Spatial and temporal trends in carbon storage ofpeatlands of continental western Canada throughthe Holocene

Dale H. Vitt, Linda A. Halsey, Ilka E. Bauer, and Celina Campbell

Abstract: Peatlands of continental western Canada (Alberta, Saskatchewan, and Manitoba) cover 365 157 km2 andstore 48.0 Pg of carbon representing 2.1% of the world’s terrestrial carbon within 0.25% of the global landbase. Only asmall amount, 0.10 Pg (0.2%) of this carbon, is currently stored in the above-ground biomass. Carbon storage inpeatlands has changed significantly since deglaciation. Peatlands began to accumulate carbon around 9000 years ago inthis region, after an initial deglacial lag. Carbon accumulation was climatically limited throughout much of continentalwestern Canada by early Holocene maximum insolation. After 6000 BP, carbon accumulation increased significantly,with about half of current stores being reached by 4000 BP. Around 3000 BP carbon accumulation in continental west-ern Canada began to slow as permafrost developed throughout the subarctic and boreal region and the current southernlimit of peatlands was reached. Peatlands in continental western Canada continue to increase their total carbon storagetoday by 19.4 g m–2 year–1, indicating that regionally this ecosystem remains a large carbon sink.

Résumé: Les tourbières de l’Ouest continental canadien (Alberta, Saskatchewan et Manitoba) couvrent plus de365 157 km2 et contiennent 48,0 Pg de carbone, ce qui représente 2,1 % du carbone terrestre dans 0,25 % du territoireterrestre. Seulement une petite quantité, soit 0,10 Pg (0,2 %) de ce carbone, est présentement entreposée dans la bio-masse de surface. L’entreposage du carbone dans les tourbières a grandement changé depuis la déglaciation. Les tour-bières ont commencé à accumuler du carbone dans cette région il y a environ 9000 ans, après un premier retard dedéglaciation. L’accumulation de carbone a été limitée par le climat à travers une grande partie de l’Ouest continentalcanadien par une insolation maximum à l’Holocène précoce. Après 6000 Av. Pr., l’accumulation de carbone a aug-menté de façon significative; environ la moitié des réserves présentes a été atteinte vers 4000 Av. Pr. L’accumulation decarbone dans l’Ouest continental canadien a commencé à ralentir vers 3000 Av. Pr. lorsque le pergélisol s’est déve-loppé dans les régions boréales et subarctiques et que la limite sud actuelle des tourbières a été atteinte. Les tourbièresde l’Ouest continental canadien augmentent continuellement leur entreposage total de carbone de 19,4 g m–2a–1, indi-quant ainsi que cet écosystème demeure une grande trappe de carbone.

[Traduit par la Rédaction] Vitt et al. 693

Introduction

Peatlands have long been recognized as large sinks for at-mospheric CO2, removing an estimated 0.076 Pg (1 Pg =1015 g) of carbon from the atmosphere annually through theprocess of peat accumulation (Gorham 1991). Peat accumu-lates in wetlands where the rate of biomass production isgreater than the rate of decomposition, and it is most abun-dant in boreal and subarctic regions of the circumpolar northwhere cool and moist climatic conditions favour decreasedrates of decomposition (Gore 1983). Anaerobic decomposi-tion in northern peatlands produces methane (CH4), withroughly 0.032 Pg being released to the atmosphere annually(Frolking 1991). Clearly, peatlands are an important compo-nent of the terrestrial carbon budget, with northern peatlandstorage representing about 455 Pg of carbon, of which

6.8 Pg is living biomass (Gorham 1991). Peatland carbondynamics influence atmospheric CO2 and CH4 concentra-tions and thus, future changes in peatland carbon storagehave the potential to influence greenhouse gas-inducedwarming (Post et al. 1992). How this peatland carbon is dis-tributed across the landscape and, more importantly, how ithas accumulated through time at a regional scale is largelyunknown, with only vague statements having been appliedto long-term accumulation at the landscape level (Moore etal. 1998).

Not all peatlands are the same, they have different hydro-logical, chemical, and biotic gradients. Peatlands are eitherombrogenous and receive their surface water and nutrientssolely from precipitation, or they are geogenous and receivewater not only from precipitation but also from surface wa-ter and groundwater; the former are termed bogs, while thelatter are fens. Swamps, peat accumulators in eastern Can-ada, generally do not accumulate >40 cm of organic matterin continental western Canada (cf. Tarnocai et al. 1995) and,hence, are not included in this paper.

Fens may be acidic andSphagnum-dominated (poor fens),or alkaline, basic to neutral, and dominated by “brownmosses” (rich fens). Bogs are acidic and are dominated by

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683

Received November 23, 1998. Accepted September 13, 1999.

D.H. Vitt, 1 L.A. Halsey, I.E. Bauer, and C. Campbell.Department of Biological Sciences, University of Alberta,Edmonton, AB T6G 2E9, Canada.

1Corresponding author (e-mail: dvitt@gpu.srv.ualberta.ca).

some combination ofSphagnum, lichens, and feather mosses(Belland and Vitt 1995). Unlike domed bogs found in tem-perate and oceanic areas, continental bogs are relatively flatacross their surfaces (National Wetlands Working Group1988). Permafrost is an important component of northernpeatlands and is restricted almost exclusively to bogs withinthe boreal forest (Vitt et al. 1994). Water table depths arevariable in peatlands, generally in the range of 10 cm aboveto 40 cm below the surface for fens (cf. Gignac et al. 1991;Nicholson et al. 1997), while bogs are drier as a whole, withwater tables 40–60 cm below the surface for nonpermafrostbogs, and approximately 100 cm below the surface for per-mafrost bogs (Belland and Vitt 1995). Thus bogs have a rel-atively thick, upper aerobic zone (acrotelm), while in fensthe acrotelm is shallower; despite the differences, decompo-sition in both bogs and fens remains less than production.

Peatland distributions have been spatially and temporallyvariable throughout the Holocene, developing after an initialdeglacial lag (Halsey et al. 1998). The initiation of peat ac-cumulation is related to stabilization of seasonal water levelsand restriction of water flow through a wetland (Zoltai andVitt 1990) and, in conjunction with leaching of soluble saltsfrom the mineral substrate, allows the establishment and de-velopment of a moss layer in hydrologically conducive areas(Vitt et al. 1993). Thus, climate and geology are importantfactors controlling peatland distribution in both space andtime (Halsey et al. 1998). The stabilization of regional watertables appears to have been an important component in thesuccessional change from prairie marshes to boreal fens inthe western interior of Canada over the past 10 000 years(Zoltai and Vitt 1990).

Successional patterns through time have been docu-mented; bogs can succeed fens (though not all fens becomebogs) with the increased importance ofSphagnumcausing afundamental change in the functioning of these peatlandecosystems, leading to acidification and oligotrophication(Vitt and Kuhry 1992). Permafrost development occurred inthe later part of the Holocene, around 3000 to 4000 BP,within the subarctic and boreal forest of western Canada,once Sphagnumaccumulation had become significant andthreshold climatic conditions were established (Zoltai 1995).

While net primary production appears to be similar at aregional scale in all boreal peatland types (Campbell et al.2000), decomposition rates differ, and result in variable car-bon accumulation potentials in the acrotelm of a peatland(Thormann et al. 1999). Permafrost bogs have extremely lowcarbon accumulation potentials in the acrotelm at the cen-tury scale due to the relatively high frequency of fires on drypeatland surfaces, resulting in near-surface samples withnonrecent radiocarbon dates (cf. Zoltai 1993). Carbon accu-mulation potentials at a yearly scale, though probably highlyvariable, are unknown for permafrost bogs, while fornonpermafrost bogs and brown moss peatlands, first yearmass loss estimates range from 14% to 25–61%, respec-tively, (Thormann et al. 1999). The decomposition of peat inthe deeper anaerobic zone (catotelm) has been approximatedby a simple exponential decay model (Clymo 1984).

This paper examines how carbon is stored in peatlandswithin the boreal and subarctic landscape in continentalwestern Canada. Storage is partitioned into two components:(1) above-ground storage (including aerial components of

trees, shrubs, and herbs) and (2) surface and below-groundstorage (including living ground layer (nonvascular plants),below-ground living root biomass (vascular plants), anddead groundlayer (peat) located in the acrotelm andcatotelm). Carbon storage is examined both spatially andtemporally in 1000 year time slices throughout the Holo-cene.

Methods

Determination of current and past carbon storage inpeatlands of continental western Canada requires eight typesof data, each derived from different sources:

(1) Inventory of current peatland distribution by peatlandtype.

(2) Estimates of current maximum depth distributions.(3) Calculation of surface and below-ground storage vol-

ume by peatland type using area and maximum depth ad-justed by basin topography.

(4) Carbon content of organic matter in peat.(5) Profiles of organic matter density distinguished by

peatland type.(6) Above-ground carbon content of biomass by peatland

type on a mass–area basis.(7) Temporal patterns of peatland initiation and expan-

sion.(8) Long-term catotelm decomposition.

Current peatland distributionPeatlands and peatland complexes across continental

western Canada were inventoried by type from 1 : 40 000 to1 : 60 000 aerial photographs following the classification ofHalsey and Vitt (1997), and the data were transferred to1 : 250 000 base maps. Peatland types were distinguished onthe basis of hydrology: bog versus fen; permafrost: presenceor absence; patterning: presence or absence; and forestcover: wooded, shrubby, or open. At the scale of mappingused, individual peatlands were rarely identified, with mostpolygons composed of peatland complexes and the compo-nents identified to the nearest 10% cover. Peatland dis-tributions were determined from the 1 : 250 000 base mapsthrough digitizing onto provincial base maps. Areal extentswere calculated in ARC/INFO for 0.25° latitude and 0.5°longitude grids by peatland type. Published summaries havebeen completed for Alberta (Vitt et al. 1996) and Manitoba(Halsey et al. 1997), while summary data for Saskatchewanare available from the authors.

Current maximum depth distributionMaximum depth values for 818 peatland sites in continen-

tal western Canada were compiled from several sources(Bannatynne 1980; Zoltai et al. 2000; Vitt published and un-published data) and contoured through gridding inMacGridzo for 0.25° latitude and 0.5° longitude grid cells,with extrapolation in areas with no data along the northeast-ern margin of Manitoba (Rockware Inc. 1991).

Peatland volumeBelow-ground peatland volumes were calculated for 0.25°

latitude and 0.5° longitude grids by multiplying maximumdepth and peatland type areas (maximum volume), adjusted

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by a topography value to account for basin slope. Topogra-phy by grid cell was determined from numerous surficial ge-ology studies, soil inventories, and biophysical reports (listavailable from authors) and was divided into five types fol-lowing those established by the Canadian Soil Survey Com-mittee (1978) for surface expression. These include(1) level, with 98% of the maximum volume occupied bypeat (1–3% slope); (2) undulating, with 93.5% of the maxi-mum volume occupied by peat (4–9% slope); (3) rolling,with 87.5% of the maximum volume occupied by peat (10–15% slope); (4) hummocky – knob and kettle, with 77% ofthe maximum volume occupied by peat (16–30% slope); and(5) steep–inclined, with 70% of the maximum volume occu-pied by peat (>31% slope).

Carbon contentCarbon contents of 253 samples were determined from ten

cores recovered from across continental western Canada(Athabasca: 55°05′N; 113°15′W in an open fen, a woodedfen, and a nonpermafrost bog; Rainbow Lake: 58°17′N;119°22′W in a shrubby fen and a permafrost bog; FlintstoneLake: 50°43′N; 95°18′W in a nonpermafrost bog; Jan Lake:54°53′N; 102°48′W in a wooded fen and a nonpermafrostbog; and North Knife Lake: 58°07′N; 97°02′W in two openfens (Bauer, unpublished data, 1996, 1997).

Samples of known volume were taken from the cores at10 cm intervals, air dried for 48 h, and weighed for bulkdensity. From each of these samples two ground subsampleswere taken. From one of the subsamples, loss on ignition at550°C was calculated to determine organic matter density(= ashless bulk density) (Dean 1974). A second subsamplewas analyzed for carbon content in a Controlled EquipmentCorporation Model 440 CHN elemental analyzer comparedto a certified standard (Acetanilide). Relating carbon contentto organic matter density is preferable to comparisons of drybulk density (Wieder et al. 1994), particularly in continentalwestern Canada where aeolian activity has been extensivethroughout the Holocene in areas associated with geogenousfens (Halsey et al. 1990), and volcanic ash deposits are com-monly found within peat deposits (Zoltai 1989). For this rea-son, percent carbon is calculated on an ash-free basis.

Organic matter densityVariations in mean organic matter density determined by

stratigraphic horizon (Dean 1974) for 475 cores collectedacross western Canada were compared among peatland type,ecoregion, and whole core depths grouped by 50 cm inter-vals as main effects within a General Linear Model proce-dure in SAS (SAS Institute Inc. 1988). All main effectsdisplayed a normal distribution, thus a Student–Newman–Keuls post test was utilized to distinguish statistically simi-lar groupings for all main effects.

Modern peatland carbon storageThe amount of current carbon stored in peatlands was cal-

culated by using the mean organic matter densities for allmain effects groupings distinguished in the post test adjustedby mean carbon content. Carbon content densities were thenmultiplied by peatland type volumes obtained from the 1838land-based 0.25° latitude and 0.5° longitude grid cells fromacross continental western Canada.

BiomassA literature survey of above-ground biomass was con-

ducted for continental western Canada. Only vascular plantswere placed into above-ground biomass as it is difficult, ifnot impossible, to determine the boundary between livingnonvascular plants and dead peat. Pooled, mean biomass val-ues by peatland type were used to determine total above-ground biomass for 0.25° latitude and 0.5° longitude gridcells. When coupled with carbon content, the amount of car-bon stored in living above-ground vascular plants was deter-mined. Details are reported in Campbell et al. (2000).

Temporal pattern: calculated past carbon storageThe distribution and extent of peatlands in continental

western Canada has changed throughout the Holocene(Zoltai and Vitt 1990). Peatland volume at a given time is afunction of the timing of initial peat formation and the rateof subsequent peatland expansion (paludification) across thelandscape. As peatlands in continental western Canada arerelatively flat the elevation (depth) of the surface at onepoint in a peatland will be generally the same across thepeatland. Thus, as a peatland expands through time its eleva-tion (depth) increases. Following this pattern of lateralexpansion, paludification patterns (depth-calibrated radiocar-bon date) were established for a permafrost site (RainbowLake, AB: 58°17′N; 119°22′W) and a nonpermafrost site(Athabasca, AB: 55°03′N; 113°15′W) using curve estima-tions whose intercepts were forced through zero in SPSS(SPSS 1995). These relationships were then used to extrapo-late peatland expansion throughout the Holocene for all gridcells utilizing timing of peatland initiation determined fromHalsey et al. (1998). Carbon contents in 1000 year incre-ments were calculated for each grid cell using current storeswith depth adjusted by the paludification patterns derived forpermafrost and nonpermafrost peatlands.

Temporal pattern: modeled past carbon storage(catotelm decomposition)

Amounts of carbon based on current storage need to becorrected for long-term catotelm decomposition. Here weutilize Clymo’s (1984) exponential decay model for catotelmpeat to determine the long-term decay constant in thecatotelm (α) for nine cores located throughout continentalwestern Canada (Gypsumville: 51°46′N; 98°30′W (Kuhry etal. 1992); Site 4: 52°51′N; 116°28′W (Zoltai 1989); Site 5:53°20′N; 117°28′W (Zoltai 1989); Zoltai 81–18A: 54°45′N;115°51′W (S.C. Zoltai, unpublished data, 1981); BuffaloNarrows: 55°56′N; 108°34′W (Kuhry 1994); MarianaLakes: 55°54′N; 112°04′W (Nicholson and Vitt 1990); Leg-end Lake: 57°26′N; 112°57′W (Kuhry 1994); RainbowLake: 58°18′N; 119°17′W (Zoltai 1993); and Zama City:59°07′N; 118°09′W (Zoltai 1993)). Using the mean esti-mated value of a modeled amount of carbon (Ct) for each1000 year increment throughout the Holocene was calcu-lated from the apparent amount of carbon (C′) in each 0.25°latitude and 0.5° longitude grid cell [1].

[1] Ct/C′ = e–αt

Since acrotelm decay contributes to the actual current stor-age values, and does not add significantly to long-term de-composition, no attempt was made to incorporate acrotelmlosses through time.

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Results

Current peatland distributionContinental western Canada consists of Alberta, Saskatch-

ewan, and Manitoba (Fig. 1) and contains approximately365 157 km2 of peatlands that make up 21% of the landbase

(Table 1). Peatlands are concentrated in northern and north-eastern Alberta and northeastern Manitoba as well as alongthe northeastern shore of Lake Winnipeg (Fig. 1). Sixty-three percent of these peatlands are fens, 28% are permafrostbogs, while nonpermafrost bogs represent only 9% of allpeatlands in continental western Canada (Table 1). Perma-

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686 Can. J. Earth Sci. Vol. 37, 2000

Fig. 1. Contoured current distribution of peatlands. Contour intervals are in 10% increments of total land surface.

Fig. 2. Contoured maximum depth values for peatlands of continental western Canada. Contour interval is 50 cm.

frost bogs and open nonpatterned fens are most abundant inthe Subarctic Region, while all other peatland types reachhighest abundance in the High Boreal Region. Nearly one-half of all peatlands are in the High Boreal Region. Sixty-four percent of the peatlands of continental western Canadaare treed.

Current maximum depth distributionContoured maximum depth values for continental western

Canada are presented in Fig. 2. Peatlands are deepest in themid-boreal and along the eastern border of Manitoba. Forthis reason carbon storage does not reflect peatland distribu-tion.

Carbon contentMean carbon content of 253 samples was 47.7 ± 5.0% of

dry bulk density, and when subtraction of ash is included, ityields a mean of 51.8 ± 4.7% of carbon. This value fallswithin the range of carbon contents that can typically be ex-pected for soils (Nelson and Sommers 1996).

Organic matter densityAnalysis of the variation of organic matter density, using

the General Linear Model, shows that all main effects—peatland type, ecoregion, and whole core depth grouped into50 cm intervals—are significant in explaining variation inorganic matter density, with only the interaction betweenecoregion and depth being significant (Table 2). Ecoregionand depth have a significant interaction, as deeper peatlands(> –350 cm) are found more often within the mid- and lowboreal ecoregions. Of the main effects, only peatland type isrecognized by the Student–Newman–Keuls post test as hav-ing significantly different variation (Fig. 3). Organic matterdensity of wooded and shrubby fens is significantly differentfrom that of open fens and permafrost and nonpermafrost

bogs, with the former group having a 12% greater wholecore mean organic matter density (Fig. 3).

Current peatland carbon storageCurrent peatland carbon pools form two groups:

(1) wooded and shrubby fens with a carbon density of 0.055 ±0.003 g C@cm–3 and (2) open fens and (permafrost andnonpermafrost) bogs with a carbon density of 0.049 ±0.004 g C@cm–3. Carbon storage in continental western Cana-dian peatlands is concentrated in northern and northeasternAlberta, northeast of Lake Winnipeg, and within the HudsonBay Lowlands of Manitoba (Fig. 4). Currently, 47.9 Pg ofcarbon are stored as living groundlayer, below-ground bio-mass, and peat in continental western Canadian peatlands(Table 3). Fens contain almost twice as much carbon asbogs. Bogs contain about 35% of peatland carbon, with per-mafrost dominated systems having 27% of the total carbon.Only 9% of the stored carbon is in nonpermafrost bogs. Fenshave 65% of the total carbon, with treed fens having themost (30%). Treed systems in general contain 65% of the to-tal carbon. Fifty-one percent of peatland carbon is found inthe High Boreal Region of continental western Canada.Manitoba contains 57.8% of peatland carbon, followed byAlberta with 27.9%, and Saskatchewan with 14.3%.

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Vitt et al. 687

Peatland type Arctic Subarctic MontaneHighBoreal

MidBoreal

Aspen Parklandand Interlake Total

%total

%area

Permafrost bogs 24 66 159 0 35 749 1 112 0 103 044 28.2 5.9Nonpermafrost bogs 0 1 749 0 20 884 6 513 1 812 30 958 8.5 1.7Treed fens 0 10 324 56 68 194 16 698 6 182 101 454 27.8 5.8Shrubby fens 51 2 986 21 13 638 4 287 3 362 24 345 6.7 1.4Open nonpatterned fens 202 30 190 26 25 327 10 496 5 724 71 965 19.7 4.1Open patterned fens 0 4 156 26 15 768 11 445 1 996 33 391 9.1 1.9Total 277 115 564 129 179 560 50 551 19 076 365 157 100.0 20.8% 0.1 31.7 0.1 49.2 13.7 5.2 100.0

Note: Regions follow those defined by the Ecological Stratification Working Group (1995).

Table 1. Distribution of peatlands in continental western Canada (Alberta, Saskatchewan, and Manitoba) in square kilometres.

Source F value Pr >F

Ecoregion 3.12 0.0090**

Depth interval 3.06 0.0006**

Peatland type 6.14 0.0001**

Ecoregion*depth interval 1.84 0.0051**

Depth interval*peatland type 0.61 0.8605Ecoregion*peatland type 0.72 0.8690Ecoregion*depth interval*peatland type 0.69 0.8928

** Independent variables that are significant at the 95% level.

Table 2. Effects model significance levels of all variables. Fig. 3. Mean organic matter density for different peatland typesobtained from 475 whole cores from across continental westernCanada. Error bars represent the standard deviation of the mean.Student–Newman–Keuls post test identified three groupings A, B,and C of which two are statistically distinguishable with means of0.105 g@cm3 representing wooded and shrubby fens (A) and 0.094g@cm3 for open fens and permafrost and nonpermafrost bogs (C).

BiomassAbove-ground vascular plant biomass by peatland type for

continental western Canadian peatlands is presented in Ta-ble 4. Wooded peatlands have the highest amount of biomass

per unit area, followed by shrubby, and open fens. Of thewooded peatlands, bogs have the highest above-ground bio-mass. Carbon contents, coupled with pooled, mean biomassnumbers by peatland type, and area of peatlands results in

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688 Can. J. Earth Sci. Vol. 37, 2000

Fig. 4. Contoured current carbon storage in the surface and below-ground component of peatlands across continental western Canada.Contour interval is 20 kg@m2.

Fig. 5. Contoured current carbon storage of above-ground vascular plant biomass in peatlands of continental western Canada. Contourinterval is 50 g@m2.

0.10 Pg of carbon stored in above-ground vascular peatlandplant biomass, with its distribution following that ofpeatland distribution (Fig. 5).

Temporal pattern of past carbon storagePaludification trends from two bog–fen peatland com-

plexes show that age is linearly related to depth for thenonpermafrost site, while for the permafrost site age is re-lated to depth as a power function (Fig. 6). With the devel-opment of permafrost, peatland development follows a cycleof aggradation and degradation often related to fire (Zoltai1993). This cyclic development retards long-term carbon ac-cumulation once permafrost is initiated. The slope of thecurve fitted to the permafrost site decreases around 4000 BP(Fig. 6), corresponding to the timing of permafrost expan-sion into bogs of this area (Zoltai 1993).

Carbon stored in peatlands has increased during the Holo-cene (Fig. 7; Table 5). Peatlands began to accumulate carbonaround 9000 BP, with half (51.4%) of current stocks presentby 4000 BP. During the last 1000 years about 5.1 Pg(10.6%) of the total long-term (catotelm) carbon has de-cayed, and 12.2 Pg (25.5%) of new carbon has accumulated,with a net gain of 7.1 Pg, or 14.8%. This compares with be-tween 4000 and 5000 BP when the net carbon gain wasabout 67.2%, increasing the stock from 16.0 Pg to 24.7 Pg.During the early Holocene, between 8000 and 9000 yearsago, the percent net carbon gain was even higher (91.7%),but the actual increase in stock was minimal (1.1 Pg).

Discussion

Peatlands of continental western Canada contain a signifi-cant amount of carbon—47.9 Pg with an additional 0.10 Pg

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Vitt et al. 689

Peatland type Arctic Subarctic Montane High Boreal Mid BorealAspen Parklandand Interlake Total %

Permafrost bogs 1.77 × 1012 8.34 × 1015 0 4.17 × 1015 1.44 × 1014 0 1.27 × 1016 26.5Nonpermafrost bogs 0 2.96 × 1014 0 2.84 × 1015 8.05 × 1014 2.01 × 1014 4.14 × 1015 8.6Treed fens 0 1.36 × 1015 3.78 × 1012 9.72 × 1015 2.25 × 1015 7.47 × 1014 1.41 × 1016 29.4Shrubby fens 3.95 × 1012 4.04 × 1014 1.40 × 1012 1.77 × 1015 3.89 × 1014 2.65 × 1014 2.83 × 1015 5.9Open nonpatterned fens 1.59 × 1013 4.09 × 1015 1.71 × 1012 3.23 × 1015 9.55 × 1014 4.51 × 1014 8.74 × 1015 18.3Open patterned fens 0 7.09 × 1014 1.83 × 1012 2.63 × 1015 1.59 × 1015 4.48 × 1014 5.38 × 1015 11.2Total 2.16 × 1013 1.52 × 1016 8.72 × 1012 2.44 × 1016 6.13 × 1015 2.11 × 1015 4.79 × 1016 100% 0.1 31.7 0.0 50.9 12.8 4.4 100

Note: Regions follow those defined by the Ecological Stratification Working Group (1995).

Table 3. Distribution of carbon in peatlands of continental western Canada (Alberta, Saskatchewan, and Manitoba) in grams.

SiteAbove-groundbiomass (g m–2) Reference

Pooled mean above-ground biomass (g m–2)

Wooded bogsNonpermafrost bog 901.3 Reader and Stewart 1972 662 775Nonpermafrost bog 423.3 Reader and Stewart 1972Nonpermafrost bog (1991A) 823.2a Szumigalski 1995 887Nonpermafrost bog (1992A) 854.8 Szumigalski 1995Nonpermafrost bog (1994A) 984.0a Thormann 1995

Wooded fensRich fen (1991B) 887.3a Szumigalski 1995 750Rich fen (1992B) 613.3 Szumigalski 1995

Shrubby fensPoor fen (1991C) 364.1 Szumigalski 1995 395 275Poor fen (1992C) 424.8 Szumigalski 1995Rich fen (1991D) 224.6 Szumigalski 1995 201Rich fen (1992D) 139.8 Szumigalski 1995Rich fen (1993D) 110.0 Thormann 1995Rich fen (1994D) 328.0 Thormann 1995Rich fen (1991E) 190.0 Szumigalski 1995 230Rich fen (1992E) 269.0 Szumigalski 1995

Open fensRich fen (1991F) 105.0 Szumigalski 1995 99 254Rich fen (1992F) 92.7 Szumigalski 1995Rich fen (1993G) 339.0 Thormann 1995 410Rich fen (1994G) 480.0 Thormann 1995

Note: Swamp and marsh sites are not included. Year of collection as well as a site designator (A through G) are given for sites measured over multiple years.aTree layer biomass component is based on 1992 modeled values from Szumigalski (1995).

Table 4. Total above-ground biomass for all continental western Canadian peatland sites.

in above-ground biomass. Together this represents roughly2.1% of the world’s terrestrial carbon within 0.25% of theglobal terrestrial surface area. Other workers have calculatedthe amount of carbon stored in organic soils (peats) in Can-ada (Tarnocai 1998), with an estimated 38.1 Pg (–20.5% de-viation when compared to the amount of carbon weestimate) within Alberta, Saskatchewan, and Manitoba (C.Tarnocai and B. Lacelle, personal communication, 1998).This value is considered low by these workers, with the dis-crepancy attributed to lack of information on peat depths (C.Tarnocai and B. Lacelle, personal communication, 1998).

Gorham (1991) examined current carbon storage on ageneral circumboreal level. Comparing our more detailed ap-proach over a much smaller area, variation in the contribut-ing components is as follows: peatland area for continentalwestern Canada +14.4% (Gorham’s (1991) value is higher);+11.7% for mean carbon density (Gorham’s (1991) value ishigher); and –10.4% for depth (calculation used here provid-ing higher contribution). Thus, while depth is a major sourceof error in calculating carbon storage (cf. Botch et al. 1995),carbon densities, and peatland distributions are also variable.Detailed inventories of peatland distributions, such as thosewe present here, will reduce error in carbon storage esti-mates substantially.

Adding together the estimated variation in this study forpeatland distribution (±4%), standard error of the mean for

carbon content (±0.3%), standard error the mean for organicmatter density for treed and shrubby fens (±0.4%) that com-prise 35% of peatlands, and for bogs and open fens (±0.5%)representing 65% of peatlands. When a ±10.4% variationderived from comparing differing depth measurement meth-ods (cf. Gorham 1991) is included, the root-mean-squareerror is 11.2%, with carbon storage in peatlands of continen-tal western Canada estimated to represent 48.0 ± 5.4 Pg.

Carbon storage in peatlands has changed considerablyover the Holocene (Fig. 7). Peatlands began to accumulatecarbon around 9000 BP in this region, after an initialdeglacial lag. Carbon accumulation was climatically limited,however, as the amount of land with suitable climatic condi-tions for peatlands was restricted by early Holocene maxi-mum summer insolation (Halsey et al. 1998). As summerinsolation decreased, more land became climatically avail-able for peatland formation, with the peatland climaticthreshold transgressing southward (Halsey et al. 1998).

Over a span of 3000 years during the mid-Holocene(6000–3000 BP), about half of current stocks accumulated.Since the mid-Holocene, peatland carbon stocks have contin-ued to increase with a further one-third accumulating overthe last 3000 years. The net increase in carbon stocks overthe last 3000 years has declined relative to the mid-Holocene. This decline corresponds temporally to the expan-sion of permafrost into continental western Canada (Zoltai

© 2000 NRC Canada

690 Can. J. Earth Sci. Vol. 37, 2000

Site LocationRadiocarbon dates(years BP) Depth (cm) Reference

Decay rate(year–1)

Gypsumville 51°46′N and 98°30′W 1790±90 (AECV-1082C) 146.0 Kuhry et al. 1992 1.9 × 10–4

2710±100 (AECV-1081C) 186.04230±100 (AECV-1031C) 235.5

Site 4 52°51′N and 116°28′W 4460±170 (BGS-771) 205.0 Zoltai 1989 4.3 × 10–5

6170±140 (BGS-770) 241.08600±250 (BGS-772) 354.5

Site 5 53°20′N and 117°28′W 2800±300 (BGS-773) 190.0 Zoltai 1989 2.3 × 10–4

6140±200 (BGS-774) 332.08400±270 (BGS-775) 506.0

Zoltai 81–18A 54°45′N and 115°51′W 2820±220 (BGS-776) 243.5 Zoltai unpublished 1.4 × 10–4

6170±180 (BGS-777) 442.58940±240 (BGS-554) 551.0

Buffalo Narrows 55°56′N and 108°34′W 1480±100 (AECV-1092C) 86.0 Kuhry 1994 1.3 × 10–4

5230±90 (AECV-1739C) 120.07870±130 (AECV-1091C) 163.0

Mariana Lakes 55°54′N and 112°04′W 3170±80 (AECV-262C) 107.0 Nicholson and Vitt 1992 1.8 × 10–5

5270±90 (AECV-263C) 160.06740±100 (AECV-212C) 189.0

Legend Lake 57°26′N and 112°57′W 1180±80 (AECV-1645C) 60.5 Kuhry 1994 2.6 × 10–4

4390±91 (AECV-1738C) 90.57950±100 (AECV-1900C) 112.0

Rainbow Lake 58°18′N and 119°17′W 1070±90 (AECV-989C) 46.5 Zoltai 1993 8.9 × 10–5

3710±100 (AECV-990C) 84.57620±120 (AECV-991C) 187.0

Zama City 59°7′N and 118°9′W 1420±90 (AECV-984C) 52.0 Zoltai 1993 1.7 × 10–4

2410±120 (AECV-985C) 111.05840±100 (AECV-986C) 258.0

Mean 1.41 × 10–4

Standard deviation 8.13 × 10–5

Table 5. Sites and associated radiocarbon dates used to calculate catotelm decay rates.

1995), and to the establishment of the current southern limitof peatlands (Halsey et al. 1998).

Since the accumulation of peatland carbon in continentalwestern Canada was initially limited by global maximumearly Holocene insolation (Halsey et al. 1998), accumulationpatterns similar to those presented here would be expectedthroughout the circumboreal. High-resolution records of at-mospheric methane in Greenland show an increase in con-centration after a mid-Holocene minimum (Chappellaz et al.1997). This increase in methane concentration correspondstemporally to the highest peat accumulation rates in ourdata.

With the development of permafrost in continental west-ern Canada 3000–4000 BP, in conjunction with the southernlimit of peatlands being reached about 3000 BP, the rate ofincrease in overall carbon storage in peatlands of continentalwestern Canada declined. This decrease relates to the de-cline in the Greenland/Antarctic methane concentration ra-tio, suggesting that the contribution of methane fromnorthern sources became less important globally around thistime (Chappellaz et al. 1997). While there was more carbonstored in continental western Canada 3000 years ago than atany other time previously in the Holocene, the widespreaddevelopment of bogs and also permafrost probably resultedin decreased methane fluxes. Bogs, with their thickacrotelms, are known to produce very low methane fluxesrelative to other peatland types, with permafrost bogs pro-ducing the least (Klinger et al. 1994; Bubier et al. 1995).This suggests that overall methane flux from continentalwestern Canadian peatlands may have decreased after thewidespread development of permafrost around 3000 BP.

Although the rate of peatland carbon sequestration is lesstoday than 3000 years ago, this regional study documents

that peatlands in continental western Canada continue tofunction as a carbon sink. This finding follows a similar ob-servation made by Kuhry and Vitt (1996) at the site specificlevel. Our estimate suggests that in continental westernCanada 7.1 × 1012 g C@year–1 (19.4 g C@m–2

@year–1) havebeen sequestered during the past 1000 years. This is lowerthan Gorham’s (1991) estimate for northern peatlands of28.1 g C@m–2

@year–1, but comparable to the 21 g C@m–2@year–1

for very poorly drained soils (peatlands) estimated for borealManitoba where accumulation–loss of carbon from the soilprofile was determined (Raphalee et al. 1998), and to thoseof Finnish mires summarized by Mäkilä (1997) that rangefrom 14.1 to 22.5 g C@m–2

@year–1 The carbon stock increaseof 10.6% over the past 1000 years indicates that currentstocks are increasing at a rate of 0.011% annually. Thus,while regional storage is beginning to level off due to in-creased total catotelm decay, as predicted by Clymo (1984),more important to this decline in net carbon storage is thedevelopment of permafrost, and the establishment of the cur-rent southern limit of peatlands.

Conclusions

Peatlands cover 365 157 km2 of continental western Can-ada and store 48.0 Pg of carbon, representing 2.1% of globalterrestrial carbon over 0.25% of the landbase. Peatland car-bon stores have been highly variable through the Holocene.

© 2000 NRC Canada

Vitt et al. 691

Fig. 6. Relationship between peatland depth and calibrated, basalradiocarbon date (I. Bauer, unpublished data). Basal dates fromthe permafrost site at Rainbow Lake, AB are represented as opensquares, while dates from the nonpermafrost site at Athabasca,AB are represented by closed circles. Regression lines were de-termined from curve estimation in SPSS (SPSS 1995). Thenonpermafrost site has a linear relationship between peatlanddepth and calibrated radiocarbon date (depth = 0.052 × date, r2 =0.76), while the permafrost site displays a power relationship(depth = 4.47 × 10–7 × date2.23, r2 = 0.66).

Fig. 7. Changes in carbon storage through the Holocene forpeatlands in continental western Canada. Solid bar segments areestimated carbon present at each time interval; stippled bar seg-ments represent the modeled amount of carbon decayed since de-position (1 Pg = 1 × 1015 g).

Accumulation began after an initial deglacial lag, and in-creased around 6000 BP as more land area passed through aclimatic threshold that was previously limited in extent byearly Holocene maximum insolation. Contemporaneous in-creases in methane concentrations in Greenland ice cores(Chappellaz et al. 1997), correlate well with rates of peat ac-cumulation, not with the total carbon pool. About half ofcurrent peatland carbon was present by 4000 BP. The rate ofincrease in carbon storage in continental western Canadianpeatlands began to decline around 3000 BP, responding towidespread permafrost development and the establishmentof the current southern peatland limit. Declines in the ratioof Greenland/Antarctic methane concentration after 3000 BPsuggest a declining boreal source (Chappellaz et al. 1997)that appears to be related to the rate of carbon accumulation.

Northern peatlands have played an important role in atmo-spheric carbon budgets through the Holocene, and currentlysequester about 19.4 g C@m–2

@year–1. The changes in carbonstorage presented here are based on limited data on thedepth/age relationship of peatland expansion and should beviewed only in the context of the overall trends. Collectionof more data on peatland expansion spanning all climaticand physiographic regions is required to reach a better un-derstanding of how peatlands, and the carbon they store, re-spond to changing climates.

Acknowledgments

Funding for this project was provided by a Climate Sys-tem History and Dynamics, National Science and Engi-neering Research Council Research Network Grant and aNetwork of Centres of Excellence in Sustainable ForestManagement Grant to Dale Vitt. R. Kelman Wieder im-proved an earlier draft of this manuscript, for which we arethankful. Dr. Bob Vance and the Geological Survey of Can-ada provided the radiocarbon dates for the Rainbow LakeCore, for which we are grateful. In addition thanks are alsoextended to Nigel Roulet and an anonymous reviewer,whose thoughtful and thorough reviews greatly improved themanuscript. Graphics were produced by Laureen Snook andSandi Vitt. We dedicate this manuscript to Stephen Zoltai,our dear friend and always enthusiastic colleague, whose ex-tensive work in the peatlands of western Canada contributedsignificantly to this manuscript.

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