Determination of Deforestation Rates of the World`s Humid Tropical

Determination of Deforestation Rates of the World's Humid Tropical Forests
Author(s): Frédéric Achard, Hugh D. Eva, Hans-Jürgen Stibig, Philippe Mayaux, Javier
Gallego, Timothy Richards, Jean-Paul Malingreau
Source: Science, New Series, Vol. 297, No. 5583 (Aug. 9, 2002), pp. 999-1002
Published by: American Association for the Advancement of Science
Stable URL: http://www.jstor.org/stable/3832048
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REPORTS
of
Determination
Rates
of
Deforestation
World's
the
Tropical
Humid
Forests
Frederic Achard,l* Hugh D. Eva,1 Hans-Jurgen Stibig,1
Philippe Mayaux,1 Javier Gallego,2 Timothy Richards,3
Jean-Paul Malingreau4
A recentlycompletedresearchprogram(TREES)
employingthe globalimaging
of Earth-observing
satellitesprovidesupdatedinformation
on the stacapabilities
tus of the world'shumidtropicalforestcover.Between1990and1997,5.8 + 1.4
millionhectaresof humidtropicalforestwerelost eachyear,witha further2.3 +
0.7 millionhectaresof forestvisiblydegraded.
Thesefiguresindicatethattheglobal
net rate of changein forestcoverfor the humidtropicsis 23%lowerthanthe
of carbonfluxesin the
generallyacceptedrate.Thisresultaffectsthe calculation
sinkissmallerthanpreviously
inferred.
globalbudgetandmeansthattheterrestrial
Loss of forestcover affectsclimate.Globalforest assessmentssuch as thoseundertaken
by the
Food and AgricultureOrganization(FAO) (1)
are designed to measurethe area of and the
trendsin the extent of the world's forests.The
humidtropicalforestsdeserveour specialattention becausedemographic,economic,andsocial
changescontinueto exertconsiderablepressure
on forestcoverandconditionsin this region(2),
andourknowledgeconcerningtheirdistribution
andratesof changeremainssurprisingly
limited.
The IntergovernmentalPanel on Climate
Change(IPCC)has pointedoutthat"fortropical
estimatesareveryuncercountries,deforestation
tainandcouldbe in errorby as muchas +50%"
(3). The uncertaintyof such estimatessuggests
thattotalglobalcarbonemissionsfromland-use
changesfall withinthe rangeof +0.8 to +2.4
gigatonsof carbon(GtC)year-' for the 1990s
(4-5). Here we estimatethe changesin humid
tropicalforestcover from satelliteremotesensing imagery,with betterglobalconsistencyand
with greateraccuracythanpreviouslyavailable,
in orderto understandtheirimplicationsfor the
globalcarbonbudget.
The evergreenand seasonal forests of the
tropicalhumidbioclimaticzone coveredby our
workcorrespondclosely to thoseforestsdefined
by the FAO as closed broadleavedforests (6)
andby the WorldConservationUnionas closed
forests(7). We do not documentthe woodlands
or the forestsof the dry tropics,except for continentalSoutheastAsia, wherethe seasonalforests areintermixedwith the humidforests(table
Sl). All figuresreportedherereferto the humid
'Global Vegetation MonitoringUnit, Joint Research
Centre,TP 440, 21020 Ispra,Italy.2LandUse-Land
CoverUnit,JointResearch
Centre,TP263, 21020
TechnologyLimited,4 Old
Ispra,Italy.3Conservation
Tarnwell,SomersetBS184EA,UK.4ScienceStrategy
Directorate,Joint ResearchCentre, EuropeanCommission,B-1049 Bruxelles,Belgium.
*Towhomcorrespondence
shouldbe addressed.
Email:frederic.achard@jrc.it
tropicalforestbiome of LatinAmerica,excluding MexicoandtheAtlanticforestsof Brazil;the
humidtropicalforestbiome of Africa(GuineoCongolianzone and Madagascar);and the humid tropicalforestbiome of SoutheastAsia and
India, includingthe dry biome of continental
SoutheastAsia.
We developeda statisticalsamplingstrategy
usingsatelliteimageryto providea reliablemeasurementof changein tropicalforestcover in a
uniform,independent,and repeatablemanner.
The methodis basedon (i) the establishmentof
subcontinentalforest distributionmaps for the
early 1990s at 1:5,000,000 scale, derivedfrom
1-km2spatialresolutionsatelliteimages;(ii) the
generationof a deforestationriskmap, identifying so-called"deforestation
hot-spotareas"with
knowledge from environmentaland forest expertsfromeachregion(8); (iii) the definitionof
five strataas definedby the forestand hot-spot
proportionsobtainedfrom the previous steps;
of a stratifiedsystematic
(iv) the implementation
scheme
with
100
sampling
samplesites (Fig. 1)
covering 6.5% of the humid tropicaldomain,
which was designedfor change assessmentby
includinghighersamplingprobabilitiesin deforestationhot-spotareas;(v) the change assessof
mentfor each site, basedon the interpretation
fine spatialresolution(20 to 30 m) satelliteimageryacquiredat two datesclosestto ourtarget
years, 1990 and 1997, and performedby local
partnersusing a commonapproach;and(vi) the
statisticalestimatesof forestandlandcovertransitionsat the continentallevel usingthe datathat
between
were obtainedby linearlyinterpolating
the two referencedates.Becausewe appliedan
unequalprobabilitysamplingscheme,a nonclassical statisticalestimator(derivedfromthe Horwitz-Thompsonestimator)was used (9). The
samplingaccuracy(standarderror)was estimated with a resampling(bootstrap)method.
The resultsof our study show that in 1990
(the Kyoto Protocolbaseline year) there were
about 1150 ? 54 X 106 hectares(ha) of humid
tropicalforest(Table 1). The estimatedchange
in global humidtropicalforest areafor the period from 1990 to 1997 shows a markedreduction of dense and open naturalforests: The
annualdeforested(10) areafor the humidtropics is estimatedat 5.8 ? 1.4 X 106 ha, plus a
further2.3 ? 0.7 X 106 ha of forest where
degradationcouldbe visuallyinferredfromsatellite imagery.Largenonforestareaswere also
reoccupied by forests, but these areas were
mainly young regrowth on abandonedland,
along with some forest plantations.Both are
very differentfromnaturalforestsin ecological,
biophysical,and economictermsand therefore
to the loss
counterbalance
arenot an appropriate
of matureforests.
The threecontinentswe examinedrevealed
considerabledifferencesin percentagechange
rates(Table 1). SoutheastAsia had the highest
percentagedeforestationrate,andAfricalost its
forestsat abouthalf the rateof SoutheastAsia.
Latin America showed the lowest percentage
rate,but at a rate of 2.5 X 106 ha year-', the
annualloss of forestareawas almostthe sameas
the loss estimatedfor SoutheastAsia. Forest
degradationshows a similar overall pattern:
most prominentin SoutheastAsia, intermediate
Table 1. Humidtropicalforestcoverestimatesfor the years 1990 and 1997 and meanannualchange
estimatesduringthe 1990-1997 period.All figuresare x 106 ha. Samplefigureswere extrapolated
1991 and
linearlyto the dates 1 June1990 and 1 June1997.Averageobservationdateswere February
1989andMarch1996forAfrica,andMay1990 andJune1997for
May1997for LatinAmerica,February
SoutheastAsia.Estimatedrangesare at the 95%confidencelevel.
Southeast
Asia
Latin
America
Totalstudyarea
Forestcoverin 1990
Forest cover in 1997
Annual deforested area
Rate
Annual regrowth area
Rate
Annual net cover change
Rate
Annualdegraded area
Rate
1155
669 + 57
337
198 + 13
446
283 ? 31
653 + 56
2.5 ? 1.4
0.38%
0.28 ? 0.22
0.04%
-2.2 + 1.2
0.33%
0.83 ? 0.67
0.13%
193 ? 13
0.85 ? 0.30
0.43%
0.14 ? 0.11
0.07%
-0.71 ? 0.31
0.36%
0.39 ? 0.19
0.21%
270 + 30
2.5 ? 0.8
0.91%
0.53 ? 0.25
0.19%
-2.0 + 0.8
0.71%
1.1 ? 0.44
0.42%
www.sciencemag.org SCIENCE VOL297
9 AUGUST2002
Globa
1937
1150 ? 54
1116 ? 53
5.8 ? 1.4
0.52%
1.0 ? 0.32
0.08%
-4.9 ? 1.3
0.43%
2.3 ? 0.71
0.20%
999
R EPO RT S
Fig. 1. Locationsof the 100 observationsites aroundthe tropics.
1000
9 AUGUST2002
VOL297
in Africa, and lowest in LatinAmerica.These
estimatesrepresentonly the portionof degradation identifiableusing our methodology,which
does not include processes such as selective
logging. Reforestationwas dominantin Southeast Asia, but it occurredmainly throughthe
transitionof formermosaics and woodlandsto
forest.Reforestationoccurredless frequentlyin
LatinAmericaas comparedwith SoutheastAsia
and was very limitedin Africa.
Globally,the mainforestconversionprocess
in the humidtropicswas the transformation
of
closed, open, or fragmentedforeststo agricultureat a rateof 3.09 x 106ha year-l (Table2).
The majorforestchangeswere largelyconfined
to a numberof hot-spot areas where change
rateswere alarminglyhigh:Annualtransformation ratesof more than2.5% were measuredat
16 samplesites. In LatinAmerica,the transformationfromclosed, open,or fragmentedforests
to agriculture by clear-cutting dominated
(1.72 X 106ha year-l) (tableS2). This process
is concentratedin hot spots (Table 3), where
forests are increasingly fragmented,heavily
logged, or burned.In addition,3.61 x 106 ha
year-l of mosaics or savanna-woodlands
were
transformedinto agriculturein Latin America.
Surprisinglythe estimatedpercentagerate of
deforestationfor Africawas higherthanthatfor
Latin America, with very high local rates in
Madagascar and Cote d'Ivoire. In Africa,
310,000 ha year-l of forestswere transformed
to agriculture,with a further280,000 ha year- 1
into mosaicsand200,000 ha year-l into savannas or woodlands. For Southeast Asia, the
change estimateindicatesa high annualdeforestation rate and a substantialannual rate of
detectabledegradation.In total, 1.06 x 106 ha
year-l of forestswereconvertedintoagriculture
and 650,000 ha year-l into mosaics. A further
550,000ha year- 1of forestswere degradedinto
savannaor woodlands.At the same time, about
650,000ha year-l of mosaicsor savanna-woodlandschangedto agriculture.
How do our estimates of forest area and
forestareachangecompareto the FAO figures
(1)? The latterare widely used in spite of the
highlightedinternalinconsistencies[chapter46
in (11)] arisingfromthe difficultiesin standardizingnationaldataobtainedfromdifferentcountries(12). Forcomparison,we adjustedthe FAO
figures to the humid tropical domain for the
countriesincludedin our survey(13). Our 1990
globalforestareaestimate(indicatedas TREESII in Table4) shows only a 1.9%relativedifference as comparedwith the FAO estimate(+3%
for Latin America, -9% for Africa, and -6%
for SoutheastAsia). More striking,our global
estimate of net forest area change duringthe
1990-1997 periodis 23% lower thanthe FAO
estimate.
The use of secondaryinformation,expert
opinions,andoutdatedcountrydataby the FAO
may explainthese differences(14). Already,the
FAO forestareaestimatesfor the year 1990 (1)
SCIENCE www.sciencemag.org
REPORTS
Table2. Forestcoverchangesin the humidtropicsfromJune1990 to June
1997. Allareafiguresare x 106 ha.Theforestclassdefinitionswere made
accordingto those applied by the FAO Forest ResourceAssessment
Exercise(11) usingtwo parameters:tree cover (canopydensitywithin a
forest stand) and forest proportion(forest stand density within the
mappingunit).An areaassignedto one of the forest classes had a forest
proportionof morethan 40%in whichthe foreststandshavea tree cover
of morethan 10%.Whenthe forest proportionwas at least 70%,the area
was consideredclosed forest if the tree cover was more than 40% and
open forest if the tree coverwas between 10 and 40%.Whenthe forest
proportionwas between40 and 70%,theareawas definedas fragmented
forest. Plantationsand forest regrowthare groupedas nonnaturalforest.
Referringto the nonforestclasses, mosaicswere definedas containinga
forest proportionbetween 10 and40%.Othernaturalvegetationsuch as
shrubor grassland,but also agriculturalland,may have still containeda
forest proportionor a tree coverup to 10%.Forforest covercalculations,
we appliedforest cover weights per class as determinedby an independent postassessmentof the observationsite results(8). The total forest
coverestimates in 1990 and 1997 were derivedby the additionperclass
of the weightedforest cover areas.Boldfiguresindicatethe total forest
cover in 1990 and 1997; underlinedfiguresindicatethe unchangedarea
for each land coverclass between the two dates.
NonforestclassesForest
Forestclasses
1997
Closed Open Fragmented Plant/regrowMosaics Natural AgricultureUnvegetated
1990
Coverweight
Forest
classes
Nonforest
classes
Closed
Open
Fragmented
Plant/regrow
Mosaics
Natural
Agriculture
Unvegetated
Forestcoverin 1997
in
Forestcover
coverin
100
100
75
100 902.3 11.2
100
1.7 120.6
75
1.8
1.0
100
0.0
0.1
25
0
0
0
0.9
1.0
0.6
0.0
Perclass 908
total
were foundto be muchhigherthanthe previous
FAO estimatesfor the same year (6), with the
exceptionof SouthAmerica(12). Furthermore,
ourTREES-I forestareaestimatesfor 1990 are
very close to our estimates from a previous
TREES-I study (15) that used coarse-spatialresolutionmaps calibratedwith a sample of
maps (16). Our forest
high-spatial-resolution
area changeestimatesare lower thanthe FAO
estimatesthat were adjustedto the humid domain (17) by an amount of -0.5 x 106 ha
year-l foreachcontinent.In SoutheastAsia, the
FAO estimatefor Indonesia(which represents
39% of the forestareaof this region)is largely
based on nationalremote sensing-derived informationfor earlieryears(1985 and 1997) and
does not include the exceptionalfire event in
Indonesiain 1997-1998 (18, 19) (neitherdoes
our survey). In Africa, the differencecan be
explained by the very low in-countryforest
monitoringcapacitiesof most countries.
In LatinAmerica,our estimatesreferto two
subregions:the BrazilianAmazonandGuyanas
subregion and the pan-Amazon and Central
Americasubregion.OurBrazilianAmazonand
Guyanassubregionestimates(420 ? 37 X 106
ha of forest areain 1990 and -1.32 + 0.74 x
106ha year- of forestareachange)areclose to
estimatesfromothersources(401 X 106ha and
-1.43 x 106 ha year-1) (20), with small relative differences(5 and 9%). Because the latter
regional estimateswere derived from wall-towall assessmentsusing high-resolutionsatellite
images, the similarityin estimatesprovidesan
independentconfirmationthat our method allows for a determinationof global humidtropical forestcover changein a more reliableway
thanwas previouslyavailableandhighlightsthe
0.1
0.4
0.3
0.0
134
100
25
0
0
0
4.1
2.4
37.8
0.0
1.1
0.1
0.1
72
4.6
1.2
3.0
0.1
3.4
1.6
1.0
0.3
16.3
2.3
3.1
1.2
1.1
0.2
0.2
0.1
0.5
0.2
0.3
0.0
0.1
0.3
0.3
0.1
108.5
4.1
2.6
0.0
3.2
377.1
3.6
0.8
10.4
21.6
232.9
0.5
0.6
1.4
0.6
33.7
31
0
0
34
9
Perclass Total
944
130
36
9
31
0
0
0 1150
0
1116
Table3. Annualdeforestationrates,as a percentageof the 1990forestcover,forselectedareasof rapid
forestcoverchange(hot spots)withineachcontinent.
Annualdeforestationrateof
samplesites withinhot-spot
area(range)
Hot-spotareasby
continent
0.38%
0.8-1.5%
LatinAmerica
CentralAmerica
Brazilian
Amazonian
belt
Acre
Rondonia
MatoGrosso
Para
Colombia-Ecuador
border
Peruvian
Andes
Africa
Madagascar
Coted'lvoire
SoutheastAsia
SoutheasternBangladesh
CentralMyanmar
CentralSumatra
SouthernVietnam
SoutheasternKalimantan
4.4%
3.2%
1.4-2.7%
0.9-2.4%
-1.5%
0.5-1.0%
0.43%
1.4-4.7%
1.1-2.9%
0.91%
2.0%
-3.0%
3.2-5.9%
1.2-3.2%
1.0-2.7%
importanceof this new estimateof forest area mates is suggested to be as high as ?30
to ?60%. Carbonwas assumedto be 50% of
change in the humidtropics.
biomass(3). Theresultingregionalestimatesare
Our data can help reduce the amount of
from
129 tons of carbon(tC) ha-1 for the pan-Amain
net
carbon
flux
uncertainty calculating
deforestation(21) and regrowthin the humid zon and CentralAmericaregion, 190 tC ha-1
tropics.To estimatenet carbonflux, we consid- for the BrazilianAmazon forests (23), 179 tC
ered existing regional figures of total carbon ha-~ for tropicalmoist Africa,and 151 tC ha-'
vegetationbiomassderivedfromthe actualbio- for SoutheastAsia. Carbonfluxes can then be
mass density without roots (22) as a starting computedusingthe fractionsof biomassthatare
point. These figuresare weightedby the 1990 assumedto be convertedto CO2as a resultof
forest area, and we added 20% for below- the deforestationand regrowth carbon rates,
groundvegetation(root)biomass,acceptingthat which are proportionalto initialforestbiomass
rootbiomassvariesconsiderablyin tropicalfor- (24). The fractionsof biomassconvertedare0.2
ests (22). The errorrangeof such biomassesti- frominitialforestbiomassburned,0.008 annual
www.sciencemag.org SCIENCE VOL297
9 AUGUST2002
1001
REPORTS
humidtropicalforest cover estimates with FAOestimates.TREES-II,
this study;
Table 4. Comparisonof TREES
TREES-I,
previousstudy (15). FAOcountryestimatesare derivedfromthe countrytables (1). Indiawas included
with SoutheastAsia but not 41 x 106 ha of India'sdry forest. ForAfricaand LatinAmerica,we correctedthe
countryestimatesto the humiddomainby multiplyingthe forest area by the proportionof rainand mountain
forests,excludingthe moist and dryforests [appendix3 in (11)]. Mexicowas excludedfrom LatinAmerica.The
estimates of net change in forest coverwere interpolatedto the June 1990-June 1997 period.Average
TREES
observationdates were June1990 and March1997 forthe TREES
study.FAOforestcovernet changeestimates
are reportedfor the 1990-2000 period.The averagereferenceyears for the latest area data used by the FAO
are 1991 for Africaand South Americaand 1995 for Asiaand CentralAmerica.Estimatedintervalsare at the
95% confidencelevel.
Southeast Asia
Africa
LatinAmerica
Global
283 ?
198 ?
669 ?
1150 ?
31
13
57
54
TREES-I
281
207
671
1,158
ratefromdecay of wood removedfromthe site
for a 10-yearperiod,and0.07 initialannualrate
with an exponentialdecreasein time from the
decay of biomassleft as slash.The initial(firstyear)totalfractionof 0.28 increasesto 0.72 over
a 10-yearperiod,when includingfuturesources
embodiedin first-yeardecaypools, and to 0.97
over a 75-year period. The accumulationof
carbonon abandonedlandsthatrevertedto forests (24) is taken as 2.8, 5.5, 5.0, and 3.8 tC
ha-1 year-1 forthe pan-Amazon,Brazilian,African,andSoutheastAsianregions,respectively,
with a maximum accumulationof 129, 190,
179, and 151 tC ha-1.
FAO
country
302
218
652
1,172
TREES-II
-2.0
-0.7
-2.2
-4.9
?
?
?
+
0.8
0.3
1.2
1.3
FAO
country
-2.5
-1.2
-2.7
-6.4
(3). Consideringthatthenetchangein forestarea
is lower in the dry tropicsthan in the humid
tropics(11) andthatthe biomassof drytropical
forests is less than half that of humid tropical
forests(22, 26), a maximumestimateof global
net emissionsfromland-usechangein the tropics wouldbe about0.96 GtCyear-'. Evenif this
figuredoesnot includeloss of carbonfromforest
degradation,which is much more difficult to
estimate,this resultleads us to believe thatthe
residualterrestrialuptakemust be smallerthan
previouslyinferred.
17.
18.
19.
20.
21.
22.
23.
References and Notes
Fromour annualdeforestationandregrowth
estimates,we can computethree estimatesof
carbonflux: an initial flux for the first year, a
flux for the next 10 years (includ"committed"
future
sources
andsinks),anda "committed"
ing
flux forthenext75 years.Thefirst-yearfluxwill
the impactof the landobviouslyunderestimate
cover change.The 75-yearcommittedflux implies that the deforestationand regrowthrates
that we have measuredhave been constantfor
the past 75 years. The 10-yearcommittedflux
hasthereforebeenassumedto be morerepresentativethanthe 75-yearcommittedflux. For the
BrazilianAmazon,comparisonwith otherstudies supportsthis assumption:Our 10-yearand
75-yearcommittedflux estimatesfor this region
are 0.19 + 0.12 GtC year-' and 0.24 + 0.18
GtC year-1, which correspondwell with the
estimatesof 0.18 GtCyear-' of annualnet flux
overtheperiodfrom1989to 1998(24) and0.26
GtC year-' of annual100-yearcommittedflux
(25).
Using our 10-yearcommittedflux figureas a
good estimateof the actualannualnet flux leads
to a globalestimateof 0.64 + 0.21 GtC year-1
for the periodfrom 1990 to 1997. This estimate
is farlowerthanthe estimateof totalannualnet
emissionfromland-usechanges,primarilyin the
tropics, for the period from 1989 to 1998 as
reportedby the IPCC (1.6 ? 0.8 GtC year-1)
1002
16.
Annualforest area
change, 1990-1997
(106 ha year-')
Forest area for the year 1990 (106 ha)
TREES-II
15.
1. State of the World'sForests2001 (FAO,Rome, 2001).
2. H. J. Geist, E. F. Lambin,WhatDrivesTropicalDeforestation? (Louvain Univ., Louvain-Da-Neuve,Belgium,
2001).
3. R.T.Watsonet al., Eds.,LandUse,LandUse Changesand
Forestry(CambridgeUniv.Press,Cambridge,2000).
4. D. S. Schimel et al., Nature 414, 169 (2001).
5. R. A. Houghton, Eos 81, S281 (2000).
6. ForestResourcesAssessment 1990: TropicalCountries
(FAO,Rome, 1993).
7. C. S. Harcourt,J. A. Sayer, The ConservationAtlas of
Tropical Forests: The Americas (Macmillan,London,
1996).
8. F. Achardet al., Eds.,Identificationof Deforestation
Hot Spot Areasin the HumidTropics(EuropeanCommission, Luxembourg,Luxembourg,1998).
9. Materials and methods are available as supporting
material on Science Online.
10. Deforestation is defined as the conversion from forest (closed, open, or fragmented forests; plantations;
and forest regrowths) to nonforest lands (mosaics,
natural nonforest such as shrubs or savannas, agriculture, and nonvegetated). Reforestation (or regrowth) is the conversion of nonforest lands to forests. Degradationis defined as a process within forests that leads to a significantreduction in either tree
density or proportion of forest cover (from closed
forests to open or fragmented forests).
11. Global Forest Resources Assessment 2000 Main Report (FAO,Rome, 2001).
12. E. Matthews, Understandingthe FRA2000 (World
Resources Institute, Washington, DC, 2001).
13. The FAOestimates were extractedfor the corresponding countries,restrictedto the humiddomain(usingthe
FAO definitions of rain and mountain ecofloristic
zones), and aggregatedto the continentallevel
14. As "inmanycountries,primaryinformationon forestarea
was not availableor was not reliable,"FAO"hadto rely
on secondaryinformationand/orexpertestimates"(11).
9 AUGUST 2002
VOL 297
24.
25.
26.
27.
28.
29.
30.
In particular,the averagereferenceyears for the latest
area data are 1991 for Africaand South Americaand
"a
1995 for Asiaand CentralAmerica(11). Furthermore,
high proportionof developingcountrieshad to rely on
expertopinionfor the latest area estimates"(11). Comparisonsmay be less informativebecause of the expert
extrapolationsto the 1990-2000 period.
P. Mayaux,F. Achard,J. P. Malingreau,Environ.Conserv. 25, 37 (1998).
Inthe previousmethod (27), the 1-km resolutionbaseline forest continentalinputmaps were the same as for
the presentstudy,butthe set of high-resolutionimagery
was different(30 sites were selected insteadof 100, and
at differentlocations).Also,the baselinemapswere used
in a differentway in the previousstudy, to derivethe
two calibrationcovariables-forest areaand fragmentation-for all 100-km2grid cells of the humid tropical
zone, ratherthan to derive two samplingcovariables,
forest area and hot-spot area,for the sampleframeand
for the regions.
The FAO net change estimate for Africa before adjustment, mainly from the contributions of a few
countries that include a large proportion of dry forests (Cameroon, C6te d'lvoire, Democratic Republic
of Congo, and Nigeria),is -1.9 x 106 ha year-'. The
LatinAmerica FAO net change estimate before adjustment, including contributions of deciduous forests from Bolivia, Colombia, Peru, Venezuela, and
Brazil,is -3.6 x 106 ha year-' (11).
H.-J.Stibig,J. P. Malingreau,R.Beuchle,Int.J. Remote
Sens. 22, 503 (2001).
F. Siegert, G. Ruecker,A. Hinrichs,A. A. Hoffman,
Nature 414, 437 (2001).
We used two other estimates for the BrazilianAmazon: the LANDSATPathfinder1988 forest area estimate (28) normalized to 1990 (362 x 106 ha) and
the Brazilianaverage estimate of net change (29)
correctedfor deciduous forest contributions(-1.38 x
106 ha year-'). For the Guyanas region, the FAO
estimates were used.
R.J. Scholes, I. R. Noble, Science 294, 1012 (1993).
S. Brown,EstimatingBiomassand BiomassChange of
TropicalForests (FAO,Rome, 1997).
The BrazilianAmazon estimate is the average of two
estimates: 186 tC ha-' and 195 tC ha-'. The first
estimate was derived from 310 tC ha-1 of actual
biomass density without roots (22), and the second
was an average of three estimates: 145, 210, and 232
tC ha-' (24).
R. A. Houghton et al., Nature 403, 301 (2000).
P. M. Fearnside,Clim. Change 35, 321 (1997).
Q. Zhang, C. 0. Justice,Ambio 30, 351 (2001).
P. Mayaux,E. F. Lambin,Remote Sens. Environ.59, 29
(1997).
D. Skole, C. J. Tucker,Science 260, 1905 (1993).
Deforestation 1995-1997 Amazonia(Instituto Nacional de Pesquisas Espaciaisand Institute Brasileirodo
Meio Ambiente e dos RecursosNaturals Renovaveis,
Brazil,1997).
of
Supportedby the DirectorateGeneralfor Environment
the EuropeanCommission.We thankthe local partners
fromtropicalcountrieswho interpretedthe imageryover
the sample sites, in particularthe regionalcoordinators:
G. Leclerc,CentroIntemacionalde Agricultura
Tropical,
Colombia;A. Dorado,Ecoforca,Brazil;P. S. Roy, Indian
Instituteof Remote Sensing;U. R. WasrinSyafii,Bogor
BelUniversity,Indonesia;M. Massart,IMAGE-Consult,
MadaFoiben-Taosarintanin'i
gium;A. M. H. Faramalala,
The contributionof R. Drigo,Istigasikara,Madagascar.
tuto Agronomicoper l'Oltremare,
Italy,to the consistency assessmentis also acknowledged.We thankP.Janvier
and A. Brinkfor assistancein developingthe Geographic
InformationSystem (GIS)database;S. Fritzfor assistance
with the GISrepresentationof Fig.1; and A. Belward,P.
Kennedy,G. Matteucci,andthe anonymousreviewersfor
theirconstructivecommentson the manuscript.
Supporting Online Material
www.sciencemag.org/cgi/content/full/297/5583/999/
DC1
Materialsand Methods
Fig. S1
Tables S1 and S2
7 February2002; accepted 8 July 2002
SCIENCE www.sciencemag.org