‘Solid–fluid–gas’:
the
state
of
knowledge
on
carbon-sequestration
potential
of
agroforestry
systems
in
Africa
PK
Ramachandran
Nair
1and
Vimala
D
Nair
2Abstract
Theperceptionthatagroforestrysystemshavehigherpotential tosequestercarbonthancomparablesingle-speciescrop systemsorpasturesystemsisbasedonsolidscientific foundation.However,theestimatesofcarbonstockof agroforestrysystemsinAfrica—reportedtorangefrom1.0to 18.0MgCha 1inabovegroundbiomassandupto
200MgCha 1insoils,andtheirCsequestrationpotentialfrom 0.4to3.5MgCha 1yr 1–arebasedongeneralizationsand vagueorfaultyassumptionsandthereforeareofpoorscientific value.Althoughagroforestryinitiativesarepromisingpathways forclimate-changemitigation,rigorousscientificproceduresof carbonsequestrationestimationsareneededforrealizingtheir fullpotential.
Addresses
1
SchoolofForestResourcesandConservation,UniversityofFlorida, 118N-ZHall,POBox110410,Gainesville,FL32611-0410,USA
2SoilandWaterScienceDepartment,UniversityofFlorida,Gainesville,
2181McCartyHallA,POBox110290,Gainesville,FL32611-0290,USA Correspondingauthor:Nair,PKRamachandran([email protected])
CurrentOpinioninEnvironmentalSustainability2014,6:22–27 ThisreviewcomesfromathemedissueonSustainabilitychallenges EditedbyCheikhMbow,HenryNeufeldt,PeterAkongMinang, EikeLuedelingandGodwinKowero
ForacompleteoverviewseetheIssueandtheEditorial
Received20May2013;Accepted8July2013 Availableonline11thOctober2013
1877-3435/$–seefrontmatter,#2013PKRamachandranNair. PublishedbyElsevierLtd.Allrightsreserved.
http://dx.doi.org/10.1016/j.cosust.2013.07.014
Introduction
Theimportanceofcarbon(C)sequestrationinmitigating climatechangeneedsnospecialintroduction.Numerous reports are available on C sequestration potential of various agroforestry systems(AFS) from differentparts oftheworldincludingAfrica[1,2].Theyallportraythe perceptionthatAFShavehigherpotentialtosequesterC than comparable single-species crop systemsor pasture systems.Theunderlyingpremiseofthisperceptionisthe niche complementarity hypothesis, which states that a larger array of species in a system leads to a broader spectrumofresourceutilizationmakingthesystemmore productive [3: Tilman 1990], and implies that plant speciesinamixedsystemuseresourcesina
complemen-taryway[4:Kahmenetal.,2006].However,inspiteofthis
commonality in the perception and its underlying pre-mise,enormousvariabilityexistsinthesereportsinterms of the nature, rigor, and details of studies, so that it becomes difficult to compare the datasets based on uniform criteria to draw widely applicable conclusions. Inotherwords,thepoorquality(highdegreeofvariability and lack of rigor in the reported results) of available reportsseriously limittheir potentialuse for arriving at widely applicable land-management decisions and recommendations.Theobjectiveofthispaperisto sum-marizethereportedresultsofCsequestrationpotentialof AFSinAfricaandhighlightthecommonmethodological weaknessesand drawbacks in thereported dataso that futureeffortscouldendeavortoovercomesuchproblems.
Carbon
sequestration
potential
of
agroforestry
systems:
what
the
literature
shows
A comprehensive literature search was conductedon C sequestrationin AFS,including reports thataredirectly relatedtoAFSaswellasthoseofamethodologicalnature thatarerelevanttoAFS:seehttp://sfrc.ufl.edu/pdf/faculty/
nair_afcsliterature.pdf for the nearly 600 references.
Amongthese, themost frequentlyquoted and/orrecent references that focused on AFS include [1,2,5,6,7,8,9,
10,11,12,13,14,15,16–28], whereas those of a broader
and methodological nature include [29–48,49,50–
52,53,54–56]. Following that compilation, the various
AFSreported in the literaturewere groupedunder five subgroups[Table1].ThereportedCsequestrationvalues from 22 countries representing different ecoregions of Africawerethencompiledincludingdetailssuchas agroe-cologicalcharacteristics(location,climate,majortreesand crops),andvaluesofCsequestrationabovegroundandin soils[PKRNair,unpublished].TheestimatedC seques-trationratesforthedifferentAFsystemsacrossAfricafrom thisdatasetaresummarizedinTable2[18].
Common
problems
and
weaknesses
in
the
reported
data
Ingeneral,thereportedvalues[Table2]aremostly specu-lative,basedoncircumstantialandexperientialratherthan empiricalandexperimentalevidence.Eventhefewthat are empiricalare not based onuniform or rigorous pro-cedures,andhavehighvariability.Althoughthisis com-montomanyaspectsofagroforestry,itisparticularlysoin thecaseofcarbonsequestrationand climate-change-miti-gation discussions partly because these are trendy and fashionablesubjectstotalkabout.Theextreme site-speci-ficityofAFSalsocontributestothelackofuniformityin
assessment methodologies. The systems even within a region vary considerably in structure (arrangements of components),function(expectedoutputs),species diver-sity(ofcropsandtrees),management,andsocioeconomics, suchthatnotwoagroforestryfieldsareidentical.Asaresult, the reported research results vary extremely in the methodsusedand/orlevelofdetailsreported.Therefore itisdifficulttosubjectsuchresultstointegratedanalyses suchasmeta-analysisandotherwell-knownstatisticaltools usedtoelucidatetrendsamongadisparatesetofstudies with different experimental approaches and methods. Furthermore,mostpublishedstudiesareofshortduration, which cannot be used for predicting long-term con-sequences.Difficultytomodeldiscontinuousmultispecies stands also adds to the problem. Most models used in forestry(forestimatingstandvolume,Ccontent,growth patterns,amongothers)havebeendevelopedfor continu-ous,single-speciesstands;agroforestrysystemsrepresent discreetstandsofmultiplespecies;tryingtoapplyavailable forestrymodelstostudyAFSpresentsthe ‘round-peg-in-square-hole’situation.
Eachoftheabovedifficultiescouldbediscussedindetail. Forreasonsofbrevity,however,onlysomeofissuesthat are specific to C sequestration studies in AFS are dis-cussed insomedetailhere.
Biomass
Treebiomass
AsdiscussedbyNairet al.[1]andMalmeretal.[53], extensiveestimationsofglobalforestbiomassarebased onrough estimations: mostly estimatingthestem-wood volume and multiplying it with species-specific wood density,andother‘correctionfactors’togetanestimation of whole-tree biomass. Such estimations have mostly been donefor forestecosystems withattemptsto extra-polating them to AFS at a global scale [e.g. 40: Dixon etal.,1993].Carboncontentisassumedtobe50%ofthe estimatedwhole-treebiomass,androotbiomassis gener-ally excluded. Although whole-tree harvesting method, whichinvolvessumminguptheamountofharvestedand standing biomass, has traditionally been used for more accurate estimations of tree biomass, the extremely Table1
SubgroupsofAgroforestrySystems(AFS):majorformsofagroforestryandtheiragroecologicaldistributioninthetropics. AFSsubgroup Majorformsofagroforestry Majoragroecologicaldistributioninthetropics Treeintercropping Alleycropping,improvedfallows Regionswith>800mmrain/yr;
Multipurposetrees(MPTs)onfarmlands Throughoutthetropics
Multistratasystems Homegardens Tropicalwet(mostlyelevationsupto1000masal) Shadedperennials Wet,Moist&Montaneregionswith>1000mmrain/yr Silvopasture Browsing,cut-and-carry Tropicalwetandmoistregions
Treesonpasture/grazinglands Semiaridtoaridregions
Protectivesystems Windbreaks,shelterbelts Tropicaldry(arid,semiarid),coastalregions Soilconservationhedges Slopingareas:moist,montane
Boundaryplanting Throughout
Agroforestrytreewoodlots Woodlotsforfirewood,fodder, landreclamation
Dry:firewood;landreclamationtropwet&moist:fodder;Land reclamation:eroded/degradedlands)
Source:Nair[18].
Table2
Estimatesofcarbonstockandcarbonsequestrationpotentialundermajoragroforestrysystems(AFS)inAfricaa.
AFSsubgroup Majoragroforestrypractices Estimatedcarbonstock(Mgha 1) Carbonsequestration
potential(Mgha 1yr 1)
Aboveground Soils Aboveground Soils
Treeintercropping Alleycropping,improvedfallows Upto15 Upto150 0.5–4.0 1.5–3.5
MPTsonfarmlands Upto12 Verylowto150 0.2–2.5 1.5–3.5
Multistratasystems Homegardens 2–18 Upto200 0.5–3.0 1.5–3.5
Shadedperennials 5–15 Upto300 1.0–4.0 1.0–5.0
Silvopasture Browsing,cutandcarry 1.8–3.0 1.5–3.5lowto80 0.3–4.0 1.0–2.5
Treesonpasture/grazinglands 1.5–8.0 Verylowto60 0.3–2.0 0.4–1.0
Protectivesystems Windbreaks,shelterbelts Soilconservationhedges Boundaryplanting
1.5–7.0 Verylowto60 0.7–2.0 0.4–1.0
AFTreeWoodlots Woodlotsforfirewood,fodder, landreclamation
(Highlyvariable) Verylowto60 1.0–5.0 1.0–6.0
Source:Nair[18].
a
tedious nature of the method limits its application to researchpurposesonly.
Allometric equations developed based on biophysical propertiesof treesandvalidatedbyoccasional measure-mentsofdestructivesamplingarewidelyusedinforestry for estimating volumes of standing forests. These equations are developed as regression models with the measured variables such as diameter at breast height (DBH), total tree height or commercial bole height, and sometimes wood density, as the independent vari-ables and total dry weight as the dependent variable. Various allometric equations have been developed for differentforest types and forestry species[53]; similar studiesarenowbeingundertakenforsometreesinAFS
aswell[46–48,54].ICRAF’sdatabasesfortree
character-istics such as wood density are valuable resources for suchefforts(http://www.worldagroforestrycentre.org/sea/
products/afdbases/af/asp/SpeciesInfo).
Cropbiomass
UnlikethemultipurposetreesthatarecommoninAFS, therearenospecificagroforestrycrops.Most,ifnotall,of thecommonagriculturalcrops growninagivenlocality are grownin AF systemsas well. Justas in agricultural systems, choice of crops is determined by the local ecological(climate,soil)andsocioeconomicfactors.Most cropsinAFSareherbaceousannuals(excepttheshaded perennialssuchascacao[Theobromacacao],coffee[Coffea spp.], tea [Camellia sinensis], and black pepper [Piper nigrum]).Theseannuals havehighharvestindexvalues (proportion of harvested economic productivity to total biologicalproductivityaboveground),andthereforetheir biomasscontributiontototalCsequestrationinanAFSis relativelyless,comparedwithtreesandperennialshrubs.
Belowgroundbiomass
Ingeneral,rootsarebelievedtoaccountforathirdofthe totalNPP(netprimaryproductivity).However,itisvery difficulttomeasurethisfraction.Theroot-to-shootratiois thereforecommonlyusedtoestimatebelowgroundliving biomass. The ratios differ considerably among species and across ecologicalregions. The livingmicrobial bio-massconstitutesroughly1%ofthetotalSOC(soilorganic C).InspiteofthelowtotalamountofCinvolvedinthis pool, it is an important indicator of organic matter decomposition and C sequestration though the break-down or tying-up of C and their relationship to soil aggregates.
Soils
SoilsplayavitalroleintheglobalCcycle[7,35]andsoilC that traditionally has been a sustainability indicator of agriculturalsystemshasnowacquiredtheadditionalrole as anindicator of environmental health[45,46].Recent studies have confirmed the niche complementarity hy-pothesisinrelationtosoilCsequestration(SCS)inAFS
[1]. However, as mentioned before in the context of AFSingeneral,theestimatedvaluesofSCSinAFSvary greatly depending on biophysical and socioeconomic characteristicsof thesystem parametersandbecause of thelackofuniformityinstudyproceduressuchasdepth ofsamplingandsoilanalyticalprocedures.Anothermajor drawback is thelack of essentialinformation aboutthe soils in many reports (e.g. soil bulk density) that are crucial for comparison and extrapolation of data [15] onSCS.
Erroneousassumptions
An important part of the UNFCCC (United Nations Framework Convention on ClimateChange) definition ofCsequestrationisthe securestorage C(CO2)thatis removedfromtheatmosphereinlong-livedpools.There is considerable ambiguity in the understanding of this concept,especiallywhenit comesto ‘long-lived’ pools. The literature on C sequestration in land-use systems, especiallyAFS,isnotclearonthis.MostreportsequateC stocktoCsequestration.
Furthermore, the estimations and computations of C stockinAFSareapproximationsandarebasedonseveral assumptions,at least some of which are erroneous. For example:
‘Carboncontentinbiomassis50%.’Oftenitislessthan that.
‘All biomass represents sequestered C.’ All biomass doesnotendupin‘long-lived’pools.Thefoliagethat fallsongrounddecomposesrapidlyand releasesCO2 backto the atmosphere.The fraction of thebiomass that can be considered as sequestered C is variable depending on a number of factors including the species,plant part,andecologicalconditions.
‘Tree biomass (and C) estimates based on existing biomass equations are applicable to agroforestry situ-ations.’Mostoftheexistingbiomassequationsarebased ontreesgrowingeitherincloselyspacedplantation or naturalstands;theydonotgivegoodbiomassestimates foropen-grown(widelyspacedorscattered)agroforestry treesthatcouldbedifferentintheirgrowthform. ‘All C in soil represents sequestered C.’ Recent C
additionsto surfacesoilthroughlitterfallandexternal additions are subject to rapid decomposition and release of CO2, with only a small percentage of C becoming stable C in ‘long-lived’ pools. If C stocks increasethroughtime,thatisaformof sequestration. ‘Carbon stock is the same as C sequestration.’ C sequestrationisarateprocessinvolvingthetimefactor (e.g.MgCha 1yr 1);Cstock(Mgha 1)doesnothave thetime factor.
‘Growthformoftreeshaslittletodowithrootbiomass.’ Differencesingrowthformof treesand management practices can lead to under-estimations or over-estimationsofrootbiomass.
‘Amount of C sequestered is generally uniform for a given agroforestry practice.’ High levels of spatial heterogeneity exist among similar agroforestry prac-tices at different locations such that extrapolation across systemsand locationscanbemisleading.
Future
directions
Onthebasisofthesynthesispresentedabove,thecurrent state of knowledge on carbon sequestration in agrofor-estrysystemsinAfricacanbesaidtobeina‘solid–fluid– gas’ situation as depicted in Figure 1. The gist of the figure isthat while thescientificprinciples support the premise that agroforestry systems have higher carbon sequestrationpotentialthansingle-speciescrop systems or pasture systems, the proceduresused for estimating CSPareina‘fluid’stateastheyarebasedon generaliz-ationsandvagueorfaultyassumptions,andthereforethe estimateslackscientificcredibility(‘hotair’).
Several uncertainties and deficiencies need to be addressedforresolvingthisproblem.Theseincludeissues thatareofageneralnaturecommontoallland-usesystems,
andothersthatarespecifictoAFS.Amajorgeneralissueis thelackofunderstandingaboutcarbondynamicsinsoils.It is not clearly known if the residence time of C that is sequesteredinitiallyinasystemdiffersfromthatofCthat issequesteredlater.ArethecyclesthattheinitialCand laterCadditionsgothroughthesame?SincechangesinC stockisunlikelytobelinearthroughtime,understanding thenatureofthecurveofCstorageovertimeisimportant tounderstandtheperiodswhenmostCisbeing seques-tered.Well-planned,process-orientedresearchisneeded togainclearinsightsintosuchissues.
ComingtoissuesthatarespecifictoAFS,allstudiesonC sequestration under AFS referredto in the References sectionofthisreportareofashort-termnature(lessthan fiveyears).Chronosequencestudiesareveryimportantto monitorlong-termchangeinsoilCinland-usesystems; but nosuch studiesinvolvingAFS havebeen reported. Furthermore,thelackofuniformmethodsfordescribing area under agroforestry is a problem in gauging the importance of agroforestryin carbon sequestration ona regionalorlargerscale.EstimatesofareaunderAFSthat haverecentlybecomeavailable[15,16,29]suggestthat while the areaunder AFS is alittle over 1.0 billion ha (2009),thereispotentialtobringupto1.6billionhaunder AFSin thenearfutureglobally[15].
Alargenumber ofsuchquestionsneedtobeanswered forrealisticallyassessingtheimpactofagroforestryand other management practices on C sequestration. First and foremost, the methodologies for estimating C sequestration need to be standardized. At the same time, efforts could be initiated to set up a ‘Carbon Reference Database for Agroforestry Systems (CRD-AFS)’ as an approach toestimating carbon benefits of AFS [PKR Nair, unpublished]. The CRD-AFS is an ecological approach based on the premise that the productivity of aland-use system is determined prim-arily byits ecological features,especiallyfor low-man-agement, low-input systems such as agroforestry. Its essence is aStandard ReferenceGuide(SRG), a com-puterized database, which will include relevant drop-down menu for narrowing down to the ecological and system characteristics of any AFS in any region or country. The anticipated range of values for C stock and Csequestration potentialfor the systemcould be deducedbyreferringtotheSRG.Thedatabaseshould be updatedcontinuously basedon fieldmeasurements andnewscientificdata.Thetoolispracticalandeasyto use; it does not involve complicated on-site measure-mentsand computationseverytimeit isused.If prop-erlyconstructedandrigorouslymaintained,thetoolwill beasignificantcontributiontoAFScarboncalculations worldwide. Organized global efforts are needed to undertake such efforts,whichcouldpossibly be under the auspices of IPCC (Intergovernmental Panel on ClimateChange)withICRAF(the WorldAgroforestry Figure1
SOLID: The science of high C sequestration potential of Agroforestry Systems
Carbon SequestrationPotential of Agroforestry Systems: The Current State of Knowledge in
“Solid-Fluid-Gas” Stages
Rigorous and standardized measurement-and-estimation
procedures are needed for realizing the potential
Lack of rigor and uniformity in methods used
Current Opinion in Environmental Sustainability The practice of estimating the potential:
Most ofthe reported results are of poor scientific credibility (“hot air”)
Thecurrentstateofknowledgeoncarbonsequestrationpotentialof agroforestrysystemsinAfrica.Althoughthescienceissolid,the methodsofestimationare‘fluid’(imprecise),andthereforetheavailable estimatesconstitutemostly‘hotair’.
Centre), the only international institution for agrofor-estry,providing theneededstimulus and leadership.
Conclusions
Thelackofrigorousbutsimplescientificproceduresfor estimatingandreportingthecarbonsequestration poten-tial of agroforestry systemsseriously affectsthe quality and usefulness of the available reports and makes it difficultto compare thedifferencesundervarious man-agementpractices,soils,environments,socialconditions, amongothers.Inordertocapitalizeonthehighpotential forclimatechangemitigationthroughcarbon sequestra-tion offered byagroforestry systems, the procedures of measuring and estimating C sequestration need to be rigorousandstandardized.Internationaleffortsshouldbe steppedup toaddressthis issue.
References
and
recommended
reading
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