tropical fire ecology across the african continent: a - pages
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Figure 2: A) Detrended microscopic charcoal (green line) and oxygen isotope record (purple line) for Eski AcÄągĂśl Lake (Turkey). Correlation analysis (gray bars) shows statistically significant correlations between microscopic charcoal influx (particles cm-2 a-1) and δ18O (â°) for most of the record (modified from Turner et al., 2008); B) Lago dellâAccesa (Tuscany, Italy) lake-level fluctuations (blue line; modified from Magny et al., 2007) and the fire-return interval (red line) reconstructed from fire-event detection based on sedimentary macroscopic charcoal (modified from Van-nière et al., 2008). Peaks in the fire-return interval curve correspond with high lake-level stands before 4 cal ka BP.
acrossâ theâ Mediterraneanâ (CarriĂłnâ etâ al.,â2003,â2007;âSadoriâetâal.,â2008).
ConclusionTheâ paleofireâ recordâ fromâ theâ Mediter-raneanâ isâ paradoxical.â Climaticâ variationsâhaveâ certainlyâ actedâ asâ oneâ ofâ theâ mainâpacemakersâ ofâ fireâ regimes,â particularlyâinâ theâ firstâ halfâ ofâ theâ Holocene.â Underâdifferentâ climateâ conditionsâ (e.g.,â season-alityâ ofâ precipitation),â theâ southernâ andânorthernâ Mediterraneanâ mayâ haveâ beenâ
differentiallyâ impactedâ byâ fire.â Similarly,âhumanâ actionsâ (e.g.,â directlyâ viaâ ignitionâorâ indirectlyâ viaâ fuelâ management)â haveâbothâincreasedâandâdecreasedâfireâactivityâduringâtheâHolocene.â Increasedâsedimen-taryâ charcoalâ influxâ isâ oftenâ associatedâwithâ pre-â andâ proto-historicâ forestâ clear-anceâbutâinâtheâlateâHolocene,âwildfireâfre-quencyâoftenâreachedâaâmaximumâduringâphasesâofâlandâabandonmentâandâsecond-aryâ scrub-woodlandâ development,â e.g.,âduringâtheâ lastâcenturyâ inâmuchâofâMedi-
terraneanâ Europe.â Evenâ apparentlyâ well-establishedâ relationships,â suchâ asâ ever-greenâoaksâbeingâfavoredâbyâfire,âturnâoutâtoâ beâ wrongâ whenâ viewedâ overâ decadalâtoâ centennialâ timescales.â Theseâ complexâlong-termâresponsesâareâsignificantâinâtheâcontextâofâincreasingâaridityâandâwarming,âasâwellâasâmajorâregionalâland-useâchangesâlinkedâ toâ agriculturalâ andâ tourismâ devel-opmentâ aroundâ theâ Mediterraneanâ Sea.âUnderstandingâthemâwillâhelpâusâtoâbetterâmanageâandâpreserveâoneâofâtheâmostâfire-proneâregionsâofâtheâworld,âcharacterizedâbyâextraordinaryâplantâdiversity.
DataThe data are submitted to the Global Palaeofire Working Group Database (http://www.gpwg.org) and are available by contacting B. Vannière ([email protected])
AcknowledgementsWe are greatly indebted to M. Magny, M. Power, W. Tinner and R. Turner who have contributed to these results and for enriching discussions about paleofire. Many thanks also to all contrib-utors to the Mediterranean part of the Global Charcoal Database.
ReferencesColombaroli, D., Tinner, W., van Leeuwen, J.F.N., Noti, R., Vescovi, E.,
Vannière, B., Magny, M., Schmidt, R. and Bugmann, H., 2009: Response of broad-leaved evergreen Mediterranean forest veg-etation to fire disturbance during the Holocene: insights from the peri-Adriatic region, Journal of Biogeography, 36: 314-326.
Colombaroli, D., Vannière, B., Chapron, E., Magny, M. and Tinner, W., 2008: Fire-vegetation interactions during the Mesolithic-Neo-lithic transition at Lago dell'Accesa, Tuscany, Italy, The Holocene, 18: 679-692.
Turner, R., Roberts, N. and Jones, M.D., 2008: Climatic pacing of Mediter-ranean fire histories from lake sedimentary microcharcoal, Global and Planetary Change, 63: 317-324.
Vannière, B., et al., in press: Circum-Mediterranean fire activity and cli-mate changes during the mid Holocene environmental transition (8500-2500 cal yr BP), The Holocene.
Vannière, B., Colombaroli, D., Chapron, E., Leroux, A., Tinner, W. and Mag-ny, M., 2008: Climate versus human-driven fire regimes in Medi-terranean landscapes: the Holocene record of Lago dellâAccesa (Tuscany, Italy), Quaternary Science Reviews, 27: 1181-1196.
For full references please consult:http://www.pages-igbp.org/products/newsletters/ref2010_2.html
Tropical fire ecology across the African continent: A paleoecological perspectivedaniele Colombaroli1,2 and dirk versChuren2
1Institute of Plant Sciences and Oeschger Center for Climate Change Research, University of Bern, Switzerland; [email protected] Unit, Department of Biology, Ghent University, Belgium
High-resolution charcoal records from African lake sediments provide new insights for longstanding research questions on fire-climate-human interactions in tropical and subtropical ecosystems.
Everyâ year,â theâ tropicsâ experienceâ moreâfireâ thanâ anyâ otherâ regionâ inâ theâ worldâ(Fig.â1).âTropicalâgrasslandâ(savannah)âfiresâareâ theâ dominantâ sourceâ ofâ carbonâ fromâbiomassâ burningâ andâ provideâ moreâ thanâ60%âofâtheâglobalâtotalâ(MouillotâandâField,â2005).â However,â muchâ ofâ whatâ isâ knownâaboutâ tropicalâ fireâ ecologyâ isâ basedâ onâ
monitoringâ programsâ startedâ withinâ theâlastâ decade,â withâ highlyâ fragmentaryâ his-toricalâ dataâ extendingâ onlyâ toâ theâ earlyâ20thâ century.â Theseâ dataâ doâ notâ allowâ usâtoâassessâwhetherârecentâtrendsâinâfireâfre-quencyâandâmagnitudeâareâunusualâinâtheâcontextâ ofâ naturalâ long-termâ ecosystemâdynamicsâ(GillsonâandâWillis,â2004).â
Natural or anthropogenic fire regime? Seventyâpercentâofâtropicalâandâsubtropi-calâ areasâ worldwideâ areâ consideredâ toâhaveâ ecologicallyâ degradedâ fireâ regimesâ(Shliskyâ etâ al.,â 2007).â Inâ Africa,â theâ recentâincreaseâ inâ fireâ frequencyâ isâ attributedâ toâhumanâecosystemâdisturbanceâassociatedâ
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Figure 1: A) Satellite image of world fire activity detected by MODIS in the early spring of 2010 (red dots, data from Fire Information for Resource Management System FIRMS, http://maps.geog.umd.edu/firms). B) Worldwide geographi-cal distribution of paleoecological fire-regime records currently in the Global Charcoal Database (modified from Power et al., 2008). Despite the great fire activity in tropical and subtropical ecosystems, few paleofire records are available from regions such as Africa. Dashed lines delimit the tropical region, bounded by latitudes 23.5°N and S.
withâintensifyingâagricultureâ(Fig.â2;âDavid-sonâetâal.,â2003).âForâ instance,â inâtheâ low-landârainforestsâofâWestâAfricaâandâinâmoistâmontaneâ forestsâ atâ higherâ elevationsâ inâEastâAfrica,ânaturalâfireâisâuncommonâ(Gol-dammer,â1990).âYetâwidespreadâclearanceâofânaturalâvegetationâhasâconvertedâlarge,âformerlyâ forestedâ areasâ intoâ highlyâ flam-mableâgrasslandsâ(Roberts,â2000;âGoldewi-jk,â2001).âStudiesâ ofâglobal-scaleâpatternsâinâhistoricalâlandâuseâ(e.g.,âArchibaldâetâal.,â2005)âtendâtoâassumeâthatâhumanâimpactâonâ tropicalâ Africanâ ecosystemsâ wasâ lim-itedâ beforeâ 1700â ADâ becauseâ populationâdensitiesâ ofâ indigenousâ peopleâ wereâ lowâ(Goldewijk,â 2001;â Ramankuttyâ andâ Foley,â1999).âThisâ perspectiveâ contrastsâ withâ ar-cheologicalâandâpaleoecologicalâevidenceâthatâ indicatesâ thatâ anthropogenicâ forestâclearanceâ inâ partsâ ofâ Eastâ Africaâ startedâatâleastâ2.5âkaâago,âinâassociationâwithâtheâintroductionâ ofâ ironâ smeltingâ technologyâ(Robertshawâ andâTaylor,â 2000).â Otherâ au-thorsâ suggestâ thatâ humansâ haveâ alteredâAfricanâ forestâ ecosystemsâ overâ aâ longerâtimeâ (Willisâ etâ al.,â 2004).â Ifâ increasingâ fireâactivityâ duringâ theâ Holoceneâ wasâ indeedârelatedâ toâ intensifyingâ humanâ impactâ(Lejjuâ etâ al.,â 2005;â Rynerâ etâ al.,â 2008),â theâtimingâ andâ extentâ toâ whichâ humansâ al-teredâ localâ ecosystemsâ variedâ regionallyâandâ amongâ ecosystems.â Forâ instance,â re-centâ studiesâ indicateâ thatâ deforestationâassociatedâ withâ sedentaryâ agricultureâstartedâ onlyâ ~0.35â kaâ agoâ inâ theâ moistâhighlandsâ ofâ centralâ Kenyaâ (Lambâ etâ al.,â2003)âbutâatâleastâ~0.8â1âkaâagoâinâsub-hu-midâ westernâ Ugandaâ (Ssemmandaâ etâ al.,â2005;âRussellâetâal.,â2009).âInâdrierâenviron-ments,â agriculturalâ activityâ oftenâ beganâ~0.12â kaâ ago,â duringâ colonialâ times,â yetâlandscapesâ mayâ haveâ beenâ significantlyâmodifiedâ byâ pastoralistâ culturesâ wellâ be-foreâ then.â Detailedâ charcoalâ studiesâ withâadequateâspatialâcoverageâareâneededâ toâdetermineâ whetherâ currentâ fireâ regimesâareâwithinâtheârangeâofâhistoricâvariabilityâ(WillisâandâBirks,â2006),âorâwhetherâfireâfre-quencyâ hasâ increasedâ inâ responseâ toâ theâdifferentâ typesâ andâ intensitiesâ ofâ humanâimpactâassociatedâwithâpastoralistâandâag-riculturalistâsocieties.
Fire regime response & feedback to past climate variabilityHigh-resolutionâ charcoalâ studiesâ haveâshownâ howâ fireâ canâ beâ aâ âcatalystââ forâclimate-changeâ effectsâ onâ vegetation.âForâinstance,âmoistâconditionsâlimitâfireâtoâspreadâinâpresentâtropicalâforests,âbutâdur-ingâ drierâ periodsâ inâ theâ pastâ wildfireâ wasâlikelyâ moreâ common,â causingâ changesâinâ ecosystemâ structureâ andâ degradationâ(e.g.,â Willisâ andâ Birks,â 2006;â Bushâ etâ al.,â
2008;â Fig.â 2).â Climate-proxyâ informationâfromâ theâ sedimentsâ ofâ Eastâ Africanâ lakesâdocumentâ majorâ variationsâ inâ moistureâbalance.âForâexample,âinâtheâlateâ18thâcen-tury,â anâ episodeâ ofâ severeâ droughtâ com-pletelyâ desiccatedâ allâ butâ oneâ lakeâ inâ theâEasternâRiftâValleyâofâKenya,âsouthâofâLakeâTurkanaâ(Verschuren,â2004;âBessemsâetâal.,â2008).â Inâ theâ lastâ fewâ millennia,â century-longâ periodsâ ofâ bothâ significantlyâ drierâandâ wetterâ conditionsâ thanâ todayâ haveâoccurredâoverâmostâofâequatorialâEastâAf-rica.â Thereâ haveâ alsoâ beenâ periodsâ (e.g.,âfromâ~1500âtoâ1750âAD)âwhenâclimateâwasâunusuallyâ dryâ inâ theâ normallyâ sub-humidâwesternâpartsâofâtheâregionâwhileâremain-ingâunusuallyâwetâinâsemi-aridâregionsâfur-therâ eastâ (Verschurenâ etâ al.,â 2000;â Russellâandâ Johnson,â 2007).â Researchâ document-ingâtheâresponseâofâterrestrialâecosystemsâtoâthisâclimateâvariabilityârevealsâtheâhighâsensitivityâ ofâ vegetationâ transitionâ zones,âsuchâ asâ theâ forest/savannahâ ecotone,â toâevenâ modestâ decadal-scaleâ variationsâ inârainfallâ (Lambâ etâ al.,â 2003;â Ngomandaâ etâal.,â 2007).â Additionalâ charcoalâ recordsâ ofâhighâ temporalâ resolutionâ areâ neededâ toâshowâhowâfireâregimesâhaveârespondedâtoâcontrastingâclimateâtrendsâatâtheâregionalâscaleâ(Fig.â1b).
Tropical ecosystems resilience to fireInâ theâseasonallyâdryâclimateâregimeâpre-vailingâ throughoutâ mostâ ofâ Eastâ Africa,âfireâisâtheâdominantâdirectâcontrolâonâveg-etationâ distributionâ (Bondâ etâ al.,â 2005;âGillsonâ andâ Duffin,â 2007)â butâ naturalâ fireâfrequencyâ decreasesâ fromâ semi-aridâ cen-tralâandâeasternâKenyaâtoâsub-humidâwest-ernâ Uganda.â Howâ doesâ fireâ controlâ theâlandscape-scaleâ ecotoneâ betweenâ savan-nahâ andâ forest?â Recentâ studiesâ postulateâthatâ tropicalâ rainforestsâ andâ grassâ savan-nahâmayâexistâasââstableâstatesââinâwhichâaâgrass-dominatedâecosystemâisâmaintainedâbyâ frequentâ firesâ whileâ aâ tree-dominatedâecosystemâhelpsâcreateâaâwetâmicroclimateâandâlowâgroundâcover/fuelâloadâthatâlimitsâfireâ(Sankaranâetâal.,â2005;âGillsonâandâDuf-fin,â2007;âGillson,â2008).âAâshiftâfromâsmallâpatchyâfiresâsetâbyâindigenousâpeoplesâtoâtheâ largerâfiresâcharacteristicâofâEuropeanâlandâ managementâ hasâ stronglyâ alteredâtheâ savannah-forestâ ecotone,â byâ favoringâhighlyâ flammableâ annualâ grasses.â Thus,âbyâ increasingâ theâ flammabilityâ ofâ grassâcommunities,âthisâhistoricalâchangeâinâfireâmanagementâmayâhaveâcausedâaâpositiveâfeedbackâ withâ fireâ (Cochrane,â 2009).â Thisâhypothesisâ needsâ toâ beâ rigorouslyâ testedâ
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Figure 2: A) Human-set fire in a rural area close to Mozambique border. Here, fire is used to clear land for agriculture and livestock ranching. B) Wooded savannah stand in Kruger National Park (South Africa). During the dry season (Apr-Oct), most of the biomass dries, increasing the fire hazard. (Photos by D. Colombaroli). C) Pollen and char-coal record from Malahlapanga Lake (Kruger National Park) showing the transition from grassland to a fire-maintained wooded savannah after 1100 AD (modified from Gillson and Ekblom, 2009). The authors suggested that increased regional rainfall promoted biomass production (i.e., wooded savannah), allowing fire activity to increase.
onâ longâ timescalesâ andâ inâ multipleâ re-gionsâtoâdetermineâwhetherâthisâfeedbackâisâ characteristicâ ofâ presentlyâ highlyâ dis-turbedâ conditions,â orâ whetherâ itâ alsoâ oc-curredâduringânaturalâcyclesâofâ long-termâhydrologicalâ change.â Paleoinformationâonâ theâ resilienceâ ofâ tropicalâ moistâ forestsâtoâoccasionalâfireâand,âspecifically,âtheârateâatâwhichârainforestsârecoverâfromâdestruc-tiveâ fireâ wouldâ alsoâ beâ highlyâ instructiveâforâ futureâ conservationâ (Cochrane,â 2003).âNewâ reconstructionsâ ofâ pastâ fireâ regimesâbasedâ onâ fossilâ charcoalâ analysisâ thatâquantifyâ theâ localâ frequencyâ ofâ fireâ (e.g.,âWhitlockâ andâ Larsen,â 2001;â Gavinâ etâ al.,â2006;âHigueraâetâal.,â2008),âcombinedâwithâmodernâ calibrationâ studiesâ (Duffinâ etâ al.,â2008),â shouldâ revealâ howâ Africanâ ecosys-temsârespondâtoâfireâvariabilityâatâdecadalâtoâcenturyâtimescales.â
Research outlook Coupledâ atmosphere/ocean/biosphereâclimateâ modelsâ projectâ futureâ tempera-turesâ acrossâ tropicalâ Africaâ toâ increaseâfromâ 0.2â toâ 0.5°Câ perâ decade,â andâ pre-
cipitationâinâEastâAfricaâtoâincreaseâduringâtheâ shortâ rainyâ seasonâ (Northernâ Hemi-sphereâ winter)â andâ decreaseâ duringâ theâmainârainyâseasonâ (NorthernâHemisphereâspring)â(Hulmeâetâal.,â2001;âIPCC,â2007).âInâaddition,âchangesâinâtheâteleconnectedâElâNiĂąo/SouthernâOscillationâ(ENSO)âareâpro-jectedâ toâ causeâ pronouncedâ droughtâ inâsomeâregionsâandâincreasedâriskâofâflood-ingâ inâ othersâ (Waraâ etâ al.,â 2005).â Ifâ meanâannualâprecipitationâoverâEastâAfricaâdoesâincreaseâ (IPCC,â 2007),â itsâ beneficialâ effectâonâ forestâecosystemsâwillâ likelyâbeâ lostâ inâareasâ withâ frequentâ anthropogenicâ firesâandâ increasingâ demographicâ pressure.âInsightsâ intoâ howâ Africaâsâ forestâ andâ sa-vannahâ ecosystemsâ willâ respondâ toâ theâmultipleâstressorsâofâfutureâglobalâclimateâchangeârequiresâanâunderstandingâofâpastâecosystemâ responsesâ toâ large-magnitudeâenvironmentalâ changes.â Currentlyâ stillâveryâ rareâ inâ Africaâ andâ elsewhereâ inâ theâtropics,â high-resolutionâ paleoecologicalârecordsâ ofâ (pre-)historicalâ humanâ impact,âfireâ andâ vegetationâ canâ provideâ suchâ ho-listicâinformation.
AcknowledgementsWe thank Lindsey Gillson for useful comments and Mitchell J. Power for the global map of sites in the Global Charcoal Database. The project âFire, climate change and human impact in tropical ecosystems: paleoecological insights from the East African regionâ is funded by the Swiss National Science Foundation (FNS âAmbi-zioneâ Project no. PZ00P2_126573).
ReferencesCochrane, M.A., 2003: Fire science for rainforests, Nature, 421: 913-919. Duffin, K.I., Gillson, L. and Willis, K.J., 2008: Testing the sensitivity of
charcoal as an indicator of fire events in savanna environments: quantitative predictions of fire proximity, area and intensity, The Holocene, 18: 279-291.
Gillson, L. and Ekblom, A., 2009: Resilience and thresholds in savannas: nitrogen and fire as drivers and responders of vegetation transi-tion, Ecosystems, 12: 1189-1203.
Sankaran, M., Hanan, N. and Scholes, R., 2005: Determinants of woody cover in African savannah, Nature, 438: 846-849.
Willis, K.J., Gillson, L. and Brncic, T.M., 2004: How "virgin" is virgin rain-forest? Science, 304: 402-403.
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