XDepartment of Geosciences, University of. by the American Geophysical Union. Stephen C. Whalen and William S. Reeburgh I.

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1 GLOBAL BOGEOCHEMCAL CYCLES, VOL. 6, NO. 2, PAGES , JUNE 1992 NTERANNUAL VARATONS N TUNDRA METHANE EMSSON: A 4-YEAR TME SERES AT FXED STES Stephen C. Whalen and Wllam S. Reeburgh nsttute of Marne Scence, Unversty of Alaska Farbanks Abstract. Ths paper summarzes 4 years ( ) of weekly net CH4 flux measurements at permanent stes representng mportant plant components of Arctc tundra. The data concde wth varatons n precptaton and temperature of nterest n regonal and global modelng efforts and are useful n placng bounds on the role of tundra n the global C-h budget. Precptaton n the study area durng the summer emsson perod ranged from twce to half the long-term mean, and ar temperature anomales were about +2 øc. Ths data set also permts consderaton of temporal (seasonal to nterannual) and spatal varablty n CH4 flux. We studed the relatonshp between the net C-h flux and subsurface propertes (water table depth, thaw depth, sol temperature, pch4 dstrbutons) at these permanent stes durng the 1988 and 1989 emsson perods. Net C-h emsson and subsurface propertes are largely unrelated. Relatonshps between sol temperature (or any sngle varable) and emsson are ste specfc and are of lttle value as flux predctors. Parameters that ntegrate condtons nfluencng flux appear to be the best flux predctors over the emsson perod. We estmate that Arctc wet meadow XDepartment of Geoscences, Unversty of Calforna, rvne. Copyrght 1992 by the Amercan Geophyscal Unon. Paper number 92GB /92/92GB-43. $1. and tussock:shrub tundra presently emt about Tg CH4 yr-1 to the atmosphere. Ths estmate has a North Amercan bas, but t s supported by measurements n a range of locatons, transect studes, and model calculatons. NTRODUCTON Tundra envronments occupy some 7% of the Earth's land area and contan about 13% of ts stored sol carbon [Post et al., 1982]. The fate of ths stored sol carbon under altered clmate s a major queston [Bllngs, 1987]. The role of hgh-lattude wetlands n the global CH4 budget was poorly known untl recently. Pror to 1986, only three publcatons [Svensson, 1976; Svensson and Rosswall, 1984; Sebacher et al., 1986] reported measurements of CH4 fluxes from tundra envronments. Reports of ncreases n atmospherc C-4 concentratons [Rasmussen and Khall, 1981, 1984; Blake and Rowland, 1988], conf'n'maton of the ncrease n ce cores [Raynaud et al., 1988; Stauffer et al., 1985], and the realzaton that trace gases could have an effect on greenhouse wnmng [Dcknson and Ccerone, 1985; Ramanathan et al., 1985] have stmulated a broad reexamnaton of the global CH4 budget [Ccerone and Oremland, 1988; Fung et al., 1991]. Several approaches have been used n assessng the role of hgh lattude wetlands n the global CH4 budget. These nclude vegetaton assessments and applcaton of emsson data from characterstc areas

2 14 Whalen and Reeburgh: Tundra Methane Emsson [Matthews and Fung, 1987; Aselmann and Crutzen, 1989; Fung et al., 1991], ntensve feld campagns n representatve areas over relatvely shortme perods (e.g., ABLE 3A (Yukon-Kuskokwm delta, 1988), ABLE 3B/NOWES (Hudson's Bay lowlands, 199), flux transects [Sebacher et al., 1986; Whalen and Reeburgh, 199a], and tme seres flux measurements at representatve stes over several months [Crll et al., 1988; Harrss et al., 1982; Moore and Knowles, 199; Moore et al., 199] or a year [Whalen and Reeburgh, 1988]. The work reported here extends the latter study to 4 years, 1987 through 199. Durng ths perod the summer precptaton vared by twce and half of the long-term (--5 yr) mean, and ar temperatures were generally 2 øc above the long-term mean. The data allow the fn:st assessment of nterannual varablty n hgh-lattude C-t4 flux and are useful n placng bounds on tundra CH4 emsson. Further, the nterannual varatons n precptaton and ar temperature cover a range of nteresto global clmate change modelers. METHODS Ths study s a contnuaton of our earler work [Whalen and Reeburgh, 1988] and uses the same samplng stes and methods. Stes Samplng stes were located n a muskeg and along a pond margn (Smth Lake) n the Unversty of Alaska Arboretum (64 ø 52' N, 147 ø 51' W, m elevaton). Physcal and vegetatve characterstcs of the study area are smlar to those of Arctc tundra; poorly draned softs are undedan by permafrost and an Erophorum vagnarum L. tussock plant communty [Kummerow et al., 1983; Wen and Blss, 1974] domnates the muskeg, whle extensve stands of Carex spp. lne the margn of Smth Lake. Three permanent samplng statons were located wthn each of four florstc unts or stes: (1) E. vagnaturn tussocks (T statons), (2) ntertussock depressons ("black hole" or BH statons) that are often waterlogged and have a surface layer of detrtus or Sphagnum spp., (3) mossy areas (M statons) whch are elevated relatve to the BH statons and contan no Sphagnum, and (4) Carex stands (C statons). Examples of a T, BH, and M staton were grouped or clustered wthn 1 to 2 m at three locaton separated by 1 to 2 m wthn the muskeg, and C statons were establshed n three dstnct stands of Carex aquatls Wahlenbach separated by about 2 m on the margn of Smth Lake. The stes were dspersed over such a large area that elevated walkways were mpractcal; frm sols mnmzed staton dsturbance durng samplng. Paths were worn to the stes, and the statons became somewhat shopworn after 3 years of year-round study; new statons were establshed n the same muskeg and pond margn before freeze-up n 1989 and were sample durng 199. We determned that wnter C-h emssons were nsgnfcant and suspended wnter measurements n December, Net CH4 Flux Measurements Net CH4 fluxes were determned usng statc chambers. Each flux chamber conssted of a skrted alumnum base, stackable lucte vertcal sectons, and a lucte ld. Each ld was ftted wth a capllary bleed to equalze pressure and a Swagelok O-seal fttng equpped wth a septum for syrnge samplng. Waterflled channels provded seals between the chamber components. The alumnum bases were permanently set n the sol at M, BH, and C stes, and solated an area of.75 m2. Smaller bases (.23 m2) were permanently nserte drectly nto the tops of Erophorum tussocks at T stes from 1987 to 1989; regular bases (.75 m2) were used to surround each tussock at the sol surface when new statons were establshed n Chamber headspaces were sampled wth glass-barrel syrnges over a.5 to 24 hour tme course, and C'h concentratons were det ed wthn a few hours of sample collecton by gas chromatography. A Shmadzu GC Mn-2 gas chromatograph equpped wth a flame onzaton detector and samplng valve was used for the analyses. Analytcal precson was 2%; workng standards were relatable to Natonal Bureau of Standards standards. Methane fluxes were calculated from chamber geometry and the lnear change n CH4 concentraton wth tme. Other Measurements All supportng envronmental measurements were made at prmary statons (T- 1, M- 1, BH- 1, and C- 1 n 1987 through 1989; T-4, M-4, BH-4, and C-11 n 199) whenever CH4 fluxes were determned; less comprehensvenvronmental data were collected smultaneously at other statons. Sol envronmental varables are measured relatve to the sol surface. The water table level was measured between 1988 and 199. Shallow wells were bored adjacento all statons, were cased wth perforated PVC ppe to prevent collapse, and were screened to avod decmatng the local vole populaton. Sol temperatures were measured wth permanently deployed multthermster probes extendng from the sol surface to permafrost at all prmary statons

3 Whalen and Reeburgh: Tundra Methane Emsson 141 except C-1 and C-11, where thermstors extended to 4 and 7 cm, respectvely. Mean sol temperatures n the thawed layer at T, M, and BH prmary statons were calculated by ntegratng temperature data from the sol surface to permafrost. Mean sol temperatures to the deepest thermstor were calculated at C prmary statons. Thaw depths were measured by nsertng a 3-mm- dameter stanless steel rod to the frozen sol horzon. The thaw season reported at each staton (Tables 1-4) TABLE Summary Data for CH 4 Flux and Envronmental Varables Durng Thaw Season at Permanent Samplng Statons n the Unversty of Alaska Arboretum CH 4 Flux, mg m -2 d -1 Ste n Mean SEM a Maxmum Medan BH BH BH C-1 b C C M M M T-1 c T T asem s the standard error of mean. Maxmum Thaw Thaw Depth, Duraton, cm days Mean Sol Temperature, o C 1. ball data for Carex (C) stes taken from May 26 (- 3 weeks after sprng thaw) untl freeze-up. CThaw depth from top of tussock TABLE Summary Data for CH 4 Flux and Envronmental Varables Durng Thaw Season at Permanent Samplng Statons n the Unversty of Alaska Arboretum CH 4 Flux, mg m -2 d -1 Ste n Mean SEM a Maxmum Medan Maxmum Water Table Thaw Thaw Depth, Range, Duraton, cm cm days BH to5 148 BH to BH to C C C M to 155 M to 155 M to 155 T-1 b to T to T to asem s the standard error of mean. bthaw and water table depths from top of tussock and sol surface, respectvely. Mean Sol Temperature, o C

4 142 Whalen and Reeburgh: Tundra Methane Emsson s the perod from ntal rod penetraton n the sprng to surface sol freeze-up n the fall. The "warm rm effect" [Lkens and Johnson, 1969] lmted permafrost formaton n the lake margn, so no thaw depth determnatons were made at C statons. Sol C-n profles were sampled wth equlbraton samplers [Hesslen, 1976] wthn.5 rn of prmary statons durng the 1988 and 1989 thaw seasons. The 1989 sol C-h measurements were made at staton C-7. The samplers were made of lucte and had equlbraton wells (3 cm3) spaced at 2.5-cm ntervals along the sample probe. The equlbrafon TABLE Summary Data for CH 4 Flux and Envronmental Varables Durng Thaw Season at Permanent Samplng Statons n the Unversty of Alaska Arboretum CH4 Flux, mg m -2 d -1 Ste n Mean SEMa Maxmum Medan Maxmum Water Table Thaw Mean Sol Thaw Depth, Range, Duraton, Temperature, cm cm days øc BH to BH to BH to7 139 C C C M to M to M to 151 T-1 b to T to T to asem s the standard error of mean. bthaw and water table depths from top of tussock and sol surface, respectvely. TABLE Summary Data for CH 4 Flux and Envronmental Varables at Permanent Samplng Statons n the Unversty of Alaska Arboretum, May 21 through September 22, 199 CH 4 Flux, mg m -2 d -1 Ste n Mean SEM a Maxmum Medan Maxmum Water Table Mean Sol Thaw Depth, Range, Temperature, cm cm øc BH to BH-$ to 12 BH to 18 C to48 C to C to65 M to M to-9 M to-25 T to T to 1 T to 11 asem s the standard error of mean. bthaw and water table depths from top of ressock and sol surface, respectvely.

5 _ Whalen and Reeburgh: Tundra Methane Emsson 143 wells were flled wth degassedstlled water and were covered wth a gas-permeable membrane (Teflon FEP, 2 ml thckness), whch was held n place wth a perforated plate. The samplers were nserted n a snug plot hole made prevously wth another sampler and were allowed to equlbrate n the feld for 2 weeks. Equlbrated water (2 cm3) was wthdrawn by puncturng the membrane wth two hypodermc needles, one attached to a syrnge for sample collecton and one that served as an ar nlet. The syrnge contents were njected nto capped, Heflled serum vals (3 cm3) that were allowed to equlbrate 24 hours wth perodc shakng before analyss of headspace C-h. The water table poston sometmes fell below the topmost equlbrafon wells durng deployment, and snce these wells remaned water flled, they sampled gases n the most sol zone. Bunsen solublty coeffcents at 1 øc [Yamamoto et al., 1976] were used to calculate the dssolved C-4 concen afon. Sol C-4 dstrbutons are expressed as pc-h (matm) to present data from both the waterlogged and most sol zones n comparable unts. RESULTS Methane Flux Tme Seres Fgures 1-4 show net C-4 flux at T, M, BH, and C statons, respectvely, from 1987 through 199. Negatve fluxes (net CH4 consumpton) were never observe durng the study. Methane emsson vared seasonally at all statons, although the seasonalty s less evdent wth the log flux scale. n general, hghest fluxes occurred n mdsummer, and lowest fluxes were recorded n late wnter. Wnter emssons were at or near zero at T and BH statons but perssted at 1 to >1 mg C-4 m-2 d-1 at some M and C statons. Wnter flux characterstcs at actve staton showed no year-to-year consstency. For example, M-3 showed notable wnter C-4 emsson only durng , when November through Aprl fluxes vared from 5.7 to mg m-2 d-1. Smlarly, C-3 exhbted lttle wnter actvty untl 1989, when more lmted data (November through December) showed C-h emsson rangng from 2.5 to 11.1 mg m-2 d-1. Thaw season C-h emsson! - o, lo'. lo ø O- /,x\\ a!/ / -lo ß V 1988 V V V Fg. 1. Net C-4 flux to atmosphere and sol temperatures for tussock (T) stes n Unversty of Alaska Arboretum, (Top) Net C-h flux versus tme for permanent tussock statons. Note log flux scale; fluxes of mg m-2 d-1 plotted on abscssa. (Bottom) Sol sotherms (4 øc contour nterval) on depth versus tme plot for statons T-1 ( ) and T-4 (199). Wnter measurements were suspended after December 1989.

6 144 Whalen and Reeburgh: Tundra Methane Emsson lo'.- o -M1? a k o -M4 2 1' = ---- M2 / ] = M5 'E,", = M3 ',,:,,. 't" ).!: "= M :,..A.A "' ' 1 2 ' j".":' E -1-2O -3-4! ll V ll V ll V ll 199 V Fg. 2. Net CH4 flux to atmosphere and sol temperatures for moss (M) stes n Unversty of Alaska Arboretum, (Top) Net CH4 flux versus tme for three permanent moss statons. Note log flux scale; fluxes of mg m-2 d-] plotted on abscssa. (Bottom) Sol sotherms (4 øc contour nterval) on depth versus tme plot for statons M-1 ( ) and M-4 (199). Wnter measurements were suspended after December accounted for 77 to 92%, 33 to 88%, 83 to 95%, and 65 to 88% of annual emsson at T, M, BH, and C stes durng the calendar years 1987 through Thaw season emssons would have been >9% at all prmary statons f the thaw seasons were consdered to extend untl sols froze to permafrost. Ths usually occurred 2 to 3 weeks after freezng of surface sols. Summary data for C-4 flux durng the 1987 through 199 thaw seasons are gven n Tables 1-4. Thaw season C-4 emsson at each staton s so varable that both mean (and standard error) and medan fluxes are gven as measures of central tendency. Mean and medan fluxes for T statons for all years and means and medans for C statons durng 1987 and 199 generally ranked hghest when wthn-year comparsons of data from all statons were made. These statons consstently showed hgh emsson throughout the thaw season. Mean and medan fluxes at M statons (except M-1) were consstently low. Black hole (BH) statons frequently showed hgh mean, but low medan fluxes. Ths resulted from epsodc emsson on one or two dates (e.g., BH-3 n 1988; Table 2). Mnmum fluxes approached zero n most cases and are not gven. Wthn-ste and spatal varatons n thaw season CH4 fluxes were remarkably hgh. Coeffcents of varaton (CV) for CH4 emsson from T, M, BH, and C stes averaged 84, 118, 93, and 67% (n = 62 to 73; staton separated by 1 to 2 m). The CV for CH4 flux from T-M-BH clusters (all data) averaged 96% (n = 198; statonseparated by 1 to 2 m). Methane fluxes (mg m-2 d-) for each staton (Fgures 1-4) were averaged, and these were ntegrated (trapezodal role) over tme to gve annual emsson estmates for each ste (Table 5). Methane emsson from the T ste showed lttle nterannual varablty, rangng over a factor of only 1.7. n contrast, annual emssons at the BH and M stes ranged over factors of 7 and 1, respectvely, and the data for the C ste vared by a factor of 74. Annual flux estmates for each tundra component n Table 5 were weghted as n the work by Whalen and Reeburgh [1988] to estmate global tundra CH4 emsson for 1987 through 199 (Table 6). Overlap n percent ground cover occuped by each ste type causes total areal coverage to exceed 1%. Nonetheless, three ponts are clear n Table 6. Frst, tussocks (T ste, Erophorurn) are responsble for

7 Whalen and Reeburgh: Tundra Methane Emsson ,,! ll ll V Fg. 3. Net C-4 flux to atmosphere and sol temperatures for ntertussock depresson or black hole (BH) stes n Unversty of Alaska Arboretum, (Top) Net CH4 flux versus tme for three permanent black hole statons. Note log flux scale; fluxes of mg m-2 d-1 plotted on abscssa. (Bottom) Sol sotherms (4 øc contour nterval) on depth versus tme plot for statons BH-1 ( ) and BH-4 (199). Wnter measurements were suspended after December most of the CH4 emtted from tussock:shrub tundra. Second, the C ste (Carex) accounts for most of the C-4 emsson from wet meadow tundra; t s the domnant plant form and annual CHn emsson was roughly equal to or exceeded that of the M ste n all stud years except 1988 (Table 5). Fnally, the annual source strength of tundra n the global C-h budget s hghly varable. The mean global emsson calculated from the estmates s Tg C-h yr-1. ncludng the 199 global emsson estmate, whch contans the unusually hgh Carex emsson rate (Tables 5 and 6), gves a global emsson estmate of of Tg yr-1. Envronmental Varables Ar temperature and precptaton data from the study area are presented n Fgures 5 and 6. Ar temperatures durng the study were generally hgher than the long-ter mean. Only 9 of 48 months showed negatve temperature anomales. Ar temperature anomales were about +2 øc durng the June through September emsson perod. Ctmaulatve annual precptaton vared from 16 cm (1987) to 47 cm (199), or from 6 to 17% of the long-term mean of 28 cm. Summer ranfall s crtcal n regulatng water table poston n the muskeg. Sols are water-saturated freeze-up, and the snow pack s largely exported by runoff before surface sols thaw, so that lttle standng water remans. Summeranfall durng 1987 through 1989 was low; precptaton durng ths perod was only 56 to 67% of the the long-term mean value of 16 cm. n contrast, 199 summer precptaton totaled 3 cm, or about 19% of the long-term mean. Tables 1-4 contan summary data for envronmental varables at permanent samplng statons from the thaw seasons, respectvely. Annual ranks of the duraton of thaw, maxmum thaw depth, and mean sol temperature for stes followed a predctable pattern based on local topography. Tussocks are taller than the surroundng vegetaton, so they thaw earler. The lake margn experences early thaw due to the hgh heat content of the eulttoral zone. The duraton of thaw vared from 125 to 155 days, wth the longesthaw seasons observed at T and C statons. Maxmum thaw depths averaged about -85, -7 and -55 cm at T, M, and BH statons. Mean sol

8 _ 146 Whalen and Reeburgh: Tundra Methane Emsson temperatures of approxmately 9 øc at C statons were 5 øc hgher than at statons located n the muskeg. Muskeg sol temperatures averaged 1ø to 5 øc, wth means ncreasng n the order BH, M, and T. The water table poston at T, M, and BH statons was always at or above the sol surface at fall freezeup and at sprng thaw, as noted above. Low summer precptaton (Fgure 6) resulted n a progressve lowerng of the water table level through the thaw season at these statons to local mnma of-18 and -4 cm n 1988 and 1989 (Tables 2 and 3). Softs remaned saturated throughout the thaw season at only two statons (T-1 and BH-2) n 1988, whle no softs remaned saturated n The summer '7 1a- ' ß -' E1 ; _. E 1' - : - ::3 -,T 1o o =C1 o =Cll [] = C2 [] -- C12 "= C3, : "= C13 1"- 1 '2_. O- E -2 -lo [ -3 V V V V Fg. 4. Net C-n flux to atmosphere and sol temperatures for Carex (C) sms n Unversty of Alaska Arboretum, (Top) Net C-tn flux versus tme for three permanent Carex statons. Note log flux scale. (Bottom) Sol sotherms (4 øc contour nterval) on depth versus tme plot for statons C- 1 ( ) and C- 11 (199). Wnter measurements were suspended after December TABLE 5.. Methane Emsson.by Tundra Ste Types Year (T) Erophorurn (C) Carex (M) Moss (BH) Black Hole Emsson measured n g CH 4 m -2 yr -1. ntegrated annual CH 4 flux for each ste + lo. ntegraton perod for 1987, 1988, and 1989 was January 1 to December 31. No wnter measurements were made n 199; data for 199 assume CH4 flux ncreases lnearly from May 1 to May 21 (date of frst flux determnaton) and decreases lnearly from September 22 (date of last flux determnaton) to October 15.

9 Whalen and Reeburgh: Tundra Methane Emsson 147 Ste TABLE 6. Global Tundra Methane Emsson Estmate Methane Emsson. Te vr -1 Percentage Covera Tussock and Low Shrub Tundra (6.46 x 112 m2) b Erophorum Carex Moss Black Hole Z Tussock Wet Meadow Tundra (.884 xlo 2 m 2) Carex Moss Wet Meadow Z Tundra akummerow et al., 1983; Walker et al., btotal tundrarea s 7.34 x 112 m 2 [Matthews, 1983]; wet meadow tundrarea s taken as all nonforested bog north of 5øN [Matthews& Fung, 1987; Table 5]. ranfall defct was greater n 1989 than 1988 (7 cm versus 5 cm), and ths s reflected n a greater water table reducton for Data for 199 (Table 4) are not drectly comparable to prevous data due to staton relocaton. Durng ths year of elevated precptaton (Fgure 6), BH statons wthn the muskeg remaned waterlogged throughout the thaw season. The magntude and areal extent of the drop n water table level over the growng season at T and M statons was roughly comparable to that observed at smlar statons n 1988 (Table 2). Water table levels at C statons are reported for 199 only. Statons were ntally establshed on the Smth Lake margn mdway between the Fall 1986, low water mark and the hstorc hgh water mark. The water table level was below the 7-cm-deep gaugng wells late n the low summer precptaton years of 1988 and The new C statons were establshed at roughly the same elevaton for 199 samplng; the water table was above the sol surface throughout ths summer of hgh ranfall (Table 4). Rank correlatons between CH4 flux and envronmental data are gven n Table 7. The best relatonshp was found between C-4 emsson and centmeter-degrees. We defne centmeter-degrees as the absolute value of the product of thaw depth and mean sol temperature to permafrost. Eleven of 12 possble correlatons were statstcally sgnfcant, showng a postve response n CH4 emsson to ncreasng centmeter-degrees. ncreasng C-h emsson was also sgnfcantly correlated wth ncreasng depth of thaw (rs negatve; depth defned relatve to sol surface). Mean sol temperature ncreases were postvely correlated wth ncreases C-4 emsson, but the relatonshp was weak; only 4 of 12 possble correlatons were statstcally sgnfcant. Qualtatvely, a postve relatonshp between C-4 emsson and sol temperature s evdent by comparng sol temperature dstrbutons wth flux n Fgures 1-4. The relatonshp between water table poston and C-4 flux was unclear. Three of 9 correlatons were sgnfcant. A lowered water table was correlated wth enhanced emsson (rs negatve) n two cases and wth reduced emsson n one nstance (rs postve). Sol pch4 Dstrbutons and CH4 Emsson Fgures 7 and 8 gve changes n water table poston, thaw depth, sol pch4 dstrbutons, and atmospherc CH4 flux at prmary statons for the 1988 and 1989 thaw seasons. As expected, the thaw depth ncreased at all statons durng both summers. The water table level decreased ntally through both

10 . 148 Whalen and Reeburgh: Tundra Methane Emsson,. 1 ß :: ß.: ß.:.:,. ß ß ß ß ' :, -'] [7] 1987 E ] 1988.r:! Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2 m m m m m m. ',. ': ::.:'?.-::':',.!:::(::?.-,: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::. " " " " :.:, ' ' N" '.::!!:::.':.:.: ß ::'::'.:'. ' :::!::::!:":.:::.. ' " L.:'{::'::'":'!:::-:: ". ::"::::::::::.: :.!::.!:::!:-:.:-....!-:.;::":::!:!::!:! ".'!':}::!:!::.::::',::}::::::::::!:{.:' ". ", ",," : ': ::::::: : ': : : ::. : :.:' : ' : : :: : :. :: ; ;: :,, ::: - 3- _. z :. o., ',... :. " ::: -3- ' ß :,,,,,,. ::-:::.: :': ; : : " :; : : : :::.:..? '::::; :.:;: : ::::. " ; :;:: : : : :, -6- : ', x' 19, ';: :::?:..:.'":......," :.:: ::::':. :::... :':' ::-' : :::'::. :':: : : :': : : ':': ': :::::::::::::::::::::: ::: ' ::::: ':: : "' :;: : : : :[::::- :? ',,,,... '-:4:,::.:.:..?.,.:: : ::: ;-:.:?'..?:.:.?::::---- ' X ' :.; ::(:';(:'":, ''.:'-:'.;: : ::::.,':'::.:;':'. :: ;.: ' " : :': :.....? ;:::.. -12, -. - ß,...,...,..., Jan Feb Mar Apr May Jun Jul Aug Sep t Nov D Fg. 5. Ar temperatures (n degrees Celsus) n the Unversty of Alaska Arboretum. Long-termeans and condtons durng study perod. (Top) Mean monthly ar temperatures. (Bottom) Ar temperature anomaly (dfference n long-termean and monthly mean values) durng study perod. Shadng denotes the major emsson perod. Data from Natonal Oceanc and Atmospherc Admnstraton (NOAA)[1986, 1987, 1988, 1989, 199] records for Farbanks nternatonal Arport, 6 km south of study stes. summers but rose slowly after mdseason n Most early season sol pc-h dstrbutonshow a 5- to 15-cm surface zone of low concentraton (< 5 matm), a 5- to 1-cm gradent of rapdly ncreasng pch4, and a deep zone where pc-h remans relatvely constant at 2 to 4 matm. Overall, the relatonshp between sol pc-h dstrbutons and C-h flux was not consstent and clear. Data for 1988 (T-1, M-1, and BH-1; Fgures 7a-7c) show low fluxes assocated wth a relatvely small C-h pool n June and early July. A July ncrease n sol pc-h and md to late August ncreases flux suggest a 2 to 3 week lag between ncreases n sol pch4 and emsson. Late season changes n sol C-h concentratons accompany a slght rse n water table level and appear unrelated to emsson. Relatonshps n the 1989 sol C-h and flux data are even less clear. A strkng mdseason ncrease n sol C-h was observed only at M-1 (Fgure 8), when CH4 emsson and the sol pc-h were clearly out of phase; hghest emssons occurred n late July and early August when sol CH4 was low. Methane emsson declned steadly to the end of the season followng the early September ncrease the sol CH4 pool. The sol C-h pool showed a gradual

11 Whalen and Reeburgh: Tundra Methane Emsson : / // // ! / 1,...- ß o -_..-."g'a,g -,.P 'P "' "... ß Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 1 E '.' ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: x <:,......: :: :5: :5: : : : : : : : :5: : * : : : :.' ;. ;. :.... ::::::::::::::::::::::::::::::::::::::::::::::::::::::... :[:5: : : :[:[: : :[:[: :5: :[: : :[:5:[: : : : /.:::...:':. : :5 :[: : : : : : : : : : : :.... '.. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 5: : : : : : : :[: : :[: : :5: : : : : :J: : :J: :[: : : : : : :[: : : : : : : : : :[: : : :[: : : : :.:.:.:..:.:..:.:..:.:.:..:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:..:.:.:.:..:.:.:. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fg. 6. Precptaton (n centmeters, water-equvalent) n the Unversty of Alaska Arboretum. Long-term mean and condtons durng study perod. (Top) Cumulatve precptaton. (Bottom) Precptaton anomaly (dfference n long-ter mean and monthly mean values) durng study perod. Shadng denotes the major emsson perod. Data from NOAA [ 1986, 1987, 1988, 1989, 199] records for Farbanks nternatonal Arport, 6 km south of study stes. TABLE 7. Spearman's Rank Correlaton Coeffcents (rs) for Correlatons Between CH 4 Emsson Durng the Thaw Season and Envronmental Varables at Prmary Samplng Statons n the Unversty of Alaska Arboretum Prmary Samplng Staton Black Hole Moss Tussock Year n Thaw depth Thaw days a Sol temperature b.87 Water table depth ND centmeter-degrees c ND ND Only values of rs sgnfcant at a =.1 are gven. ND means no data collected. afrst day of sample collecton (May 21) taken as frst day of thaw for 199. bmean to depth of permafrost. CAbsolute value of product of thaw depth and mean sol temperature to permafrost.

12 . 15 Whalen and Reeburgh: Tundra Methane Emsson 15 1 T-l, dun Water table Thaw depth A g S p Oct 1 1 _ p CH4, matra [ 6/1-6/27 6/26-7/6 7/6-8/3 8/9-9/7 9/7-9/27 Fg. 7. (Top) Net C-4 fluxes, (mddle) water table and thaw depths, and (bottom) subsurface pch4 dstrbutons for (A) T-l, (B) BH-1, and (C) M-1 statons, June - September, Carex (C-l) staton was not sampledurng Each of the pch4 versus depth panels ntegrates condtons up to the samplng date. Water table and thaw depth were measured when the equlbraton samplers were retreved. ncrease throughouthe season at T-1, whle emsson remaned generally constant (Fgure 8). Methane emsson and sol CH4 dstrbutons appear to be uncoupled at BH-1 (Fgure 8) and C-7 (Fgure 8). DSCUSSON Global Tundra Emsson Estmates One of our goals was to determne net CH4 emsson and ts temporal and spatal varablty n representatve tundra floral assocatons.we vew the annual emsson from each ste (Table 5) as a "reagent grade" budget consttuent of known uncertanty that can be approprately combned and weghted to produce regonal and global C-h emsson estmates. We have extended ste emssons from ths study (Table 5) usng our prevous weghtng scheme [Whalen and Reeburgh, 1988] to annual global CH4 emsson estmates for n Table 6. These emsson estmates ncorporate uncertantes areal coverage as well as the varatons n annual ste fluxes (Table 5). The representatveness of stes n a heterogeneous landscape s crucal n regonal extrapolaton of fluxes to global budgets [Matson et al., 1989]. We are confdenthat our weghtng scheme, whch consders the floral or ste composton n two major tundra subdvsons (tussock:shrub and wet meadow tundra), provdes a masonable framework for several reasons. Frst, C-4 fluxes from these major tundra subdvsons are domnated by two ste types: Erophorum ressocks (T) and Carex (C). These stes account for 24-45% and 8-9% of the areal coverage and contrbute 5-94% and 89-1% of the annual CH4 flux, respectvely, from the two tundra subdvsons (Table 6). Uncertantes areal coverage alone result n 1.5- to 2.4-fold varatons n annual CH4 emsson estmates for tundra. Second, statons establshed n smlar florstc unts at the Toolk Lake Long Term Ecologcal Research ste showed smlar CH4 emsson rates (mg m-2 d- ) durng two to three mdsummer vsts durng 1987 through A more complete summer flux tme seres obtaned at these stes durng 1991 (T. Chrstensen, Cambrdge Unv., personal communcaton, 1991), was weghted by the Whalen and Reeburgh framework and gave an annual emsson of 18-3 Tg C-4. Thrd, wansect measurements [Whalen and Reeburgh, 199a], whch provde an ndependent check, gave an emsson estmate of 38 Tg C-4 yr4 for tundra, whch agrees wth the present results. Fnally, a three-dmensonal tracer model study [Fung et a1.,1991], whch adjusts CH4 source strengths as a functon of lattude and requres a match between the model output and observed seasonal CH4 varaton at

13 Whalcn and R½½burgh: Tundra Methane Emsson 151 M-l, ?s t Water table Thaw depth ,,...._..._., ( X), 2( ß 1 Al g p CH 4, matm Sep ß,, 4( Oct E -1 6/1-6/27 6/26-7/6 7/6-8/3 8/9-9/7 9/7-9/ BH-1, t Jun Water table --- Thaw depth, 1,. 2 A g sp p CH 4, matra 5 1 2OO c -2 6/1-6/27 6/27-7/6 7/6-8/3 8/9-9/7 9/7-9/27 Fg. 7. (contnued) Global Montorng for Clmate Change statons [Steele et al., 1987], agrees wth the above estmates. For hgh northern lattudes, where tundra s the only C-h source, the model requres a source strength of 35 Tg CH4 yr-]. We are confdent scalng-up from long-term emsson records at fxed stes, but there are stll uncertantes. Unusual condtons, such as the hgh summer precptaton n 199, and the resultng hgh CHn fluxes from the C ste, can dstort any emsson estmate by extendng local anomales to a regonal or larger scale. Thus, we consder the CH4 emsson estmates from 1987 to 1989 (Table 6) as most representatve. Longer tme seres and a better understandng of the controls on C-h fluxes are needed. Also, our budget consttuent stes and essentally all studes (Table 8) suffer from a North Amercan bas; tme sere studes n representatve tundra and peatland areas of Sbera are needed to nsure correct regonal weghtng n future tundra CH emsson estmates. Table 8 summarzes hgh-lattude CH4 emsson estmates. The earlest estmate [Svensson, 1976] s based on a C-h flux:mosture relatonshp.

14 152 Whalen and Reeburgh: Tundra Methane Emsson a Water table Thaw depth -75 Jun 3 A g p CH,, matra O OCt 6/22-7/12 7/12-7/31 7/31-8/17 8/17-9/4 9/4-9/2 4 M-l, 1989 t Water Thaw depth table "---" _..._,_. Oct -75 Jun,J 11,N O 1 S p p CH4, matra /22-7/12 7/12-7/31 7/31-8/!7 8/! 7-9/4 9/4-9/2 Fg. 8. (Top) Net CH4 fluxes, (mddle) water table and thaw depths, and (bottom) subsurface pch4 dstrbutons (Bottom) for T-1, BH-1, M-1, and C-7 statons, June - September, Each of the pch4 versus depth panels ntegrates condtons up to the samplng date. Water table and thaw depth were measured when the equlbraton samplers were retreved. Subsequent estmates result from transect studes [Sebacher et al., 1986; Whalen and Reeburgh,!988], tme seres flux measurements at fxed locatons [Crll et al., 1988; Whalen and Reeburgh, 1988; Moore et al., 199], lterature CH4 flux data appled to refned estmates of global wetland areas [Matthews and Fung,!987; Aselmann and Crutzen, 1989], and a model estmate [Fung et al., 1991] based on lattudnal source strengths and observatonal constrants. Varatons n study types, assumptons, and regonal def'mtons are responsble for the wde range of estmates, but recent studes seem to be convergng a tundra source strength of about 35 Tg CH4 yr-1 [Reeburgh and Whalen, 1992].

15 . 153 Whalen and Reeburgh: Tundra Methane Emsson loo BH-1, t Jun Water table Thaw depth J l Al g S p Oct 2 2 p CH4, matm /22-7/12 7/12-7/31 7/31-8/17 8/17-9/4 9/4-9/2 4 t C-7, 1989 Water table Jun J l A g S p Oct 2 1 p CH 4, matm 3O., 3,oo,,. o /22-7/12 7/12-7/31 7/31-8/17 Fg. 8. (contnued) 8/17-9/4 9/4-9/2 Varablty n CH4 Emsson Prevous hgh-lattude studes at fxed stes have reported varablty n rams of C-h emsson rangng over 3 orders of magntude on seasonal [Crll et al., 1988; Moore et al., 199; Bartlett et al., 1992] and annual [Whalen and Reeburgh, 1988] scales. Matthews and Fung [1987] note that refnement of global C-h emsson estmates wll be acheved only through C-h flux determnatons over several years at representatve wetland stes. Ths study reports the frst multyear CH4 emsson measurements for Arctc tundra plant assocatons. Ths study fortutously encompasses a range of envronmental condtons useful for modelers. Ar temperature anomales were a unform +2 øc durng all summers covered by ths study (Fgure 5), so that a temperatureffect cannot be estmated from an nterannual emsson rate comparson. Precptaton was 6 to 19% of the long-term mean durng the summers of and 199, respectvely (Fgure 6), allowng meanngful comparson of

16 z c o z o z

17 Whaten and Reeburgh: Tundra Methane Emsson annual emsson estmates for a precptaton effect. The tundra CH4 flux vared from 28 to 52 Tg yr4 durng and was 12 Tg yr- n 199 (mdranges of data n Table 6) The data suggest a range of varatons n annual C-4 emsson of about twofold for years wth summer precptaton smlar to the long-term mean and about fourfold for years wth unusually hgh summer precptaton. t s unlkely that dfferences between the 199 annual flux estmates and those from prevous years resulted from staton relocafon. Parallel samplng of new and old statons on a sngle occason durng 199 showed no statstcally sgnfcant dfferences n fluxes for new and old statons of any ste type (Mann-Whtney Test; p>.5; data not shown). New C statons were establshed at roughly the same elevaton as prevous statons. We conclude that the ncreased emsson at C statons n 199 (Tables 5 and 6) resulted from a -4 cm rse n the summer water table due to record ranfall. ncreased C-h emsson from ths ste type was largely responsble for the hgh 199 emsson estmate, as noted above. We also presenthe frst data for nterannual varatons n C-h emsson from representatve tundra CH4 budget consttuent stes. Although summer ar temperature and precptaton were roughly comparable n , there were obvous dfferences n the ste fluxes (Table 5) and the relatve contrbuton of C, M, and BH ste types to the annual CH4 emsson estmate (Table 6). We have no ready explanaton for these dfferences but suspect they are due to nterannual varatons n the submeter scale nteractons of bologcal, physcal, and chemcal parameters and processes. mproved understandng of these nteractons s necessary to further resolve the twofold nterannual dfferences n overall annual CH 4 emsson. Correlatons Wth Envronmental Varables Other nvestgators have reported strong relatonshps between C-h flux and temperature. We had lmted success n correlatng mean sol temperature wth C-h emsson at prmary stes (Table 7). Correlaton coeffcents were not mproved by selectng temperature at a sngle depth or by use of a runnng mean temperature as the ndependent varable (data not shown). Lnear [Svensson and Rosswall, 1984], polynomal [Baker-Blocker et al., 1977], log-lnear [Holzapfel-Pschorn and Seler, 1986; Crll et at., 1988; Moore and Knowles, 199; Bartlett et al., 1992], and exponental [Moore et at., 199] relatonshps between C-h flux and ar or sol temperature have been reported. The varety of functons used suggests that the relatonshp between 155 C-h flux and temperature s not unque or straghtforward. The lack of statstcal correlaton between water table depth and seasonal C-h emsson at our muskeg stes (Table 7) appears to result from analyss of data for the entre thaw season. A low late summer water table corresponds wth reduced C-h flux (Fgures 7 and 8). However, early season (May) emssons from waterlogged sol were also low as a result of a thn thawed sol zone. Combnng all data for a season results n a low rs. Hghest CH4 emssons are generally observe durng mdsummer, when the thawed zone s rapdly ncreasng, but the water table s fallng. Seasonal studes n temperate swamp [Wlson et at., 1989] and wetland [Yavtt et al., 199] sols also report poor correlaton between water table level and C-h flux. n contrast, transect stude showed a closer relatonshp between C-h flux and water table poston (r =.42 to.54) [Sebacher et al., 1986; Hatrss et al., 1988; Whalen and Reeburgh, 199a] or sol mosture content (r =.77 to.93) [Svensson, 1976, 198]. t s lkely that the C-h emsson response to changng envronmental condtons occurs rapdly and s masked n whole-season correlatons. The parameters thaw depth and centmeter-degrees show the best correlaton wth C-h emsson (Table 7) because these varables ntegrate condtons mportant n determnng net C-h emsson. Specfcally, centmeter-degrees s a reasonable ndex of net mcrobal actvty because t accounts for the mean sol temperature and the depth of the actve zone. Controls on CH4 Emsson The measurements of net CH4 emsson reported here are the dfference between CH4 producton and mcrobally medated CH4 oxdaton. Ths oxdaton occurs before the CH4 s emtted to the atmosphere, and s an mportant control on tundra CH4 emsson. Reeburgh et al. (The Role of Mcrobally-Medated Oxdaton n the Global CH4 Budget, submtted to Nature, 1992) estmate that global CH4 oxdaton s about 2 Tg yr-1 larger than the estmated, 5 Tg yr-1 emtted to the atmosphere. Methane oxdaton by sol mcrobal communtes has been regarded a neglgble term n the global CH4 budget, but a body of evdence from most sols [Born et at., 199; Keller, et at. 199; Steudter et at., 199] s accumulatng that suggests t plays an mportant role n the global C-4 budget. Sols equlbrate rapdly wth the atmosphere [Whalen and Reeburgh, 199b], and the mcrobal CH4-oxdzng communty has a hgh capacty and low threshold for CH4 oxdaton

18 156 Whalen and Reeburgh: Tundra Methane Emsson [Whalen et al., 199]. Whalen et al. [1992] estmate that about half of the CH4 producton n tundra systems s oxdzed n the most aerobc zone and show that the extent of CH4 oxdaton s senstve to changes n water table level, whch drectly affects the extent of the oxdzed surface zone. For example, laboratory studes [Whalen et al., 1992] suggest that lowerng the tundra water table 1-2 cm n areas free of vascular plants would reduce the net flux of CH4 to zero, and further reductons n water table could permt oxdaton of atmospherc CH4. Thus oxdaton could provde a negatve feedback on CH4 ncreases n portons of the wetland term. The CH4 flux record from the Great Dsmal Swamp [Hatrss et al., 1982] also llustrates ths process. Sol pch 4 Dstrbutons and CH 4 Flux Sol pch4 dstrbutonsuggest a surface zone of rapd C-4 oxdaton and a subsurface zone of CH4 producton (Fgures 7 and 8). Methane profles (1 Vl CH matm C-4 at 1 øc) were smlar to those reported for Mnnesota peatlands [Cfll et al., 1988] and subarctc tundra [Bartlett et al., 1992]; a surface zone of low CH4 concentraton (<1 X,) overles a zone of rapdly ncreasng CH4 and a deep zone of 2-5 Vl CH4. Methane concentraton solnes for boreal fen sols [Moore et al., 199] suggest a smaller vertcal gradent, wth concentratons rangng from about 5 to 3 vl over smlar depths. Sequental sol CH4 dstrbutons are an ntegratng ndcator of mcrobal actvty. We expected CH4 emsson to track changes n the sol CH4 pool, whch can be seen n temporal sequences from sngle statons (Fgures 7 and 8). nstead, we found that C4_n flux, nd pool sze were generally unrelated over tme. Smlar results have been reported for a boreal fen [Moore et al., 199] and Subarctc tundra [Bartlett et al., 1992]. Ths decouplng could be the result of comparng measurements that respond to dfferent tme scales. The statc chamber flux determnatons are essentally nstantaneous rate measurements (snapshots), whle the sol CH4 determnatons result from ntegraton o er the equlbraton perod (tme exposure). However, the boreal fen C-h dstrbutons cted above were nstantaneous (pore waters syrnge-sampled) and C4_n pool szes determned n a salt marsh wth equlbrafon samplers were well correlated wth CH4 flux [Bartlett et al., 1987]. t s lkely that ths decouplng s an addtonal ndcaton of the complex botc and abotc factors determnng net CH4 emsson. Predctors and Feedbacks Severalnes of evdence suggest that the tundra system s too complex for a a sngle envronmental varable to serve as a unversal and successful ndcator of CH4 flux. Frst, a varety of functons have been used to relate CH4 flux and temperature (see above). Second, sgnfcant relatonshps often nvolve specal condtons. For example, Sebacher et al. [ 1986] reported good correlaton between water table depth to 1 cm and CH4 emsson n a transect study, and Wlson et al. [1989] requred a step functon to relate sol temperature to emsson. Thrd, relatonshps reported n Table 7 were best when data were smply ranked usng a nonparametrc statstc. Fnally, lx)olng all data for one varable by staton or year or by staton and year resulted n few sgnfcant correlatons, despte the fact that the ncrease n sample sze allows a smaller value of rs for statstcal sgnfcance. Moore et al. [ 199] noted that correlaton between CH4 flux and a sngle envronmental varable n hgh-lattude fens may, n part, be specous because varables nfluencng flux are not entrely ndependent. Ths vew s supported by our observaton that CH4 emsson was best correlated wth varables that serve as envronmental ntegrators (Table 7). Attempts to dentfy predctors of C-h emsson should focus on parameters that ntegrate factors mportant n CH4 producton and consumpton. Recent papers that have addressed feedbacks on trace gas emssons [Khall and Rasmussen, 1989; Lashof, 1989] assume that ncreased temperatures wll result n hgher CH4 fluxes,.e., a postve feedback. These studes focus on the nfluence of a sngle varable (temperature) on a sngle bologcal process (methanogeness) and do not consder other effects of clmate change, namely regonal changes n water table level [Mtchell, 1989] and rates of CH4 oxdaton. Functons that successfully relate CH4 flux to temperature are largely lmted to nundated wetlands (see above) where C-4 producton s the domnant bologcal process nfluencng emsson. We expect that CH4 flux-temperature relatonshps wll deterorate as water tables fall below the sol surface and oxdaton becomes mportant. The rato of oxdzed:reduced sol zones wll ncrease, favorng both aerobc decomposton and CH4 oxdaton. Laboratory studes wth packed peat columns [Moore et al., 1989] showed that CH4 emsson decreased logarthmcally, whle CO2 emsson ncreased lnearly as water table decreased. Moreover, changes n sol temperature wll not be unform wth depth and the effect of ncreased sol temperature wll be greater for C-4 consumpton than producton.

19 Whalen and Reeburgh: Tundra Methane Emsson 157 Fnally, the ncrease depth of thaw n permafrost areas wll not have as great a stmulatory effect on methanogeness as predcted [Khall and Rasmussen, 1989; Lashof, 1989], because deeper peat s lkely to be refractory ['Fan sh and Grgal, 1988; Yavtt et al., 1988]. A temperature feedback wll be mportant n coupled bogeochemcal-atmospherc models, but water table poston wll be a better ntegrator, waterlogged sols are zones of C-4 producton, whle most sols are areas of CH4 consumpton. Carefully executed feld experments nvolvng manpulatons of water table and temperature are needed to resolve the treatment of feedbacks n coupled bologcal models. One smple hydrologc model for northern fens ndcates that a fallng water table has a greater mpact than ncreased sol temperature on C-4 emsson [Roulet et al., 1992]. SUMMARY On the bass of a 4 year study at permanent stes we estmate that tundra contrbutes Tg yr-1 to the net atmospherc CH4 budget. Ths large, multyear data set, obtaned under a range of mosture and temperature condtons, restrcts the range of tundra CH4 source strength estmates and also shows the effects of local extrema. Results from multple permanent stes also suggest that sngle-parameter relatonshps used to predct C-4 flux are stespecfc and pont to the need for an ntegratng predctor. Acknowledgments. Ths work was supported by NASA grants NASW 841, RTOP , and DP F. S. Chapn advsed us on ste selecton at the start of ths project n Methane standards were provded by R. Rasmussen and D. Blake. Undergraduate assstants Samantha Royer (1989), Heather Andersen (199), and Krstel Kzer ( ) asssted n fabrcatng lucte chambers, feld samplng, and laboratory analyss. Unusually thorough and constructve revews by B. Svensson and an anonymous revewer mproved the manuscrpt. Contrbuton number 895 from the nsttute of Marne Scence, Unversty of Alaska Farbanks. REFERENCES Aselmann,., and P. J. Cmtzen, Global dstrbuton of natural freshwater wetlands and rce paddes, ther net prmary productvty and possble methane emssons, J. Atmos. Chem., 8, , Baker-Blocker, A., T. M. Donahue, and K. H. Mancy, Methane flux from wetland areas, Tellus, 29, , Bartlett, K. B., D. M. Bartlett, R. C. Hatrss, and D.. Sebacher, Methan emssons along a salt marsh salnty gradent, Bogeochemstry, 4, , Bartlett, K. B., P.M. Crll, R. L. Sass, R. S. Hatrss, and N. B. Dse, Methane emssons from tundra envronments n the Yukon-Kuskokwm Delta, Alaska, J. Geophys. Res., n press, Bllngs, W. D., Carbon balance of Alaskan tundra and taga ecosystems: Past, present and future, Quat. Sc. Rev., 6, , Blake, D. R., and F. S. Rowland, Contnung worldwde ncrease tropospherc methane, , Scence, 239, , Born, M., H. D6rr, and. Levn, Methane consumpton n aerated sols of the temperate zone, Tellus Ser. B, 42, 2-8, 199. Ccerone, R. J., and R. S. Oremland, Bogeochemcal aspects of atmospherc methane. Global Bogeochem. Cycles, 2, , Crll, P.M., K. B. Bartlett, R. C. Harrss, E. Gorham, E. S. Verry, D.. Sebacher, L. Mazdar, and W. Sanner, Methane flux from Mnnesota peatlands, Global Bogeochem. Cycles, 3, , Dcknson, R. E., and R. J. Ccerone, Future global wnnng from trace gases, Nature, 319, , Farfsh, K. W., and D. F. Grgal, Decomposton n an ombrotrophc bog and a mnerotrophc fen n Mnnesota, Sol Sc., 145, , Fung,., J. John, J. Lerner, E. Matthews, M. Prather, L. P. Steele and P. J. Fraser, Threedmensonal model synthess of the global methane cycle, J. Geophys. Res., 96, 13,33-13,65, Harrss, R. C., D.. Sebacher, and F. P. Day, Methane flux n the Great Dsmal Swamp; Nature, 297, , Harrss, R. C., D.. Sebacher, K. B. Bartlett, D. S. Bartlett, and P.M. Crll, Sources of atmospherc methane n the South Florda envronment, Global Bogeochem. Cycles, 2, , Hesslen, R. H., An n stu sampler for close-nterval ntersttal water studes, Lmnol. Oceanogr., 21, , Holzapfel-Pschom, A., and W. Seler, Methane emsson durng a cultvaton perod from an talan rce paddy, J. Geophys. Res., 91, 11,83-11,814, Keller, M., M. E. Mtre, and R. F. Stallard. Consumpton of atmospherc methane n sols of central Panama: Effects of agrcultural

20 158 Whalen and Reeburgh: Tundra Methane Emsson development, Global Bogeochem. Cycles, 4,21-27, 199. Khall, M. A. K., and R. A. Rasmussen. Clmatenduced feedbacks for the global cycles of methane and ntrous oxde, Tellus Ser. B, 41, , Kummerow, J. B., B. A. Ells, S. Kummerow, and F. S. Chapn, Sprng growth of shoots and roots n shrubs of an Alaskan muskeg, Am. J. Bot., 7, , Lashof, D. A., The dynamc greenhouse: Feedback processes that may nfluence future concentratons of atmospherc trace gases and clmate change, Clm. Change, 14, , Lkens, G. E., and N.M. Johnson, Measurements and analyss of the annual heat budget for the sedments n two Wsconsn lakes, Lmnol. Oceanogr., 14, , Matson, P. A., P.M. Vtousek, and D. S. Schmel, Regonal extrapolaton of trace gas flux based on sols and ecosystems, n Exchange of Trace Gases between Terrestral Ecosystems and the Atmosphere, edted by M. O. Andreae and D. S. Schmel, pp , John Wley, New York, Matthews, E., Global vegetaton and land use: New hgh-resoluton data bases for clmate studes, J. Clm. Appl. Meteorol., 22, , Matthews, E., and. Fung, Methane emsson from natural weftands: global dstrbuton, area, and envronmental characterstcs of sources,global Bogeochem. Cycles, 1, 61-86, Mtchell, J. F. B., The greenhouseffect and clmate, Rev. Geophys., 27, , Moore, T. R., and R. Knowles, The nfluence of water table levels on methane and carbon doxde emssons from peatland sols, Can. J. Sol Sc., 69, 33-38, Moore, T. R., and R. Knowles, Methane emssons from fen, bog and swamp peatlands n Quebec, Bogeochemstry, 11, 45-61, 199. Moore, T., N. Roulet, and R. Knowles, Spatal and temporal varatons of methane flux from subarctc/northern boreal fens, Global Bogeochem. Cycles, 4, 29-46, 199. Natonal Oceanc and Atmospherc Admnstraton (NOAA), Local clmatologcal data, Farbanks, Alaska, annual summary wth comparatve data, report, Ashevlle, N. C NOAA, Local clmatologcal data, Farbanks, Alaska, annual summary wth comparatve data, report, Ashevlle, N. C NOAA, Local clmatologcal data, Farbanks, Alaska, annual summary wth comparatve data, report, Ashevlle, N. C NOAA, Local clmatologcal data, Farbanks, Alaska, annual summary wth comparatve data, report, Ashevlle, N. C NOAA, Local clmatologcal data, Farbanks, Alaska, annual summary wth comparatve data, report, Ashevlle, N. C Post, W. M., W. R. Emanuel, P. J. Znke, and A. G. Stangenberger, Sol carbon pools and world lfe zones, Nature, 298, , Ramanathart, V., R. J. Ccerone, H. B. Sngh, and J. T. Kehl Trace gas trends and ther potental role n clmate change, J. Geophys. Res., 9, , Rasmussen, R. A., and M. A. K. Khall, Atmospherc methane (CH4): Trends and seasonal cycles, J. Geophys. Res., 86, , Rasmussen, R. A. and M. A. K. Khall, Atmospherc methane n the recent and ancent atmospheres: Concentratons, trends and the nterhemspherc gradent, J. Geophys. Res., 89, 11,599-11,65, Raynaud, D., J. Chappelez, J. M. Bamola, Y. S., Korotkevch, and C. Loftus, Clmatc and C-4 cycle mplcatons of glacal-nterglacal CH4 change n the Vostok ce core, Nature, 333, , Reeburgh, W. S., and S.C. Whalen, Hgh lattude systems as C-h sources, n. Trace Gas Exchange n a Global Perspectve, edted by B. Svensson and D. Ojma, Munksgaard, Copenhagen, n press, Roulet, N., T. Moore, J. Buber, and P. Lafleur, Northern fens: C-4 flux and clmate change, Tellus, n press, Sebacher, D.., R. C. Hatrss, and K. B. Bartlett, Methane emssons to the atmosphere through aquatc plants, J. Envron. Qual., 14, 4-46, Sebacher, D.., R. C. Harrss, K. B. Bartlett, S. M. Sebaeher and S.S. Grce, Atmospherc methane sources: Alaskan tundra bogs, an alpne fen and a subarctc boreal marsh, Tellus Ser. B, 38,1-1, Seler, W., A. Holzapfel-Pschom, R. Conrad, and D. Scharffe, Methane emsson from rce paddes, J. Atmos. Chem., 1, , Stauffer, B., G. Fscher, A. Neftel, and H. Oeschger, ncreases n atmospherc methane recorded n Antarctc ce core, Scence, 229, , Steele, L. P., P. J. Fraser, R. A. Rasmussen, M. A. K. Khall, T. J. Conway, A. J. Crawford, R. H. Gammon, K. A. Masare, and K. W. Thonng, The global dstrbuton of methane n the troposphere, J. Atmos. Chem., 5, , $teudler, P. A., R. D. Bowden, J. M. Melllo, and

21 Whalen and Reeburgh: Tundra Methane Emsson 159 J. D. Aber, nfluence of ntrogen fertlzaton on methane uptake n temperate forest softs, Nature, 341, , 199. $vensson, B. H., Methane producton n tundra peat, n Mcrobal Producton and Utlzaton of Gases (H2, CH4, CO), edted by H. G. Schlegel, G. Gottschalk and N. Pfenng, pp , E. Goltze, G6ttngen, Germany, $vensson, B. H., Carbon doxde and methane fluxes from the ombrotrophc parts of a subarctc mre, Ecol. Bull., 3, , 198. $vensson, B. H., and T. Rosswall, n stu methane producton from acd peat n plant communtes wth dfferent mosture regmes n a subarctc mre, Okos, 43, , Walker, D. A., N. D. Lederer, and M.D. Walker, Permanent vegetaton plots, data report, Dep. of Energy R4D Proj., NSTAAR, Unv. of Colo.,Boulder, March 31, Wen, R. W. and L. C. Blss, Prmary producton n arctc cottongrass tussock tundra communtes, Arc. Alp. Res., 6, , Whalen, $. C., and W. $. Reeburgh, A methane flux tme seres for tundra envroments, Global Bogeochem. Cycles, 2, , Whalen, $. C., and W. $. Reeburgh, A methane flux transect along the Trans-Alaska Ppelne Haul Road. Tellus Ser. B, 42, , 199a. Whalen, $. C., and W. $. Reeburgh, Consumpton of atmospherc methane by tundra softs, Nature, 346, , 199b. Whalen, $. C., W. $. Reeburgh, and K. A. Sandbeck, Rapd methane oxdaton n a landfll cover sol, Appl. Envron. Mcrobol., 56, , 199. Whalen, $. C., W. $. Reeburgh and K. $. Kzer, Methane consumpton and emsson from taga stes, Global Bogeochem. Cycles, 5, , Whalen, S.C., W. S. Reeburgh, and C. E. Remers, Control of tundra methane emsson by mcrobal oxdaton, n Landscape Functon: mplcatons for Ecosystem Response to Dsturbance, A Case Study n Arctc Tundra, Ecol Stud. Ser., edted by J. F. Reynolds and J. D. Tenhunen, Sprnger-Verlag, New York, n press, Wlson, J. O., P.M. Crll, K. B. Bartlett, D.. Sebacher, R. C. Harrss, and R. L. Sass, Seasonal varaton of methanemsson from a temperate swamp, Bogeochemstry, 8, 55-71, Yamamoto, S., J. B. Alkauskas and T. E. Crozer, Solublty of methane n dstlled water and seawater, J. Chem. Eng. Data, 21, 78-81, Yavtt, J. B., G. E. Downey, and D. M. Downey, Potental methane producton and methane oxdaton n peatland ecosystems of the Appalachan Mountans, Unted States, Global Bogeochem. Cycles, 2, , Yavtt, J. B., G. E. Lang, and A. J. Sextone, Methane fluxes n wetland and forest sols, beaver ponds, and low-order streams of a temperate forest ecosystem, J. Geophys. Res., 95, 22,463-22,4745, 199. W. S. Reeburgh and S.C. Whalen, nsttute of Marne Scence, Unversty of Alaska Farbanks, Farbanks, AK (Receved December 2, 1991; revsed February 6, 1992; accepted February 17, 1992.)