana
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alr Epiphytes Ecuador Cocoa 5,50
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any. Birds Indonesia Cocoa 1
ww.onl Epiphytes Indonesia Cocoa 127
ownloaded from w4. For personal use AARMBimnarottdspnsshkeibyisans,reptiles,andants IIMMInnndddeexxoooiinnncceeeoosssiiiaaa CCCCCooooocccccoooooaaaaa 141124844–5616544
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3. 13/ Homoptera CostaRica Coffee 106,107
2014.52:113-132.36.112 on 08/ BBMiirraddmssmanadlsotherseeddispersalagents DIEnRcoduemipaadiunobirlciacn CCCoooffffffeeeeee 174769
pathol. y 80.24 TSoeirlmcoitleesoptera IMndexiaico CCooffffeeee 4849
ob Mammals Mexico Coffee 38
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Ph Ants Mexico Coffee 95
ev. Arthropod Mexico Coffee 98
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u. Birds Mexico Coffee 99
n
An Birds Mexico Coffee 47
Antsandbirds Panama Coffee 104
Ants Panama Coffee 105
Collembola Indonesia Rubber 29
Mammals Indonesia Rubber,damar, 116
anddurian
Plants Indonesia Rubber 82
Birds Indonesia Rubber,damar, 133
anddurian
Birds,bats,butterflies,anddungbeetles Nicaragua Fallowsand 53
pastures
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ecosystemengineers.Agroforestry,asanagriculturalsystemwithhighabovegroundbiodiversity
and minimum soil disturbance, is recognized to have a diverse rhizosphere associated with the
four belowground species assemblages that together form a soil ecosystem typical of a fully
functionallatesuccessionalstage.Tillage,however,disturbsthesoil,killingmanyofthelarger
organismsandbreakingthefungalnetworks(16).Thisdisturbanceswitchesthesoilecosystem
back to an early successional stage in which some functions are destroyed and must be rebuilt
(8).
Forest clearance rapidly changes the soil microflora from one associated with the trees and
understory to one associated with the invading pioneers (77), with complete clearance of trees
havingmoreseriousimpactsthanpartialclearance.Completeclearancemakestheestablishmentof
treeplantationsmoredifficulteveninthemoisttropics,asreducedsoilinoculumoftheappropriate
mycorrhizal fungi makes it difficult for planted tree seedlings to develop the best mycorrhizal
associations (77) and hence survive and thrive. Once established, however, these trees slowly
g
or rebuildthesoilinoculumoverseveraldecades(148).
s.
ew Importantly,inthefewstudiesthathavelookedattheeffectsofconservingbiodiversityand
vi
e theproductivityofthesesystems,thereislittle,ifany,evidencethatincludingbiodiversitycon-
ualr servationwithinthesewildlife-friendlyperennialcropagroforestshasanynegativeeffectsoncrop
n
any. production. Indeed, it seems that these agroforests can often successfully combine wildlife and
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wo crop-production benefits without the need for further deforestation (20). In effect, when these
we
om al us complexagroforestsalsoincludetreesthatproducehigh-valuemarketableproducts,theyreplace
d frson the natural fallows of shifting agriculture and become income-generating commercial fallows
ownloade4. For per [(ow8g3hn)ii]cz,heiftcnhaoanttbamepaepinrretoandinnuiicantlgiavneddiavnseudrssitstauyinpipanbolaretgrsteohdfeoernleitvsaetrrlyiyhfosayorsmdtesinmogsfslpyorscotaevlmidc.eoMsmacmNwueindeietlyier&sanfgoSerchmorfoatonhpyt(i8do1en)csardefoec-rs
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3. 13/ adaptingtochangingeconomic,social,andclimaticconditions.
2014.52:113-132.36.112 on 08/ imssealhdiSinogolmhy,lafyalotddchdueorssueiergssashboelnsteht,lehaateanebd/nmoeivneaeddtufetoreoecduustsunhcodecneefrsobssctiioauondsndiovafetghrrtsheoieterycoEoclcesoyonosastfgeerbrmivicosau,dtlititoyvunperriewcsaiIitnltylhiytiiinndaotptimhvreoei,ndpiautitcoeitsndiveubeenyrffaolsartrmratgugienensatgrtoeesfeytssah,tgaeartmnoid-st
pathol. y 80.24 eascohsaybstiteamt fsourccweislsdiolinfe(6is9)t.hOatnweecosntislelqhuaevnecaeovferthyepporoerdoumndinerasnttanfodcinugsoonfmthaetuirmepaogrrtoaenccoesyosftetmhiss
ob speciesdiversityinthecontrolofpests,diseases,andweeds.However,recentresearchhasstarted
yt
Ph tofillthisknowledgegapbyfocusingontheagroecologicalfactorsthataffectthephytopathology
ev. ofcropproductivity,especiallyinmatureagroecosystems.
R
u. Regulation processes are complex, but it is recognized that important biological synergisms
n
n areafunctionofplantbiodiversity(2),which,ofcourse,isvariabledependingontemperatureand
A
rainfall,soiltypes,andlevelsofdisturbance.Oneimportantfactorinthepreventionofdisease,
pest epidemics, and weed invasions is the limitation of their dispersal. Other factors are the
densityofherbivores,predators,andothernaturalenemies;thedistanceto,andthediversityof,
naturalvegetation;andtheturnoverrateofcropswithintheseagroecosystemsandtheintensity
oftheirmanagement.Furthermore,ecologicalfunctionsareprobablyalsosubjecttovariationin
speciesabundance,thedensitydependenceoffoodchainsandlifecycles,theratesofcolonization,
reproductive success, and mortality rates, and the ability to locate hosts. In addition, there are
more species-specific factors, such as odors, repelling chemicals, feeding inhibitors, synchrony
of life cycles, etc. One conclusion from agroecological research investigating these factors has
beenthatspeciescompositionismoreimportantthanthenumberofcolocatedspecies(2).Thus,
identifying the best species assemblages needs to be the focus of future research to determine
ecologicalprinciplesforapplicationunderverydifferentcircumstancesaroundtheworld.
118 Leakey
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REBUILDINGAGROECOLOGICALFUNCTIONS
Inectomycorrhizal,andperhapsendomycorrhizal,speciestherearefungiassociatedwithrootsof
seedlingtrees,andastheseedlinggrows,thesefungiremainassociatedwithyoungrootsandradiate
outawayfromthetree’sstem,allowinglate-stagefungitocolonizeclosertothestem.Inthisway,
asuccessionalseriesoffungidevelopsovertime(66,94).Consequently,forlandrestorationand
stimulationofvegetationalcolonization,itisimportantforrapidtreeestablishmenttoinoculate
treeseedlingswithearly-stagefungiinthetreenurserypriortoplanting(65).Likewise,intrees
withendomycorrhizalsymbionts,therapiddevelopmentofthesesymbioticrelationsisimportant,
especiallyonseverelydegradedsiteswheregoodlevelsofseedlingsurvivalarehighlydependenton
mycorrhizalinoculationwiththebestsymbionts(147);however,anysymbiontseemstobebetter
thannone.Foragriculturalproductiononseverelydegradedland,treeestablishmentisaprecursor
tothereestablishmentofthesoilinoculum.Thisinoculumisthenbeneficialtotheinfectionand
g growth of associated crops (49). Mulch from leguminous tree species has also been found to
or
s. encouragethedevelopmentofsoilmicroflora,asevidencedbyincreasedmicrobialbiomassand
w
vie increasedenzymeactivity(79,134).InZimbabwe,forexample,actinomycetepopulationswere
e
alr sixtoninetimesgreaterwhenbiomassofVachelliaandCalliandraspecieswasappliedtothesoil
u
n surfacethanwhenincorporatedintothesoil(79).
any.
w.nl Leguminousfertilizertreeshavealsobeenshowntorapidlypromotethereturnofsoilfauna
wo
we eveninhighlydegradedsoils(119,121).Forexample,inZambia,earthwormdensitiesweretwoto
om al us threetimeshigherinmaizeintercroppedwithVachellia,Calliandra,Gliricidia,andLeucaenaspecies
d frson comparedwithmaizereceivingNPKfertilizer(119),whereasinmaizeplantedafteramixtureof
Downloade14. For per SneusmbPabhneyiartsospoesafbtaehnaorlatohngwdicoTarelmpkhsnrtoohswiaalnevdionggeemliiainiazdneedavlpeolinogpeeoi(nn1g2p1ter)ae.ef-abllaoswedstahgerroeecwoesryesttewmostiostshurbesetatnimtiaelslygrbeaasteedr
3. 13/ onanunderstandingoftheregulationofpests,pathogens,andweedsinfarmingsystemsandin
2014.52:113-132.36.112 on 08/ aesrheclsoriumuslybtistintSeedg.msneisnsu,bmtarhnebediebnuredcoseutfdc-ckeaisnnsosefuewsistnctuaidcdtaieioesegnsess(r2tuom,dn1iin1eca3set)iri.noeIvnanolsolpvfi(eo6thnl0ee)eg.esurIemneadnionsdtooheufeasrrtshlpyleeesgcpuuiacemrcsae.essFsist,oiiocrDneweaxselamesmdotadpgSilueetmr,sitgohasfepahplgee.rrgmoaufcmootnsritenhasoistcurayas,
hol. 0.24 repellenttostemborersofcereals,whereasNapiergrass(Pennisetumpurpureum)attractsthepests
paty 8 awayfromthecrops.Thecombinationoftheseagroecologicalfunctionshasbeendescribedas
ob
yt apush-pulltechnology(repellingandattractingpests)(24).S.sesbanhasalsobeencreditedwith
h
v. P reducingthedispersalofmaizerust(63).Sometreeandshrubspecies(e.g.,Lantanacamara,Melia
Re azedarach,Azadirachtaindica,Tephrosiaspp.)areknowntohaveinsecticidalproperties,andsome
nu. specific insecticidal chemotypes of T. vogelii have been identified (9). However, there has been
n
A concern that some leguminous shrubs also harbor insect herbivores. However, when pure and
mixed-specieslegumefallowsweretestedtherewasnoevidenceofseriousbiomasslossdueto
herbivory(41).Tephrosiafallowshadthelowestpopulationdensitiesof18speciesrecognizedas
pestsoffallows.
Predator-preyinteractionscanalsobemodifiedinearly-stageagroecosystemsbydiversifying
thehabitataroundfieldsbyplantingtreesonthefieldboundaries.Forexample,inChina,pond
cypress (Taxodium ascendens) is planted around rice to provide habitat for spiders that control
leafhoppers;inLatinAmerica,antsprotectcocoafromthearthropodvectorsofdisease.Trees
planted throughout coffee plantations provide habitat for birds, spiders, and ants, all of which
contributetopestcontrol.Thesepropertieshavebeenusedinfarmingsystemsaspartofintegrated
pestmanagementstrategies,inwhichtheneedforpesticidesisreducedbyharnessingthesenatural
processes(33).
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PY52CH06-Leakey ARI 19July2014 13:35
Table2 Publishedinvestigationsofagroecologicalfunctionsinagroforestrysystems
Organisms Country Crop Reference
Parasitoidwasps Brazil Cocoa 126
Midges CostaRica Cocoa 150,152
Antsandbeetles Indonesia Cocoa 10–13
Birds Indonesia Cocoa 20
Spiders Indonesia Cocoa 129
Birdsandbats Indonesia Cocoa 78
Birds Panama Cocoa 138–140
MoniliophthoraandPhytophthoraspp. Peru Cocoa 64
Birdsandarthropods CostaRica Coffee 100
Birds Guatamala Coffee 46
org Birds,arthropods,andfungi Jamaica Coffee 58
ws. Birds,ants,andleafminers Mexico Coffee 30
e
vi Antsandphoridflies Mexico Coffee 93
e
alr Birds Mexico Coffee 101
u
n
any. Birdsandcaterpillars Mexico Coffee 99
ww.onl Mealybug,coffeerust,andberryblotch CentralAmerica Coffee 128
we
om al us
d frson
ownloade4. For per MMAATINUTRAEINAIGNRGOLEACTOESYSSUTCECMESSSIONALOR
3. D13/1 Recently,anumberofstudieshavestartedtoseekagroecologicalunderstandingofpest,disease,
hol. 2014.52:113-130.242.36.112 on 08/ tpafoliohnabkrerdesdtelnsayrwebvopexlafertetteidshtodteeneapcpthraoaogninrbnri/doqlpeaeuatmccehrqoasesussfiyiieotrnsenirtnuesmpgmpmoaeuo.etcnurrTiaderfhteseaierro(asrmTngteaaerohnorbfadsfl.voeitneThr2egeths)oo.teirnfVssyeotseempsprryeyahesccytrietefiteixomocctpenespnansrttithenblybdoe,eaoltoawetrogndfereieec-twrhnpalrrtsceeosteeytuurfidprtdnaieheiildeansassthpeiphosareanavosvcestfethstiticbahapakenislsee,dbnnrsyepiobsmambeatapsshaeerloncedrghiv,poea.autnnnilsoadbntaoiannttfhdog-
paty 8
ob
yt
h
P
v. ShadeModification
e
R
u. Tree shade is important to provide the best growing environment for some crops, especially
n
n thoseoriginatingfromtheforestunderstory.Incocoa,thisshadecanbecontrolledtomanage
A
the incidence of diseases, such as frosty pod rot, which is caused by the fungus Moniliophthora
roreri(64).Likewise,incoffeeplantations,shadetreescanbemanagedtoprovideoptimallight
conditionstominimizetherisksfrompests[e.g.,Cercosporacoffeicola(coffeeberryandleafblotch),
Planococcuscitri(citrusmealybug),andHemileiavastatrix(coffeerust)]andmaximizeconditions
forbeneficialfaunaandmicroflora,eveninareaswithdifferentsoilsandclimate.Shadehasbeen
foundtobemorebeneficialinthedryseasonandshouldbereducedbypruninginthewetseason
(128). Predation of insect pests by canopy birds is greatest when the canopy is not intensively
managed,withtherichnessofshadetreesexplainingmuchofthevariationinbirddiversity(139).
Inanotherstudy,diseaseandinsectattacksweremoreprevalentundersinglespeciestreecanopies
thanundermixedcanopies,supportingthehypothesisthattreediversityminimizestherisksof
pestoutbreaks(11).Inaddition,shadetreesalsoprovidebreedingsitesforbeneficialinsects,such
asmidges,whicharepollinatorsofcocoa(151,152).
120 Leakey
PY52CH06-Leakey ARI 19July2014 13:35
Removalofshadetreeshasbeenfoundtolowertheabundanceandrichnessofbirdsofmost
guilds,includinginsectivorousspecies(101);conversely,theabundanceofinsectivorousbirdswas
greatestwhenthecanopycoverwasdenseandspeciesrichandtherewassomedeadvegetation.
Inconclusion,themultifunctionalroleofshadetreesforagricultureandbiodiversityconserva-
tionisnowrecognized,buttheirimportantroleinriskavoidancefrominsectpestoutbreaksis
inadequatelyunderstood.Nevertheless,itisclearthatadiversifiedfood-and-cash-croplivelihood
strategyispossible(136).
BirdExclusion
Asmallnumberofexperimentshaveinvestigatedtheimportanceofbirdandbatpopulationson
organismsfurtherdownthefoodchaininmultistratacocoaandcoffeeagroforestrysystemsby
settingupexclusionexperiments(58,78,100).Anexperimentinwhichcaterpillarswereplacedon
g
or coffeeplantswithandwithoutbirdexclusionconfirmedthatbirdsdoreducepestoutbreaks,but
s.
ew thiswasonlysignificantinassociationwithhighfloristicdiversity,indicatingtheimportanceof
vi
e speciesdiversityinthehabitat(99).Likewise,Greenbergetal.(45)foundthatexclusionofbirds
ualr alsoresultedina64–80%reductioninlarge(>5mm)arthropodsandconsequentlysomeincreased
n
any. herbivory. Other studies in cocoa and coffee have generally confirmed these findings, showing
w.nl
wo thatexclusiondoesgenerallyincreasetheabundanceoffoliage-dwellinginsectherbivores,with
we
om al us knock-onimpactsonleafdamage(138,140).InIndonesia,theexclusionofbirdsandbatsresulted
d frson ina31%reductionofcropyield,illustratingtheeconomicimportanceoftheseinsectivores(78).
ownloade4. For per FoodChainandLifeCycleStudies
D1
3. 13/ Shadedcoffeeandcocoaagroecosystemswithawell-developedcanopyofshadetreeshavehigh
2014.52:113-132.36.112 on 08/ bfocerifwoohdpeeirvprpebrroeisvsdiottuyrpce,rtsaoi.obwnHleicdomaewnsde,bvimveeerovr,ssettirthlyyiekcoesoflpymmebcpieicleecrsaxou.cinsFlietmoeorrafaetctexhtsaieomanngpsdrleeiea,nctweoprilrtoahegbdoiucaunattoldsrnahnioaccdrheeep,asanp(rdh1ao1sdi7rtiie)vd,eiarflmnsiiedptsy,acanotsefsgwtoahetneihvppearlvreyeedyasafetfaoeenncrdst,
pathol. y 80.24 acnoftfepeobpeurlariteiosn(9s,3a).nTdhbiescinaudsiecaatnestshnoewgsahtiavdeelyimafpfeacctticnogffoenepbheorrryidbfloireesrc,atnheernehiasnmceotrheedbaimolaoggeictaol
ob controlofberryborerbyants.Anotherexampleoftheshadebenefitsinvolvesparasiticwaspsof
yt
Ph thecoffeeleafminer.Leafdamagebytheminerislowerwhentwig-nestingantsareabundant,
ev. indicatingthatantpopulationscontrolleafminersinshadedcoffee(31);thus,throughtheeffects
R
u. ofshade,antsprovideimportantecosystemservicesincoffeeagroecosystems.Perhapssimilarly,
n
n astudyofparasitoidwaspsincocoaagroforestshasfoundthatwaspcommunitycompositionwas
A
influencedbyseason,disturbance,thespeciesrichnessoftheshadetrees,andthescaleofthecocoa
planting(126).However,theimpactsofsuchinteractionscanbecomplexandaffectedbysuch
thingsastheclimatezoneandfeedinghabitsofthebirdspecies.Forexample,largefrugivorous
andinsectivorousbirdsaremoreattractedtoagroforeststhansmall-to-mediuminsectivoresand
nonspecialist feeders. As might be expected, frugivores and nectarivores favored canopies with
high species richness (22). Thus, studies of bird populations need to take into account species
fromdifferentfunctionalgroups(115).
Other studies in cocoa agroforests have found that canopy thinning leads to a reduction
in the diversity of forest ant species but not of beetles (11). This contrasting effect of canopy
management illustrates the different effects of shade on different organisms and the need for
a better understanding of food chains and life cycles in agroecosystem functions. For example,
web-building spiders may be important predators. A study by Stenchly et al. (129) found that
www.annualreviews.org • TheRoleofTreesinAgroecology 121
PY52CH06-Leakey ARI 19July2014 13:35
webdensityincocoaagroforestwaspositivelyrelatedtocanopyopennessandthatlineandorb
webswerethemostcommontype.ThepresenceofPhilidrisantswasalsopositivelyassociated
withorb-webdensitybutnotwithotherwebtypes.Thepracticalimportanceofthesestudiesis
thatfruitlossesduetopathogensandinsectattackshavebeenfoundtobegreaterwhenthetree
diversityofthecanopyislow.Thissupportsthehypothesisthatdiversityintheupperstrataof
agroforestrysystemshasbeneficialeffectsonagroecologicalfunctions(12).
Meta-analysisofthebiodiversityandecosystemservicebenefitsofcacaoandcoffeeagroforests
hasconfirmedthewide-scalebenefitsofshadetreesfortheprovisionofhabitatforpestcontrol
by insectivores in Latin America and Asia, and in less-well-studied Africa (27, 102). However,
it is quite possible that these studies have only started to explore the ecological interactions of
importanceinsustainableagricultureandsofurtherecologicalresearchisneeded.
Togainabetterunderstandingofthecomplexlifecycleandfoodchaininteractionsbetween
speciesinmixed-speciesplantings,suchasagroforests,recentstudieshaveestablishedsomelong-
g
or termexperimentsinvolvingspeciesmixturesthatvaryinthediversityoftheplannedbiodiversity
s.
ew (theplantedcropsandtrees),theirspeciesdensities,andtheirconfigurations.Thiscreatesvery
vi
e differentmicroclimates(irradiance,temperature,humidity,etc.)withinboththegroundfloraand
ualr thecanopies,whichinturnaffectthecolonizationoftheecologicalnichessoformed(117).To
n
any. trytodevelopsomedeeperunderstandingoftheimportanceofallthisvariabilityonthedynamics
w.nl
wo of the incidence, severity, and dynamics of pest, disease, and weed organisms, Leakey (71) has
we
om al us proposedtwoexperimentaldesigns.Theseexperimentsweredesignedtoexploreagroecosystem
d frson breakdownwithregardtowitchesbroomdiseaseincocoa.
ownloade4. For per e(TcoaTlbohlgeeics3ae)l,dsieunsccicgluendsssiinsoegneatkhlteforiomamnpsapwcietosrnoesfoemrthteeomamguralottueidcrioesscpyiphstlaeisnmeasr,oyin.reet.h,seefarhorcemhaltpqhlua,enystitieinoldgn,stahsnprdoaunsugnhsintatgiontahmbeailtaiutgyrriooty-f
D1
3. 13/ cocoaproduction.Itisrecognizedthatinthelongtermtheresultsofexperimentslikethesewill
2014.52:113-132.36.112 on 08/ pitttorrseoaidbmtimaspbteallenynmtcbsee;entthovteuwmrsyiaitnhtdudoifriufveetirdfetouhnraeetlsdcrte.oeppnItlefiocnisuadtnaiendlssgioonofgrneeecaftocfhhegecnttilrszeeevoadetfmltcoheofanmttlapsanuerdtecihndsugeergxrrprooaeoudrtnaimdtsiyeoedsnntetbsamytasgrtheubeaeertxpdwtlraerenoemntwiensjlguyoxfsdtiasitipfemfiocaisulnealddrt
pathol. y 80.24 speTciehsecfiormstpdoessitiigonn.involvesacomplexmultistrataagroforestryexperimentlaidoutasaNelder
ob Fan (88), and the second experiment is a complex replacement series (32). Specifically, these
yt
Ph designs seek to examine the relationships in mixed-species cocoa agroforests between spacing,
ev. microclimate,theplannedandunplannedbiologicaldiversity,theincidenceofpestsanddiseases
R
u. incocoa,andtheoverallproductionandeconomicreturnsfromcocoaandtheothercompanion
n
n trees.Thereplacementseriesstudyaddsanassessmentoftheimpactsofdifferentconfigurationsof
A
companionspeciesgrownindifferingproportionswithinthecanopy.Boththeseexperimentsare
alsoexamininghowarangeofindigenouscompanioncropsproducingagroforestrytreeproducts
canadditionallyprovideincomegenerationandlivelihoodbenefits.
TheNelderFandesignaimstoexaminesomeecologicalquestions(Table3)thatarisefrom
growingasinglecrop(cocoa)atdifferentdensities(60–2,000treesperhectare)underaconstant
densityofacanopycomposedeitherofoneunderstoryshrubspeciesandonecanopytreespecies
orofsixteenunderstoryshrubspeciesandeightcanopytreespecies(Figure1).
The second design modifies the replacement series design to test the replacement of cocoa
with five other cash crops under a canopy of five tree species and five subcanopy species
(Figure 2).Theupperstorytreesandsubcanopytrees/shrubsareatthesamespacing(9m ×
9m),eachofwhichissurroundedbyeightcocoaplants,twoplantseachoffourcashcrops,ora
mixtureoffourcashcropswithfourcocoaplants(3m × 3m),dependingonthetreatmentwithin
122 Leakey