GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 6, NO. 2, PAGES 139-159, JUNE 1992
INTERANNUAL
VARIATIONS
IN TUNDRA
METHANE
EMISSION:
A 4-YEAR TIME SERIES
AT FIXED SITES
Stephen
C. WhalenandWilliamS. Reeburgh
I
Instituteof Marine Science,Universityof Alaska
Fairbanks
Abstract.This papersummarizes
4 years(19871990) of weekly net CH4 flux measurements
at
permanent
sitesrepresenting
importantplant
components
of Arctictundra.The datacoincidewith
variationsin precipitation
andtemperature
of interest
in regionalandglobalmodelingeffortsandareuseful
in placingboundson theroleof tundrain theglobal
CI-h budget.Precipitationin the studyareaduring
the summeremissionperiodrangedfrom twiceto
half the long-termmean,andair temperature
andtussock:shrub
tundrapresentlyemit about42 +
26 Tg CH4 yr-1to the atmosphere.
This estimatehas
a North Americanbias,but it is supportedby
measurements
in a rangeof locations,transect
anomalies were about +2 øC. This data set also
Earth's land area and contain about 13% of its stored
permitsconsideration
of temporal(seasonal
to
interannual)and spatialvariabilityin CH4 flux. We
studiedtherelationship
betweenthenetCI-h flux and
subsurface
properties(watertabledepth,thaw depth,
soiltemperature,
pCH4 distributions)
at these
permanentsitesduringthe 1988 and 1989emission
periods.Net CI-h emissionandsubsurface
properties
arelargelyunrelated.Relationships
betweensoil
temperature
(or anysinglevariable)andemissionare
sitespecificandareof little valueasflux predictors.
Parameters
thatintegrateconditions
influencingflux
appearto be thebestflux predictors
overthe
emissionperiod.We estimatethatArcticwet meadow
soil carbon [Post et al., 1982]. The fate of this stored
XDepartment
of Geosciences,
University
of
California, Irvine.
Copyright1992
by theAmericanGeophysical
Union.
Paper number92GB00430.
0886-6236/92/92GB-00430. $10.00
studies, and model calculations.
INTRODUCTION
Tundraenvironments
occupysome7% of the
soilcarbonunderalteredclimateis a majorquestion
[Billings, 1987]. The role of high-latitudewetlands
in the globalCH4 budgetwaspoorlyknown until
recently.Prior to 1986,only threepublications
[Svensson,1976; Svenssonand Rosswall, 1984;
Sebacheret al., 1986] reportedmeasurements
of
CH4 fluxesfrom tundraenvironments.
Reportsof
increasesin atmosphericCI-I4concentrations
[Rasmussenand Khalil, 1981, 1984; Blake and
Rowland, 1988], conf'n'mationof the increasein ice
cores[Raynaudet al., 1988; Staufferet al., 1985],
andtherealizationthattracegasescouldhavean
effecton greenhouse
winming[Dickinsonand
Cicerone, 1985; Ramanathanet al., 1985] have
stimulateda broadreexaminationof the globalCH4
budget[CiceroneandOremland,1988;Funget al.,
1991].
Severalapproaches
havebeenusedin assessing
the
role of highlatitudewetlandsin the globalCH4
budget.Theseincludevegetationassessments
and
applicationof emissiondatafrom characteristic
areas
140
[MatthewsandFung, 1987;AselmannandCrutzen,
1989;Funget al., 1991],intensivefield campaigns
in representative
areasoverrelativelyshorttime
periods(e.g., ABLE 3A (Yukon-Kuskokwimdelta,
1988), ABLE 3B/NOWES (Hudson'sBay lowlands,
1990), flux transects[Sebacheret al., 1986; Whalen
andReeburgh,1990a],andtime seriesflux
measurements
at representative
sitesover several
months [Crill et al., 1988; Harriss et al., 1982;
Moore and Knowles, 1990; Moore et al., 1990] or a
year [WhalenandReeburgh,1988].The work
reportedhereextendsthelatterstudyto 4 years,1987
through1990.Duringthisperiodthesummer
precipitation
variedby twiceandhalfof thelong-term
(--50 yr) mean,andair temperatures
weregenerally2
øCabovethelong-termmean.Thedataallowthefin:st
assessment
of interannual
variabilityin high-latitude
CI-t4flux andareusefulin placingboundson tundra
CH4 emission. Further, the interannualvariations in
precipitation
andair temperature
covera rangeof
interestto globalclimatechangemodelers.
METHODS
This studyis a continuation
of ourearlierwork
[WhalenandReeburgh,1988]andusesthe same
samplingsitesandmethods.
Sites
Samplingsiteswere locatedin a muskegandalong
a pondmargin(SmithLake) in theUniversityof
Alaska Arboretum (64 ø 52' N, 147ø 51' W, 158.5 m
elevation).Physicalandvegetativecharacteristics
of
the studyareaaresimilarto thoseof Arctictundra;
poorlydrainedsoftsareundedainby permafrostand
an EriophorumvaginarumL. tussockplant
community[Kummerowet al., 1983;Wein and
Bliss, 1974] dominatesthe muskeg,while extensive
standsof Carex spp.line the marginof SmithLake.
Threepermanentsamplingstationswerelocated
within eachof four floristic unitsor sites:(1) E.
vaginaturntussocks(T stations),(2) intertussock
depressions
("blackhole"or BH stations)thatare
oftenwaterloggedandhavea surfacelayerof detritus
or Sphagnumspp.,(3) mossyareas(M stations)
which are elevated relative to the BH stations and
containno Sphagnum,and(4) Carex stands(C
stations).Examplesof a T, BH, andM stationwere
groupedor clusteredwithin 1 to 2 m at three
locationsseparated
by 10 to 20 m withinthemuskeg,
and C stations were established in three distinct
standsof CarexaquatilisWahlenbach
separated
by
about20 m on the marginof SmithLake. The sites
weredispersed
oversucha largeareathatelevated
walkwayswereimpractical;firm soilsminimized
WhalenandReeburgh:
TundraMethaneEmission
stationdisturbance
duringsampling.Pathswere
worn to the sites, and the stationsbecame somewhat
shopwornafter3 yearsof year-roundstudy;new
stations
wereestablished
in thesamemuskegand
pondmarginbeforefreeze-upin 1989andwere
sampledduring1990.We determinedthatwinter
CI-h emissions
wereinsignificant
andsuspended
winter measurementsin December, 1989.
Net CH4 Flux Measurements
Net CH4 fluxesweredetermined
usingstatic
chambers. Each flux chamber consisted of a skirted
aluminumbase,stackablelucite vertical sections,and
a lucitelid. Eachlid wasfittedwitha capillarybleed
to equalizepressure
anda SwagelokO-sealfitting
equippedwith a septumfor syringesampling.
Waterfilled channelsprovidedsealsbetweenthechamber
components.
The aluminumbaseswerepermanently
setin the soil at M, BH, and C sites,and isolatedan
areaof 0.075 m2. Smallerbases(0.023 m2) were
permanently
inserteddirectlyintothetopsof
Eriophorumtussocksat T sitesfrom 1987 to 1989;
regularbases(0.075 m2)wereusedto surroundeach
tussock at the soil surface when new stations were
established
in 1989. Chamberheadspaces
were
sampledwith glass-barrel
syringesovera 0.5 to 24
hour time course, and CI'h concentrationswere
det•ed
withina few hoursof samplecollection
by gaschromatography.
A ShimadzuGC Mini-2 gas
chromatograph
equippedwith a flameionization
detectorandsamplingvalve wasusedfor the
analyses.
Analyticalprecisionwas2%; working
standards were relatable to National Bureau of
Standards standards. Methane fluxes were calculated
fromchambergeometryandthelinearchangein CH4
concentration with time.
Other Measurements
All supporting
environmental
measurements
were
madeat primarystations(T- 1, M- 1, BH- 1, andC- 1
in 1987 through1989; T-4, M-4, BH-4, and C-11 in
1990) wheneverCH4 fluxes were determined;less
comprehensive
environmentaldatawerecollected
simultaneously
at otherstations.Soil environmental
variables are measured relative to the soil surface.
The water table level was measured between 1988
and1990.Shallowwellswereboredadjacentto all
stations,
werecasedwith perforatedPVC pipeto
preventcollapse,andwere screenedto avoid
decimating
thelocalvolepopulation.
Soiltemperatures
weremeasured
withpermanently
deployed
multithermister
probesextending
fromthe
soilsurfaceto permafrost
at all primarystations
141
WhalenandReeburgh:
TundraMethane
Emission
to thedeepestthermistor
werecalculated
at C primary
exceptC-1 andC-11,wherethermistors
extended
to
40 and70 cm, respectively.
Mean soiltemperatures
in thethawedlayerat T, M, andBH primarystations
werecalculated
by integrating
temperature
datafrom
thesoilsurfaceto permafrost.
Mean soiltemperatures
stations.
Thaw depthsweremeasured
by insertinga 3-mmdiameter stainless steel rod to the frozen soil horizon.
The thawseasonreportedat eachstation(Tables1-4)
TABLE 1. 1987Summary
DataforCH4FluxandEnvironmental
Variables
DuringThawSeason
at
Permanent
SamplingStations
in theUniversityof AlaskaArboretum
CH4Flux,mgm-2d-1
Site
n
Mean
SEM a
Maximum
Median
Maximum
Mean Soil
Thaw
ThawDepth, Duration, Temperature,
oC
cm
days
4.4
2.6
1.5
2.5
1.3
0.5
33.6
16.0
5.6
-45
-52
-49
0.8
1.1
0.7
1.0
136
125
131
BH-1
BH-2
BH-3
14
12
13
C-1b
11
14.1
3.2
36.7
C-2
C-3
M-1
M-2
M-3
11
11
16
15
15
36.5
50.0
1.7
0.5
0.6
5.5
7.6
0.5
0.2
0.3
75.2
104.9
5.5
2.1
3.8
32.4
38.4
0.4
0.2
0.1
-62
-55
-62
146
141
141
3.0
T-1 c
T-2
T-3
16
16
16
51.4
8.7
28.3
15.9
2.7
8.6
166.6
30.1
90.3
15.5
0.6
8.5
-71
-83
-76
146
146
146
3.3
10.5
7.7
aSEM is the standard error of mean.
balldataforCarex
(C)sites
taken
fromMay26(-•3weeks
afterspring
thaw)untilfreeze-up.
CThawdepthfrom topof tussock.
TABLE2. 1988
Summary
DataforCH4FluxandEnvironmental
Variables
During
Thaw
Season
at
Permanent
Sampling
Stations
intheUniversity
ofAlaska
Arboretum
Site n
BH-1
BH-2
BH-3
C-1
C-2
C-3
M-1
M-2
M-3
T-1b
T-2
T-3
CH4Flux,mgm-2d-1
Mean SEMa MaximumMedian MaximumWaterTable Thaw
18 18.6
18
8.8
18 48.2
19
2.1
19
5.3
19
2.1
19 23.1
19
1.9
19
5.3
19 28.2
19 27.5
19 126.7
ThawDepth, Range,
8.5
7.0
18.3
1.3
0.5
0.4
7.8
0.8
2.7
8.3
8.6
43.7
139.5
127.9
292.0
11.1
11.9
5.2
146.4
14.0
43.2
134.8
143.3
653.2
1.6
0.8
2.3
0.7
4.0
1.9
11.4
0.3
0.3
5.3
7.4
4.3
cm
cm
-53
-57
-58
-3 to5
0to 15
-6to8
-71
-64
-70
-80
-88
-85
-6 to0
-16to0
-18to0
0 to9
-10to8
-6to2
Mean Soil
Duration, Temperature,
days
148
148
148
155
155
155
155
155
155
155
155
155
aSEM is the standarderror of mean.
bThaw
andwater
tabledepths
fromtopoftussock
andsoilsurface,
respectively.
oC
1.3
8.6
3.1
4.1
142
WhalenandReeburgh:
TundraMethaneEmission
is theperiodfrominitialrodpenetration
in thespring
to surfacesoilfreeze-upin thefall. The "warmrim
samplers[Hesslein,1976] within0.5 rn of primary
stationsduringthe 1988 and 1989 thaw seasons.
effect" [Likens and Johnson,1969] limited
The 1989 soil CI-h measurements were made at
permafrostformationin thelakemargin,sono thaw
depthdeterminations
weremadeat C stations.
SoilCI-Inprofilesweresampledwithequilibration
stationC-7. The samplers
weremadeof luciteand
hadequilibration
wells(3 cm3)spacedat 2.5-cm
intervalsalongthesampleprobe.The equilibrafion
TABLE 3. 1989Summary
DataforCH4FluxandEnvironmental
Variables
DuringThawSeason
at
PermanentSamplingStationsin theUniversityof AlaskaArboretum
CH4Flux, mgm-2d-1
Site
n
Mean
SEMa
Maximum
Median
Maximum
Water Table
ThawDepth,
cm
BH-1
BH-2
BH-3
C-1
C-2
C-3
M-1
M-2
M-3
19
19
19
20
20
20
19
19
20
9.6
10.4
17.1
12.8
15.7
40.4
76.3
3.9
6.9
4.9
2.7
7.7
2.8
5.4
5.9
20.6
0.9
1.8
T-1b
20
42.9
T-2
T-3
20
20
23.5
79.9
6.8
4.9
28.2
82.7
40.3
145.3
39.8
104.1
80.7
366.9
13.6
28.2
85.5
82.5
445.2
0.3
8.1
7.2
10.9
6.6
46.2
43.5
3.0
4.5
-55
-62
-61
Range,
cm
Thaw
Mean Soil
Duration, Temperature,
days
øC
-22to8
-27 to 10
-29 to7
139
139
139
151
151
151
139
139
151
2.9
10.1
-71
-75
-72
-30 to 0
-40 to 4
-29 to 0
5.0
35.5
-80
-23 to7
151
20.3
21.6
-96
-95
-32 to 9
-29 to 7
151
151
4.9
aSEM is the standard error of mean.
bThaw
andwater
table
depths
fromtopoftussock
andsoilsurface,
respectively.
TABLE4. 1990Summary
DataforCH4FluxandEnvironmental
Variables
atPermanent
Sampling
Stations
in theUniversityof AlaskaArboretum,May 21 throughSeptember
22, 1990
CH4Flux,mgm-2d-1
Site
n
Mean
SEM a Maximum
Median
Maximum
Thaw Depth,
BH-4
BH-$
BH-6
C-11
C-12
C-13
M-4
M-5
M-6
17
17
17
17
17
17
17
17
17
14.3
2.9
0.4
299.6
644.9
411.3
7.2
3.0
1.1
5.1
1.2
0.2
60.0
145.7
79.8
1.7
0.7
0.3
57.4
20.1
2.9
930.9
2216.4
1289.3
26.1
8.7
4.3
1.6
1.3
0.2
212.3
478.0
371.1
5.9
2.7
0.4
T-40
T-5
T-6
17
17
17
89.8
136.3
64.4
17.1
21.8
11.2
190.8
301.9
198.4
92.0
126.3
65.0
cm
-74
-71
-58
Water Table
Range,
Mean Soil
Temperature,
øC
2.5
-74
-70
-60
cm
6 to 20
0 to 12
0 to 18
1 to48
13 to62
15 to65
-2 to 15
-21 to-9
-7 to-25
-91
-81
-95
-10 to 10
-12 to 1
-7 to 11
4.7
aSEM is the standard error of mean.
bThaw
andwatertabledepths
fromtopofressock
andsoilsurface,
respectively.
11.2
3.2
143
WhalenandReeburgh:
TundraMethane
Emission
wellswerefilled with degassed
distilledwaterand
werecoveredwith a gas-permeable
membrane
(Teflon FEP, 2 mil thickness),which was held in
placewith a perforated
plate.The samplers
were
insertedin a snugpilot holemadepreviouslywith
anothersamplerandwereallowedto equilibratein
thefield for 2 weeks.Equilibratedwater(2 cm3)was
withdrawnby puncturingthemembranewith two
hypodermic
needles,oneattached
to a syringefor
samplecollectionandonethatservedasanair inlet.
The syringecontentswereinjectedinto capped,He-
RESULTS
Methane Flux Time Series
Figures1-4 shownet CI-I4flux at T, M, BH, and
C stations,respectively,from 1987 through1990.
Negativefluxes(netCH4 consumption)
werenever
observedduringthe study.Methaneemissionvaried
seasonally
at all stations,
althoughthe seasonality
is
lessevidentwith the log flux scale.In general,
highestfluxesoccurredin midsummer,andlowest
fluxes were recorded in late winter. Winter emissions
were at or near zero at T and BH stations but
filled serumvials (30 cm3)that were allowed to
equilibrate24 hourswith periodicshakingbefore
analysisof headspace
CI-h. The watertableposition
sometimes
fell belowthetopmostequilibrafionwells
duringdeployment,
andsincethesewellsremained
waterfilled, theysampledgasesin themoistsoil
zone.Bunsensolubilitycoefficients
at 10 øC
[Yamamotoet al., 1976] were usedto calculatethe
persistedat 1 to >100 mg CI-I4m-2d-1at someM
and C stations. Winter flux characteristics at active
stationsshowedno year-to-yearconsistency.
For
example,M-3 showednotablewinterCI-I4emission
only during1988-1989,whenNovemberthrough
April fluxesvariedfrom 5.7 to 128.9mg m-2d-1.
Similarly,C-3 exhibitedlittlewinteractivityuntil
1989, when more limited data (Novemberthrough
December)showedCI-h emissionrangingfrom 2.5
to 11.1mg m-2d-1.Thaw seasonCI-h emission
dissolvedCI-I4 concen•afion. Soil CI-I4 distributions
areexpressed
aspCI-h (matm)to presentdatafrom
boththe waterloggedandmoistsoil zonesin
comparableunits.
_
!
-
o, lO'
. lOø
O-
/ ,x\\•a!/
/
-lO
ß
-•
•
-20-
-30
-40
i
I
i
II
III
1987
IV
I
II
III
1988
IV
I
II
III
1989
IV
I
II
III
1990
IV
Fig.1.NetCI-I4
fluxtoatmosphere
andsoiltemperatures
fortussock
(T)sites
inUniversity
ofAlaska
Arboretum,
1987-1990.
(Top)NetCI-hfluxversus
timeforpermanent
tussock
stations.
Notelogfluxscale;
fluxes
of0 mgm-2d-1plotted
onabscissa.
(Bottom)
Soilisotherms
(4øCcontour
interval)
ondepth
versus
time
plotforstations
T-1(1987-1989)
andT-4(1990).
Winter
measurements
weresuspended
after
December
1989.
WhalenandReeburgh:
TundraMethaneEmission
144
lO'
'
.2•10'
'E
•101
i•.•.
•:,•
1•2 •
o-M1 ? a
k o-M4
=---M2 /• •
•]
==M5
,",
=M3• • '•,,:,,.'t"•
).!: "=M6
.•.A.A
• "'
•'•__j"•
• •.":'•
-10
E
-2O
-30
i i i !
-40
I
II
Ill
1987
IV
I
II
Ill
1988
IV
I
II
Ill
1989
i
IV
I
II
ill
IV
1990
Fig. 2. Net CH4 flux to atmosphere
andsoiltemperatures
for moss(M) sitesin Universityof AlaskaArboretum,
1987-1990.(Top)Net CH4flux versustimefor threepermanent
mossstations.
Notelog flux scale;fluxesof 0
mg m-2d-] plottedon abscissa.
(Bottom)Soilisotherms
(4 øCcontourinterval)ondepthversustimeplotfor
stations
M-1 (1987-1989)andM-4 (1990).Wintermeasurements
weresuspended
afterDecember1989.
accountedfor 77 to 92%, 33 to 88%, 83 to 95%, and
65 to 88% of annualemissionat T, M, BH, and C
sitesduringthecalendaryears1987through1989.
Thaw season emissions would have been >90% at all
primarystationsif the thaw seasons
wereconsidered
to extenduntilsoilsfrozeto permafrost.
Thisusually
occurred2 to 3 weeksafterfreezingof surfacesoils.
Summarydatafor CI-I4flux duringthe 1987
through1990thawseasons
aregivenin Tables1-4.
Thaw seasonCI-I4emissionat eachstationis so
variablethatbothmean(andstandarderror)and
medianfluxesaregivenasmeasures
of central
tendency.Mean andmedianfluxesfor T stationsfor
all yearsandmeansandmediansfor C stations
during1987and1990generallyrankedhighestwhen
within-yearcomparisons
of datafromall stations
weremade.Thesestations
consistently
showed
high
emissionthroughout
thethawseason.
Mean and
medianfluxesat M stations
(exceptM-1) were
consistently
low.Blackhole(BH) stations
frequently
showedhigh mean,but low medianfluxes.This
resultedfrom episodicemissionon oneor two dates
(e.g., BH-3 in 1988;Table 2). Minimum fluxes
approached
zeroin mostcasesandarenotgiven.
Within-siteandspatialvariationsin thaw season
CH4 fluxeswereremarkablyhigh. Coefficientsof
variation (CV) for CH4 emissionfrom T, M, BH,
andC sitesaveraged84, 118,93, and67% (n = 62
to 73; stationsseparated
by 10 to 20 m). The CV for
CH4flux fromT-M-BH clusters(all data)averaged
96% (n = 198;stationsseparated
by 1 to 2 m).
Methanefluxes(mg m-2d-I) for eachstation
(Figures1 - 4) wereaveraged,andthesewere
integrated
(trapezoidal
role)overtimeto giveannual
emissionestimatesfor eachsite(Table5). Methane
emission from the T site showed little interannual
variability,rangingovera factorof only 1.7.In
contrast,annual emissionsat the BH and M sites
rangedoverfactorsof 7 and 10,respectively,
andthe
datafor theC sitevariedby a factorof 74.
Annualflux estimates
for eachtundracomponent
in
Table5 wereweightedasin theworkby Whalenand
Reeburgh[1988] to estimateglobaltundraCH4
emission
for 1987through1990(Table6). Overlap
in percentgroundcoveroccupied
by eachsitetype
causestotalarealcoverageto exceed100%.
Nonetheless,
threepointsareclearin Table 6. First,
tussocks
(T site,Eriophorurn)areresponsible
for
WhalenandReeburgh:
TundraMethaneEmission
•
145
-15 -20
-25
,
,
I
!
II
II
1987
1988
i
Ill
1989
II
Ill
1990
IV
Fig. 3. Net CI-I4flux to atmosphere
andsoiltemperatures
for intertussock
depression
or blackhole (BH) sitesin
Universityof AlaskaArboretum,1987 1990. (Top) Net CH4 flux versustime for threepermanentblackhole
stations.Note log flux scale;fluxesof 0 mg m-2d-1plottedon abscissa.
(Bottom)Soil isotherms(4 øC contour
interval)on depthversustime plot for stationsBH-1 (1987-1989) andBH-4 (1990). Winter measurements
were
suspended
afterDecember1989.
most of the CH4 emitted from tussock:shrubtundra.
Second,the C site (Carex) accountsfor mostof the
CI-I4emissionfrom wet meadowtundra;it is the
dominantplantform andannualCHn emissionwas
roughlyequalto or exceededthatof theM sitein all
studyyearsexcept1988(Table5). Finally,the
annualsourcestrength
of tundrain theglobalCI-h
budgetis highlyvariable.Themeanglobalemission
calculated from the 1987-1989 estimates is 42 + 26
Tg CI-h yr-1.Includingthe 1990globalemission
estimate,
whichcontains
theunusually
highCarex
emission
rate(Tables5 and6), givesa global
emission
estimateof of 57 + 39 Tg yr-1.
Environmental Variables
Air temperature
andprecipitation
datafromthe
studyareaarepresented
in Figures5 and6. Air
temperatures
duringthestudyweregenerally
higher
thanthelong-term
mean.Only9 of 48 months
showednegativetemperature
anomalies.
Air
temperature
anomalies
wereabout+2 øCduringthe
JunethroughSeptember
emission
period.
Ctmaulative
annualprecipitation
variedfrom16cm
(1987) to 47 cm (1990), or from 60 to 170% of the
long-termmeanof 28 cm. Summerrainfall is critical
in regulatingwatertablepositionin themuskeg.
Soilsarewater-saturated
at freeze-up,andthesnow
packis largelyexportedby runoffbeforesurface
soilsthaw,sothatlittle standingwaterremains.
Summerrainfallduring1987through1989waslow;
precipitationduringthisperiodwasonly 56 to 67%
of the thelong-termmeanvalueof 16 cm. In
contrast,1990summerprecipitation
totaled30 cm, or
about190%of thelong-termmean.
Tables1-4 containsummarydatafor environmental
variablesat permanentsamplingstations
fromthe
1987-1990thawseasons,
respectively.
Annualranks
of thedurationof thaw,maximumthawdepth,and
meansoiltemperature
for sitesfolloweda predictable
patternbasedon localtopography.
Tussocks
are
tallerthanthesurrounding
vegetation,
sotheythaw
earlier.The lakemarginexperiences
earlythawdue
to thehighheatcontentof theeulittoralzone.The
duration
of thawvariedfrom125to 155days,with
the longestthaw seasons
observedat T andC
stations.
Maximumthawdepths
averaged
about-85,
-70 and -55 cm at T, M, and BH stations.Mean soil
146
WhalenandReeburgh:TundraMethaneEmission
precipitation
(Figure6) resultedin a progressive
loweringof thewatertablelevelthroughthethaw
temperatures
of approximately
9 øCat C stations
were
5 øChigherthanat stations
locatedin themuskeg.
Muskegsoiltemperatures
averaged1øto 5 øC,with
meansincreasingin the orderBH, M, andT.
The watertablepositionat T, M, andBH stations
wasalwaysat or abovethe soilsurfaceat fall freezeup andat springthaw, asnotedabove.Low summer
•
ß
E10•;
-'
•
_.
_
E 10' :•
-
::3
-
softs remained saturated in 1989. The summer
o =C1
[] = C2
'7 10a'0
season at these stations to local minima of-18 and
-40 cm in 1988and 1989 (Tables2 and3). Softs
remainedsaturated
throughout
thethawseason
at
only two stations(T-1 and BH-2) in 1988, while no
o =Cll
[] -- C12
"=C3
,•:• "=C13
,T 10o•
• 10"_
_
_
_
10'2_.
O-
E-lO
•
[
-20
-30
•
I
II
III
IV
I
1987
II
III
IV
I
1988
II
III
IV
I
1989
II
III
IV
1990
Fig. 4. Net CI-Influx to atmosphere
andsoiltemperatures
for Carex(C) simsin Universityof Alaska
Arboretum,
1987-1990.(Top)Net CI-tnfluxversustimefor threepermanent
Carexstations.
Notelogflux
scale.(Bottom)Soilisotherms
(4 øCcontourinterval)on depthversustimeplotfor stations
C- 1 (1987-1989)and
C-11 (1990).Wintermeasurements
weresuspended
afterDecember1989.
TABLE 5.. MethaneEmission.byTundraSiteTypes
Year
1987
(T) Eriophorurn
1988
1989
1990
8.05 + 2.5
11.38 + 2.88
8.11 + 1.80
13.64 + 1.20
(C) Carex
4.88 + 0.73
0.81 + 1.09
4.27 + 0.67
60.60 + 8.66
(M) Moss
0.47 + 0.16
4.38 + 1.35
4.78 + 1.56
0.54 + 0.12
(BH) Black Hole
0.62 + 0.28
3.90 + 1.09
2.12 + 0.66
0.79 + 0.36
Emission
measured
in g CH4 m-2yr-1.Integrated
annual
CH4 fluxforeachsite+ lo.
Integrationperiodfor 1987, 1988,and 1989wasJanuary1 to December31. No winter
measurements
weremadein 1990;datafor 1990assume
CH4fluxincreases
linearlyfrom
May 1 to May 21 (dateof firstfluxdetermination)
anddecreases
linearlyfromSeptember
22 (dateof last flux determination)to October 15.
WhalenandReeburgh:TundraMethaneEmission
TABLE
Site
147
6. Global Tundra Methane Emission Estimate
Percentage
Covera
1987
MethaneEmission.
Te vr-1
1988
1989
1990
Tussock
andLowShrub
Tundra
(6.46x 1012
m2)
b
Eriophorum
24- 45
8.6- 30.7
Carex
Moss
Black Hole
3- 7
37 -63
30
0.8 - 2.5
0.7 - 2.6
0.7 - 1.7
Z Tussock
10.8 - 37.5
13.2 - 41.4
9.8 - 28.8
19.8 - 43.1
0- 0.9
7.2- 23.3
5.4- 9.7
0.7 - 2.2
7.7 - 25.8
2.8 - 5.4
10.3 - 31.3
1.0 - 2.7
0.8 - 2.2
25.8 - 75.3
21.0 - 62.2
31.9 - 79.3
WetMeadow
Tundra(0.884xlO•2m2)
Carex
Moss
80- 90
10 - 20
Wet Meadow
Z Tundra
2.9 -4.5
0.03 - 0.11
0- 1.5
0.3 - 1.0
2.5 - 3.9
0.3 - 1.1
36.7 - 55.1
0 - 0.1
2.9 - 4.6
0.3 - 2.5
2.8 - 5.0
36.7 - 55.2
13.7 - 42.1
26.1 - 77.8
23.8 - 67.2
68.6 - 134.5
aKummerow et al., 1983; Walker et al., 1987.
bTotaltundra
areais7.34x 1012
m2 [Matthews,
1983];wetmeadow
tundra
areaistakenasallnonforested
bog
northof 50øN[Matthews& Fung, 1987;Table 5].
rainfalldeficitwasgreaterin 1989 than1988 (7 cm
versus5 cm), andthisis reflectedin a greaterwater
table reductionfor 1989. Data for 1990 (Table 4) are
notdirectlycomparable
to previousdatadueto
stationrelocation.Duringthisyearof elevated
precipitation(Figure6), BH stationswithinthe
muskegremainedwaterlogged
throughout
thethaw
season.
The magnitudeandarealextentof thedropin
watertablelevel over the growingseasonat T andM
stationswasroughlycomparable
to thatobservedat
similar stationsin 1988 (Table 2).
Watertablelevelsat C stations
arereportedfor
1990only. Stationswereinitially established
on the
SmithLake marginmidwaybetweentheFall 1986,
low watermarkandthe historichighwatermark.
The watertablelevelwasbelowthe70-cm-deep
gaugingwellslatein thelow summerprecipitation
yearsof 1988 and 1989. The new C stationswere
established
at roughlythe sameelevationfor 1990
sampling;thewatertablewasabovethe soilsurface
throughout
thissummerof highrainfall(Table4).
Rank correlationsbetweenCH4 flux and
environmental
dataaregivenin Table7. The best
relationshipwasfoundbetweenCI-I4emissionand
centimeter-degrees.
We definecentimeter-degrees
as
the absolutevalueof theproductof thawdepthand
meansoiltemperature
to permafrost.Elevenof 12
possiblecorrelations
were statistically
significant,
showinga positiveresponse
in CH4 emissionto
increasingcentimeter-degrees.
IncreasingCI-h
emissionwasalsosignificantlycorrelatedwith
increasingdepthof thaw (rsnegative;depthdefined
relativeto soil surface).Mean soiltemperature
increases
werepositivelycorrelatedwith increases
in
CI-I4emission,but therelationshipwasweak;only 4
of 12 possiblecorrelations
werestatistically
significant.Qualitatively,a positiverelationship
betweenCI-I4emissionandsoiltemperature
is
evidentby comparingsoiltemperature
distributions
with flux in Figures1-4.The relationship
between
watertablepositionandCI-I4flux wasunclear.Three
of 9 correlations
were significant.A loweredwater
tablewas correlatedwith enhancedemission(rs
negative)in two casesandwith reducedemissionin
oneinstance(rspositive).
Soil pCH4 Distributionsand CH4 Emission
Figures7 and8 give changesin watertable
position,thaw depth,soilpCH4 distributions,
and
atmospheric
CH4 flux at primarystationsfor the
1988 and 1989 thaw seasons.
As expected,the thaw
depthincreasedat all stationsduringbothsummers.
The watertablelevel decreased
initiallythroughboth
WhalenandReeburgh:
TundraMethaneEmission
148
i
i
I
I
I
i
I
I
I
i
i
i
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i
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.:
ß
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ß i
:
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Feb
I
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i
i
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i
[7] 1987
E•] 1988
.r:•!1989
i
i
i
i
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i
i
i
i
i
i
i
i
i
i
i
Apr
I
I
i
i
Mar
I
i
i
i
i
i
i
,
.:
Jan
I
'i
ß I
-30
i
i
::
•
1990
Jun
Jul
I
I
I
i
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i
I
I
i
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i
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.
May
I
I
Aug
i
Sep
Nov
Oct
Dec
i2
m
m
m
m
.
,
.
',
'
'
"
'
"
"
"
,
•"
3-•_
o.,•
m
.
. ':•::
i.i:i'?.-::':i'
,. !i:i::iii(i::•?.-,:
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
::•
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N" ' .::!!::i:i.':.:i.:
ß
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:: ' :::!:i:::!:i":i.i:::i.•.
:i.!::.!:::!:-:i.:-..i.•.!-:.;:i:"i:::!:!:i:!:
!•
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L.:'{:i:'::'":'!:::-::
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.'• !'i:}:i:!:!::.i:i:i:i:',::}:i:::i:i:i:i:i:i:!:{.:'•
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, . •::-:::.:•:':•;•:•:•"
:;•:•:•:•:::.:•
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;•:;::•:•:•:•:•
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';:•:::?:..:.'":
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:':'•::-'
•:'::•:•:•:':•:•:•':':•':
:::•'•:::::•'::•:•"'•
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... i ," :.::•::::':.
:•:::'::.i ::::::::::::::::::::::
•:;:•:•:•:•:[::::-•:?•
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Feb
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i
Apr
i
•:.;•
::(:';(:'":,
' '.:'-:'.;:•:•::::
.,':'::.:;':'•.•::
;.:•
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-12
Jan
i
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'-:4:, ::.:.:..?•.,. ::•:•:::
;-:.:?'•..?:.:.?::::----•
.• 1989 '
-9.
...... •.. . :.
i
,
May
?•;:::
, -. - ß , .........
Jun
Jul
:': :
, ........
Aug
, .......
Sep
"
. ..
.
i
,
•t
i
Nov
D•
Fig.5.Air temperatures
(indegrees
Celsius)
intheUniversity
ofAlaska
Arboretum.
Long-term
means
and
conditions
duringstudyperiod.(Top)Meanmonthly
airtemperatures.
(Bottom)
Air temperature
anomaly
(difference
in long-term
meanandmonthly
meanvalues)
duringstudy
period.Shading
denotes
themajor
emission
period.
DatafromNational
Oceanic
andAtmospheric
Administration
(NOAA)[1986,1987,1988,
1989,1990]recordsfor FairbanksInternational
Airport,6 km southof studysites.
summersbutroseslowlyaftermidseason
in 1988.
MostearlyseasonsoilpCI-h distributions
showa 5-
increases
in soilpCH4 andemission.Late season
changes
in soilCI-h concentrations
accompany
a
to 15-cm surfacezone of low concentration(< 5
slightrisein watertablelevelandappearunrelated
to
matm),a 5- to 10-cmgradientof rapidlyincreasing
pCH4, anda deepzonewherepCI-h remains
relativelyconstantat 200 to 400 matm.
Overall,therelationshipbetweensoilpCI-h
emission.
distributions and CI-h flux was not consistent and
clear. Data for 1988 (T-1, M-1, andBH-1; Figures
7a-7c) showlow fluxesassociated
with a relatively
smallCI-h poolin JuneandearlyJuly.A July
increasein soilpCI-h andmid to late August
increases
in flux suggesta 2 to 3 weeklag between
Relationships
in the 1989soilCI-h andflux data
areevenlessclear.A strikingmidseason
increasein
soilCI-h wasobservedonly at M-1 (Figure8), when
CH4 emissionandthe soilpCI-h wereclearlyoutof
phase;highest
emissions
occurred
in lateJulyand
earlyAugustwhensoilCH4waslow.Methane
emissiondeclinedsteadilyto theendof theseason
followingtheearlySeptember
increase
in thesoil
CH4pool.ThesoilCI-hpoolshowed
a gradual
WhalenandReeburgh:
TundraMethaneEmission
149
1930-86
40.
:30.
20.
....
1987
--..
1988
--.-
1989
/
•//
//
!
__ __
•
/
•10,
...-ßo•-_..-."g'a,g•-,
.P•'P•"'•".... •
I
I
Jan
I
Feb
I
Mar
Apr
I
I
May
Jun
I
Jul
I
Aug
I
Sept
I
ß
Oct
Nov
Dec
10
E
5.
<:
0
....
089 '.
.'
,
.
.
.•
.
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
x ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
.•.:•::•:5:•:5:•:•:•:•:•:
•:•:
•:•
:5:•:•
* :::•:•:
•:•:•:•
.'•;.•;.•:.
::::::::::::::::::::::::::::::::::::::::::::::::::::::
::::::::::::::::::::::::::::::::::::::::::::
.....
:[:5:•:•:•:[:[:•:•:[:[:•:5:•:[:•:•:[:5:[:•:•:•:
/ .:::.....:':•.
:•:5•:[:•:•:•:•:•:•:•:•:•:•:•:•.
.
..
'.
.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
•5:•:•:•:•:•:•:•:[:•:•:[:•:•:5:•:•:•:•:•:J:•:•:J:•:[:•:•:•:•:•:•:[:•:•:•:•:•:•:•:•:•:[:•:•:•:[:•:•:•:•
:.:.:.:.•.:.:.•.:.:.•.:.:.:.•.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.•.:.:.:.:.•.:.:.:.
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Fig.6. Precipitation
(in centimeters,
water-equivalent)
in theUniversityof AlaskaArboretum.
Long-termmean
andconditions
duringstudyperiod.(Top)Cumulative
precipitation.
(Bottom)Precipitation
anomaly(difference
in long-term
meanandmonthlymeanvalues)duringstudyperiod.Shading
denotes
themajoremission
period.
DatafromNOAA [ 1986,1987,1988,1989,1990]recordsfor Fairbanks
International
Airport,6 km southof
studysites.
TABLE7. Spearman's
RankCorrelation
Coefficients
(rs)forCorrelations
Between
CH4 Emission
During
theThaw Season
andEnvironmental
Variablesat PrimarySamplingStations
in theUniversityof AlaskaArboretum
PrimarySamplingStation
Black Hole
Year
n
87
13
88
18
Thawdepth
-0.80 -0.80
Thawdays
a
0.82 0.78
Soiltemperature
b 0.87
Watertabledepth
ND
centimeter-degrees
c 0.87
89
19
Moss
90
16
87
15
0.68
0.71
ND
0.73
0.82
88
19
-0.89
0.90
0.65
-0.73
0.77
89
19
Tussock
90
16
87
15
-0.94
0.94
88
19
-0.88
0.91
89
19
-0.69
0.66
90
15
-0.91
0.93
ND
0.74
-0.66
0.74
0.71
0.68
0.74
0.75
Onlyvaluesof rssignificantat a = 0.01 aregiven.ND meansno datacollected.
aFirstdayof samplecollection
(May 21) takenasfirstdayof thawfor 1990.
bMean
todepth
ofpermafrost.
CAbsolute
valueof product
of thawdepthandmeansoiltemperature
topermafrost.
150
WhalenandReeburgh:TundraMethaneEmission
150
T-l, 1988
100
50
0
Water table
Thaw
depth
-75
A•g
dun
S•p
Oct
p CH4,matra
0
0 i _
i
10
i
0
10
0
•1
i
i
i
200
0
400
0
.
-30
6/10-6/27
[
6/26-7/6
7/6-8/3
8/9-9/7
9/7-9/27
Fig.7. (Top)NetCI-I4fluxes,(middle)
watertableandthawdepths,
and(bottom)
subsurface
pCH4
distributions
for (A) T-l, (B) BH-1, and(C) M-1 stations,
June- September,
1988.Carex(C-l) stationwasnot
sampled
during1988.Eachof thepCH4versus
depthpanels
integrates
conditions
uptothesampling
date.
Watertableandthawdepthweremeasured
whentheequilibration
samplers
wereretrieved.
tundrasubdivisions
(tussock:shrub
andwet meadow
increasethroughoutthe seasonat T-1, while
emissionremainedgenerallyconstant
(Figure8).
MethaneemissionandsoilCH4distributions
appear
to be uncoupledat BH-1 (Figure8) andC-7 (Figure
reasons.First, CI-I4fluxesfrom thesemajortundra
subdivisions
aredominatedby two sitetypes:
8).
Eriophorum
ressocks
(T) andCarex(C). These
tundra),
provides
a masonable
framework
for several
sitesaccountfor 24-45% and 80-90% of the areal
DISCUSSION
Global Tundra Emission Estimates
coverage
andcontribute
50-94%and89-100%of the
annualCH4flux,respectively,
fromthetwotundra
subdivisions
(Table6). Uncertainties
in areal
coverage
aloneresultin 1.5-to 2.4-foldvariations
in
One of our goalswas to determinenet CH4
emissionandits temporalandspatialvariabilityin
representative
tundrafloral associations.We
view the
annualemissionfrom each site (Table 5) as a
"reagentgrade"budgetconstituent
of known
uncertainty
thatcanbe appropriately
combinedand
weightedto produceregionalandglobalCI-h
emission estimates. We have extended site emissions
from thisstudy(Table5) usingour previous
weightingscheme[WhalenandReeburgh,1988]to
annualglobalCH4 emissionestimates
for 1987-1990
in Table 6. Theseemissionestimatesincorporate
uncertainties
in arealcoverageaswell asthe
variationsin annualsitefluxes (Table 5).
The representativeness
of sitesin a heterogeneous
landscape
is crucialin regionalextrapolation
of fluxes
to globalbudgets[Matsonet al., 1989].We are
confidentthatour weightingscheme,which
considers
thefloral or sitecomposition
in two major
annualCH4 emissionestimatesfor tundra. Second,
stationsestablishedin similar floristic units at the
ToolikLakeLongTermEcological
Research
site
showedsimilarCH4 emissionrates(mg m-2d-•)
duringtwoto threemidsummer
visitsduring1987
through1989.A morecomplete
summer
fluxtime
seriesobtainedat thesesitesduring1991(T.
Christensen,
CambridgeUniv., personal
communication,
1991),wasweightedby theWhalen
andReeburghframeworkandgavean annual
emissionof 18-30 Tg CI-I4.Third, wansect
measurements
[WhalenandReeburgh,1990a],
whichprovideanindependent
check,gavean
emissionestimateof 38 Tg CI-I4yr4 for tundra,
whichagreeswith thepresentresults.Finally,a
three-dimensional
tracermodelstudy[Funget
a1.,1991],whichadjustsCH4 sourcestrengths
asa
functionof latitudeandrequiresa matchbetweenthe
modeloutputandobservedseasonalCH4 variationat
WhalcnandR½½burgh:
TundraMethaneEmission
151
M-l, 1988
Water table
t
8 -?s
Thaw
depth
•---.-..,,.•..._.__.__._.__•
Al•g
Sep
Oct
p CH4, matm
, I(•X) , 2( 0
ß
i
i
100
i
i
ß
i
, 4( 0
,
E -10
6/10-6/27
7/6-8/3
6/26-7/6
9/7-9/27
8/9-9/7
150.
BH-1,1988
0
Water table
t
---
Thaw
depth
-75
sip
A•g
Jun
p CH4, matra
0
0
,
1,0 .
20 0
50
1• •0
i
i
i
i
2OO
i
c•-20
6/10-6/27
6/27-7/6
8/9-9/7
7/6-8/3
9/7-9/27
Fig. 7. (continued)
GlobalMonitoring
for ClimateChangestations
[Steeleet al., 1987],agreeswith theaboveestimates.
Forhighnorthern
latitudes,
wheretundrais theonly
CI-h source,themodelrequires
a sourcestrength
of
35 Tg CH4 yr-].
We areconfident
scaling-up
fromlong-term
emissionrecordsat fixed sites,but thereare still
uncertainties.
Unusual
conditions,
suchasthehigh
summer
precipitation
in 1990,andtheresulting
high
CHnfluxesfromtheC site,candistortanyemission
estimate
byextending
localanomalies
toa regional
or
largerscale.Thus,we considerthe CH4 emission
estimatesfrom 1987 to 1989 (Table 6) as most
representative.
Longertimeseriesanda better
understanding
of thecontrolson CI-h fluxesare
needed.Also, ourbudgetconstituent
sitesand
essentially
all studies
(Table8) sufferfroma North
Americanbias;time seriesstudiesin representative
tundraandpeatlandareasof Siberiaareneededto
insurecorrectregionalweightingin futuretundra
CH• emission estimates.
Table8 summarizes
high-latitude
CH4emission
estimates.The earliestestimate[Svensson,1976] is
basedon a CI-h flux:moisture
relationship.
WhalenandReeburgh:
TundraMethaneEmission
152
a
Water table
Thawdepth
-75
A•g
Jun
OCt
p CH,•,matra
300 0
300
0
0 i
i
300
i
7/12-7/31
6/22-7/12
....
O0
7/31-8/17
8/17-9/4
9/4-9/20
400
M-l, 1989
0t
Thaw
depth
Water
table
"---"-.---...._..._,_.____
-75
Jun
,J•11
,N•i
O
S•p
Oct
p CH4,matra
0
10
6/22-7/12
20
0
7/12-7/31
i
7/31-8/!7
8/! 7-9/4
0
100
200
9/4-9/20
Fig. 8. (Top)Net CH4fluxes,(middle)watertableandthawdepths,and(bottom)subsurface
pCH4
distributions
(Bottom)for T-1, BH-1, M-1, andC-7 stations,
June- September,
1989.Eachof thepCH4versus
depthpanelsintegrates
conditions
upto thesampling
date.Watertableandthawdepthweremeasured
whenthe
equilibrationsamplerswereretrieved.
Subsequent
estimates
resultfromtransectstudies
[Sebacher
et al., 1986;WhalenandReeburgh,
!988], time seriesflux measurements
at fixed
locations
[Crillet al., 1988;WhalenandReeburgh,
1988;Mooreet al., 1990],literatureCH4flux data
appliedto refinedestimates
of globalwetlandareas
[MatthewsandFung, !987; AselmannandCrutzen,
1989],anda modelestimate[Funget al., 1991]
basedonlatitudinalsourcestrengths
and
observational
constraints.
Variations
in studytypes,
assumptions,
andregionaldef'mitions
areresponsible
for thewiderangeof estimates,
butrecentstudies
seemtobeconverging
ona tundrasource
strength
of
about35 Tg CH4 yr-1[Reeburgh
andWhalen,1992].
153
WhalenandReeburgh:
TundraMethaneEmission
lOO
BH-1, 1989
50
0
Water table
t
Thawdepth
J•l
Jun
Al•g
S•p
p CH4,matm
200 0
0
200
0
Oct
400 0
200
200
-30
7/12-7/31
6/22-7/12
7/31-8/17
9/4-9/20
8/17-9/4
400
t
C-7, 1989
Water table
.
J•l
Jun
A•g
p CH4, matm
0
200
I
01
.
,
3,0oo,
,
.
S•p
30O
Oct
o
0
-10
-20
-30
6/22-7/12
7/12-7/31
8/17-9/4
7/31-8/17
9/4-9/20
Fig. 8. (continued)
Variability in CH4 Emission
Previous
high-latitude
studies
atfixedsiteshave
reported
variability
inramsof CI-hemission
ranging
over3 ordersof magnitude
on seasonal
[Crill et al.,
1988;Mooreet al., 1990;Bartlettet al., 1992]and
annual[WhalenandReeburgh,1988]scales.
MatthewsandFung[1987]notethatrefinement
of
globalCI-hemission
estimates
willbeachieved
only
through
CI-hfluxdeterminations
overseveral
years
atrepresentative
wetland
sites.
Thisstudy
reports
the
firstmultiyearCH4emission
measurements
for
Arctic tundraplantassociations.
This studyfortuitouslyencompasses
a rangeof
environmental conditions useful for modelers. Air
temperature
anomalies
werea uniform+2 øCduring
all summerscoveredby this study(Figure5), sothat
a temperature
effectcannotbeestimated
froman
interannual
emissionratecomparison.
Precipitation
was 60 to 190% of the long-termmeanduringthe
summersof 1987-1989 and 1990, respectively
(Figure6), allowingmeaningfulcomparison
of
i
•
•
i
z
c•
o
z•
•
o
i
z
155
WhatenandReeburgh:
TundraMethane
Emission
annualemissionestimates
for a precipitation
effect.
The tundraCH4 flux variedfrom 28 to 52 Tg yr4
during1987-1989andwas102Tg yr-• in 1990
(midranges
of datain Table6) Thedatasuggest
a
rangeof variationsin annualCI-I4emissionof about
twofoldfor yearswith summerprecipitation
similar
to thelong-termmeanandaboutfourfoldfor years
withunusually
highsummer
precipitation.
It is unlikelythatdifferencesbetweenthe 1990
annualflux estimates
andthosefromprevious
years
resultedfromstationrelocafion.
Parallelsampling
of
newandold stations
on a singleoccasion
during
1990showedno statistically
significant
differences
in
fluxesfor newandold stations
of anysitetype
(Mann-WhitneyTest;p>0.05;datanot shown).
New C stations
wereestablished
at roughlythesame
elevationaspreviousstations.We concludethatthe
increasedemissionat C stationsin 1990 (Tables5
and 6) resultedfrom a -40 cm rise in the summer
water table due to record rainfall. Increased CI-h
emission
fromthissitetypewaslargelyresponsible
for the high 1990 emissionestimate,asnotedabove.
We alsopresentthefirst datafor interannual
variations
in CI-h emission
fromrepresentative
tundraCH4budgetconstituent
sites.Although
summerair temperature
andprecipitation
were
roughlycomparable
in 1987-1989,therewere
obviousdifferencesin the site fluxes (Table 5) and
therelativecontributionof C, M, andBH sitetypes
to the annualCH4 emissionestimate(Table 6). We
haveno readyexplanationfor thesedifferencesbut
suspecttheyaredueto interannualvariationsin the
submeterscaleinteractions
of biological,physical,
andchemicalparameters
andprocesses.
Improved
understanding
of theseinteractions
is necessary
to
further resolve the twofold interannual differences in
overallannualCH4 emission.
Correlations With Environmental Variables
Otherinvestigators
havereportedstrong
relationships
betweenCI-h flux andtemperature.
We
hadlimited success
in correlatingmeansoil
temperature
with CI-h emissionat primarysites
(Table7). Correlationcoefficientswerenotimproved
by selectingtemperature
at a singledepthor by useof
a runningmeantemperature
astheindependent
variable (data not shown).Linear [Svenssonand
Rosswall,1984],polynomial[Baker-Blockeret al.,
1977],log-linear[Holzapfel-Pschorn
andSeiler,
1986; Crill et at., 1988; Moore and Knowles, 1990;
Bartlettet al., 1992], andexponential[Moore et at.,
1990]relationships
betweenCI-h flux andair or soil
temperature
havebeenreported.The varietyof
functionsusedsuggests
thattherelationshipbetween
CI-h flux andtemperature
is not uniqueor
straightforward.
The lack of statistical correlation between water
tabledepthandseasonal
CI-h emissionat our
muskegsites(Table7) appearsto resultfrom
analysisof datafor theentirethawseason.
A low late
summerwatertablecorresponds
with reducedCI-h
flux (Figures7 and8). However,early season(May)
emissionsfrom waterloggedsoilwere alsolow as a
resultof a thin thawedsoilzone.Combiningall data
for a seasonresultsin a low rs.HighestCH4
emissionsare generallyobservedduringmidsummer,
whenthe thawedzoneis rapidlyincreasing,but the
watertableis falling. Seasonalstudiesin temperate
swamp[Wilsonet at., 1989]andwetland[Yavitt et
al., 1990] soilsalsoreportpoorcorrelationbetween
water table level and CI-h flux. In contrast,transect
studiesshoweda closerrelationshipbetweenCI-h
flux and water tableposition(r = 0.42 to 0.54)
[Sebacheret al., 1986; Hatriss et al., 1988; Whalen
andReeburgh,1990a]or soilmoisturecontent(r =
0.77 to 0.93) [Svensson,1976, 1980]. It is likely
thatthe CI-h emissionresponseto changing
environmental
conditionsoccursrapidlyandis
masked in whole-season correlations.
The parameters
thawdepthandcentimeter-degrees
show the bestcorrelationwith CI-h emission(Table
7) becausethesevariablesintegrateconditions
importantin determiningnet CI-h emission.
Specifically,centimeter-degrees
is a reasonable
index
of netmicrobialactivitybecauseit accounts
for the
meansoiltemperatureandthe depthof the active
zone.
Controls on CH4 Emission
The measurements
of netCH4 emissionreported
herearethedifferencebetweenCH4 production
and
microbiallymediatedCH4 oxidation.This oxidation
occursbeforetheCH4 is emittedto theatmosphere,
andis an importantcontrolon tundraCH4 emission.
Reeburgh
et al. (TheRoleof Microbially-Mediated
Oxidationin theGlobalCH4 Budget,submitted
to
Nature, 1992) estimatethatglobalCH4 oxidationis
about200Tg yr-1largerthantheestimated
,• 500Tg
yr-1emittedto theatmosphere.
Methaneoxidationby
soilmicrobialcommunities
hasbeenregarded
a
negligibletermin theglobalCH4budget,buta body
of evidencefrom moistsoils[Born et at., 1990;
Keller, et at. 1990; Steudteret at., 1990] is
accumulating
thatsuggests
it playsanimportant
role
in theglobalCI-I4budget.Soilsequilibrate
rapidly
with theatmosphere
[WhalenandReeburgh,1990b],
andthemicrobialCH4-oxidizing
community
hasa
highcapacityandlow thresholdfor CH4 oxidation
156
[Whalen et al., 1990]. Whalen et al. [1992] estimate
thatabouthalf of the CH4 productionin tundra
systemsis oxidizedin the moistaerobiczoneand
show that the extent of CH4 oxidation is sensitive to
changesin watertablelevel,whichdirectlyaffects
theextentof theoxidizedsurfacezone.For example,
laboratorystudies[Whalenet al., 1992] suggest
that
loweringthetundrawatertable10 - 20 cm in areas
free of vascularplantswouldreducethenetflux of
CH4 to zero, and further reductionsin water table
couldpermitoxidationof atmospheric
CH4.Thus
oxidationcouldprovidea negativefeedbackon CH4
increases
in portionsof the wetlandterm.The CH4
flux recordfrom theGreatDismalSwamp[Hatrisset
al., 1982] alsoillustratesthisprocess.
SoilpCH4 Distributions
andCH4 Flux
SoilpCH4distributions
suggest
a surfacezoneof
rapid CI-I4oxidationanda subsurface
zoneof CH4
production
(Figures7 and8). Methaneprofiles(1
•Vl CH4 --0.52 matm CI-I4 at 10 øC) were similarto
thosereportedfor Minnesotapeatlands
[Cfill et al.,
1988] andsubarctictundra[Bartlettet al., 1992];a
surfacezoneof low CH4 concentration
(<10 •X,I)
overliesa zoneof rapidlyincreasing
CH4anda deep
zoneof 200-500 •Vl CH4. Methaneconcentration
isolinesfor borealfen soils[Mooreet al., 1990]
suggesta smallerverticalgradient,with
concentrations
rangingfrom about50 to 300 •vl over
similardepths.
Sequential
soilCH4distributions
areanintegrating
indicatorof microbialactivity.We expected
CH4
emissionto trackchanges
in thesoilCH4pool,
WhalenandReeburgh:
TundraMethaneEmission
Predictors and Feedbacks
Severallinesof evidencesuggest
thatthetundra
systemis toocomplexfor a a singleenvironmental
variable to serve as a universal and successful
indicatorof CH4flux. First,a varietyof functions
havebeenusedtorelateCH4fluxandtemperature
(seeabove).Second,
significant
relationships
often
involvespecial
conditions.
Forexample,
Sebacher
et
al. [ 1986]reportedgoodcorrelation
betweenwater
tabledepthto 10 cm andCH4 emissionin a transect
study,andWilsonet al. [1989]required
a step
functionto relatesoiltemperature
to emission.
Third,
relationships
reportedin Table7 werebestwhendata
weresimplyrankedusinga nonparametric
statistic.
Finally,lx)olingall dataforonevariable
bystation
or
yearor by stationandyearresulted
in few significant
correlations,
despitethefactthattheincrease
in
samplesizeallowsa smallervalueof rsfor statistical
significance.
Moore et al. [ 1990]notedthat
correlation
betweenCH4flux anda single
environmental
variablein high-latitude
fensmay,in
part,bespecious
because
variables
influencing
flux
arenotentirelyindependent.
Thisviewis supported
by our observationthat CH4 emissionwasbest
correlated with variables that serve as environmental
integrators
(Table7). Attempts
toidentifypredictors
of CI-hemission
shouldfocusonparameters
that
integrate
factorsimportant
in CH4production
and
consumption.
Recentpapersthathaveaddressed
feedbackson
tracegasemissions[Khalil andRasmussen,
1989;
Lashof,1989]assume
thatincreased
temperatures
will resultin higherCH4fluxes,i.e.,a positive
feedback.These studiesfocus on the influenceof a
whichcanbe seenin temporal
sequences
fromsingle
single
variable
(temperature)
ona single
biological
stations(Figures7 and 8). Instead,we foundthat
CI4_n
flux ,•ndpoolsizeweregenerallyunrelated
over
process(methanogenesis)
anddo notconsiderother
effects
of climate
change,
namely
regional
changes
in
time.Similarresultshavebeenreportedfor a boreal
watertablelevel [Mitchell, 1989] andratesof CH4
fen [Moore et al., 1990] and Subarctictundra
oxidation.
Functions
thatsuccessfully
relateCH4
fluxtotemperature
arelargelylimitedtoinundated
wetlands
(seeabove)whereCI-I4production
is the
dominant
biological
process
influencing
emission.
WeexpectthatCH4flux-temperature
relationships
[Bartlettet al., 1992].Thisdecoupling
couldbethe
resultof comparing
measurements
thatrespond
to
different time scales. The static chamber flux
determinations
areessentially
instantaneous
rate
measurements
(snapshots),
while the soilCH4
determinations
resultfromintegration
o•er the
equilibration
period(timeexposure).
However,the
boreal fen CI-h distributions cited above were
instantaneous
(porewaterssyringe-sampled)
and
CI4_n
pool sizesdeterminedin a saltmarshwith
equilibrafionsamplers
werewell correlatedwith CH4
flux [Bartlettet al., 1987].It is likely thatthis
decoupling
is anadditional
indication
of thecomplex
bioticandabioticfactorsdetermining
netCH4
emission.
will deteriorateas water tablesfall below the soil
surface
andoxidation
becomes
important.
Theratio
of oxidized:reduced
soilzones
willincrease,
favoring
bothaerobicdecomposition
andCH4oxidation.
Laboratory
studies
withpacked
peatcolumns
[Moore
et al., 1989] showedthat CH4 emissiondecreased
logarithmically,
whileCO2emission
increased
linearly
aswatertabledecreased.
Moreover,
changes
in soiltemperature
willnotbeuniform
withdepth
andtheeffectof increased
soiltemperature
will be
greater
forCI-I4consumption
thanproduction.
WhalenandReeburgh:
TundraMethaneEmission
Finally, the increaseddepthof thaw in permafrost
areaswill nothaveasgreata stimulatoryeffecton
methanogenesis
aspredicted[Khalil andRasmussen,
1989;Lashof,1989],because
deeperpeatis likely to
be refractory['Fan•shandGrigal, 1988;Yavitt et al.,
1988].A temperature
feedbackwill be importantin
coupledbiogeochemical-atmospheric
models,but
watertablepositionwill be a betterintegrator,
waterloggedsoilsare zonesof CI-I4production,
while moistsoilsareareasof CH4 consumption.
Carefullyexecutedfield experiments
involving
manipulations
of watertableandtemperature
are
needed to resolve the treatment of feedbacks in
coupledbiologicalmodels.One simplehydrologic
modelfor northernfensindicatesthata fallingwater
tablehasa greaterimpactthanincreased
soil
temperatureon CI-I4emission[Rouletet al., 1992].
157
Mancy, Methaneflux from wetlandareas,Tellus,
29, 245-250, 1977.
Bartlett, K. B., D. M. Bartlett, R. C. Hatriss, and
D. I. Sebacher,
Methaneemissions
alonga salt
marshsalinitygradient,Biogeochemistry,
4, 183202, 1987.
Bartlett, K. B., P.M. Crill, R. L. Sass,R. S.
Hatriss, and N. B. Dise, Methane emissionsfrom
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Past,presentandfuture,
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Born, M., H. D6rr, and I. Levin, Methane
consumption
in aeratedsoilsof thetemperate
SUMMARY
On the basisof a 4 year studyat permanentsiteswe
estimatethattundracontributes
42 + 26 Tg yr-1to the
net atmospheric
CH4 budget.This large,multiyear
dataset,obtainedundera rangeof moistureand
temperature
conditions,
restrictstherangeof tundra
CH4 sourcestrengthestimatesandalsoshowsthe
effectsof local extrema.Resultsfrom multiple
permanentsitesalsosuggest
thatsingle-parameter
relationships
usedto predictCI-I4flux aresitespecificandpointto theneedfor an integrating
predictor.
Acknowledgments.
This work was supportedby
NASA grantsNASW 841, RTOP 147-92-41-01,and
IDP 88-032. F. S. ChapinIII advisedus on site
selectionat the startof thisprojectin 1986.Methane
standards
wereprovidedby R. Rasmussen
andD.
Blake.Undergraduate
assistants
SamanthaRoyer
(1989), HeatherAndersen(1990), and Kristel Kizer
( 1990-1991)assisted
in fabricatinglucitechambers,
field sampling,andlaboratoryanalysis.Unusually
thoroughandconstructivereviewsby B. Svensson
andan anonymous
reviewerimprovedthe
manuscript.Contributionnumber895 from the
Instituteof Marine Science,Universityof Alaska
Fairbanks.
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Biogeochemical
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Gorham,E. S. Verry, D. I. Sebacher,L. Mazdar,
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Futureglobal
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