johansson et al. 2006.pdf

Decadal vegetation changes in a northern peatland,greenhouse gas fluxes and net radiative forcing

T O R B J O R N J O H A N S S O N *, N I L S M A L M E R w , PA T R I C K M . C R I L L z, T H O M A S F R I B O R G § ,

J O N A S H . A K E R M A N *, M I K H A I L M A S T E PA N O V * and T O R B E N R . C H R I S T E N S E N *

*GeoBiosphere Science Centre (CGB), Physical Geography and Ecosystems Analysis, Lund University, Solvegatan 12, SE-223 62

Lund, Sweden, wDepartment of Ecology, Plant Ecology and Systematics, Lund University, Lund, Sweden, zDepartment of

Geology and Geochemistry, Stockholm University, Stockholm, Sweden, §Institute of Geography, University of Copenhagen,

Copenhagen, Denmark

Abstract

Thawing permafrost in the sub-Arctic has implications for the physical stability and

biological dynamics of peatland ecosystems. This study provides an analysis of how

permafrost thawing and subsequent vegetation changes in a sub-Arctic Swedish mire

have changed the net exchange of greenhouse gases, carbon dioxide (CO2) and CH4 over

the past three decades. Images of the mire (ca. 17 ha) and surroundings taken with film

sensitive in the visible and the near infrared portion of the spectrum, [i.e. colour infrared

(CIR) aerial photographs from 1970 and 2000] were used. The results show that during

this period the area covered by hummock vegetation decreased by more than 11% and

became replaced by wet-growing plant communities. The overall net uptake of C in the

vegetation and the release of C by heterotrophic respiration might have increased

resulting in increases in both the growing season atmospheric CO2 sink function with

about 16% and the CH4 emissions with 22%. Calculating the flux as CO2 equivalents

show that the mire in 2000 has a 47% greater radiative forcing on the atmosphere using a

100-year time horizon. Northern peatlands in areas with thawing sporadic or discontin-

uous permafrost are likely to act as larger greenhouse gas sources over the growing

season today than a few decades ago because of increased CH4 emissions.

Keywords: aerial CIR photography, carbon balance, greenhouse gases, GWP, northern Sweden,

peatland, permafrost, radiative forcing, sub-Arctic, vegetation change

Received 23 February 2006; revised version received 7 July 2006 and accepted 12 July 2006

Introduction

Northern wetlands are characterized by cold and wet

conditions that result in low decomposition rates for

plant litter. This promotes the sequestration of organic

matter as peat and the formation of widespread peat-

lands that, particularly in the Northern Hemisphere,

have accumulated carbon (C) by removing atmospheric

CO2 for approximately the past 10 000 years. In this

way, organic matter has accumulated since the last

glacial maximum (Smith et al., 2004) and today northern

peatlands hold 300–455 Pg C (Sjors, 1980; Gorham, 1991;

Tolonen & Turunen, 1996) representing 20–30% of all

global soil C (Post et al., 1982; Smith et al., 2004) in about

3% of the global terrestrial area (Matthews & Fung,

1987). At the same time peatlands are large sources of

atmospheric CH4 as a result of the prevailing anaerobic

soil conditions.

The stability of the atmospheric C-sink function in

peatlands is largely dependent on hydrology and tem-

perature (Gorham, 1991; Hargreaves & Fowler, 1998),

and will, therefore, vary with the climatic conditions.

Northern peatlands extends both within the sporadic

and discontinuous permafrost zone (Matthews & Fung,

1987; Brown et al., 1998). Where peatlands and perma-

frost occur together we have fragile ecosystems sensi-

tive to transformation in a changing climate. Such

changes could simultaneously have a large feedback

on the global terrestrial C balance. Houghton et al.

(2001) states that the global surface temperature has

changed 0.6 � 0.2 1C since the late 19th century. In theCorrespondence: Torbjorn Johansson, tel. 1 46 0 46 222 39 74,

fax 1 46 0 46 222 40 11, e-mail: [email protected]

Global Change Biology (2006) 12, 2352–2369, doi: 10.1111/j.1365-2486.2006.01267.x

r 2006 The Authors2352 Journal compilation r 2006 Blackwell Publishing Ltd

near future the high northern latitudes are projected to

experience larger effects on the climate with increased

temperature, precipitation, and growing season length,

than anywhere else on the globe (Kattsov et al., 2005 in

ACIA, 2005, IPCC senarios A2 and B2). Evidence for

recent climate change influencing cold regions of the

Earth is beginning to accumulate (e.g. Serreze et al.,

2000; Hinzman et al., 2005) with degrading permafrost

both in northern high latitudes and high altitude areas.

The southern border of the discontinuous permafrost

zone has retreated northward in North America since

the end of the Little Ice Age (i.e. mid-19th century) and

this process is still in progress (Halsey et al., 1995).

Recently, widespread decrease in the extent of palsa

mires in Fennoscandia has been reported from high

elevation in southern Norway (e.g. Sollid & S�rbel,

1998), northern Finland (e.g. Luoto et al., 2004), and

northern Sweden (e.g. Zuidhoff & Kolstrup, 2000;

Christensen et al., 2004). The same trend has been

observed in boreal peatlands in North America (e.g.

Camill, 2005).

Effects of permafrost thawing include both deepening

of the active layer and soil subsidence, which in turn

might cause changes in hydrology and plant cover.

Boreal and sub-Arctic peatland ecosystems have been

observed to become wetter with a subsequent vegeta-

tion change and changes in C fluxes (Camill, 1999;

Jorgenson et al., 2001; Christensen et al., 2004; Malmer

et al., 2005). The opposite, drying response due to recent

change in climate has also been reported for the tundra

of north Alaska. In the Barrow region, Oechel et al.

(1993, 1995) reports that Alaskan arctic tundra changed

from a CO2 sink to a source due to warmer and drier

soils in the 1980s. Continued warming, however, has led

to ecosystem acclimation and resulted in diminished

efflux and, in most cases, summer CO2 sink activity

(Oechel et al., 2000; Kwon et al., 2006). Measurements

from 1997 to 2000–2003 on plot scale and with eddy

covariance methods in northeast Greenland show a sink

functioning during the summer season (Christensen

et al., 2000; Friborg et al., 2000; Soegaard et al., 2000;

Groendahl et al., 2006), and the CO2 sink activity seems

to benefit from increased summer temperatures (Groen-

dahl et al., 2006).

Several authors raise concerns about the stability and

potential release of stored C as CO2 and CH4 to the

atmosphere from peatland ecosystems due to the chan-

ging climate (e.g. Gorham, 1991; Melillo et al., 1996;

Christensen et al., 1999). Numerous land-atmosphere

exchange studies of peatlands show high variability in

C fluxes at small spatial scales due to differences in

surface hydrology because of differences in plant com-

munity composition (e.g. Waddington et al., 1996; Joabs-

son et al., 1999; Bubier et al., 2003; Strom et al., 2003).

Hydrology and vegetation change as affected by the

presence or absence of permafrost also give rise to

changes in the C balance and trace gas fluxes of north-

ern peatlands.

Most studies of greenhouse gas exchange in peat-

lands are concerned only with either CO2 or CH4. Fewer

studies combine measurements of both CO2 and CH4

fluxes (e.g. Roulet et al., 1997; Christensen et al., 2000;

Friborg et al., 2003; Heikkinen et al., 2004). Some include

measurements of N2O (Nykanen et al., 1995) and also

dissolved organic C (DOC) export (Waddington &

Roulet, 2000) to arrive at C and greenhouse gas budgets.

Annual greenhouse gas budgets are important for a full

accounting of the total impact on the global radiative

forcing from a given ecosystem as a result of its ex-

changes of trace gases with the atmosphere. Laine et al.

(1996) accounted for the total impact of greenhouse

gases (CO2, CH4 and N2O) from a mire ecosystem in

Finland using IPCCs Global Warming Potentials (GWP)

indicating an increased positive radiative forcing from

northern peatlands. Friborg et al. (2003) also made a

similar calculation based on eddy covariance measure-

ments of both CO2 and CH4 fluxes in the central west

Siberian lowlands. Despite a significant C sink function

during summer months these huge wetlands showed a

strong positive radiative forcing effect due to their

substantial CH4 emissions.

Some of the first CH4 and CO2 flux measurements in

peatlands were made at the Stordalen mire in northern-

most Sweden in the early 1970s (Svensson, 1980), a site

intensively studied within the Swedish Tundra Biome

project during the International Biological Programme

(IBP) in the 1970s. In recent years this mire has been

revisited with thorough investigations of both physical

and ecological aspects allowing interdecadal compar-

isons of changes in ecosystem functioning (Svensson

et al., 1999; Christensen et al., 2004; Malmer et al., 2005).

The objective of the current study has been to analyse

how the vegetation changes resulting from thawing

permafrost in the Stordalen mire since 1970 may have

changed the net exchange of greenhouse gases CO2 and

CH4. Malmer et al. (2005) estimated the observed vege-

tation change at the Stordalen mire using species re-

cords and two colour infrared (CIR) aerial photographs

of the �17 ha mire from 1970 and 2000 with a spectral

resolution from 400 to approximately 900 nm encom-

passing the visible and near infrared region of the

spectrum. These spatial data are combined with avail-

able and published data of plot scale C fluxes from

specific vegetation types to calculate trace gas exchange

and total CO2 equivalents. The fluxes are then scaled to

the whole mire to estimate the effect vegetation change

has had on the radiative forcing of the peatland as a

whole.

D E C A D A L C H A N G E S O F C A R B O N F L U X A N D F O R C I N G 2353

r 2006 The AuthorsJournal compilation r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 2352–2369

Materials and methods

Study site

The Stordalen mire, a sub-Arctic mire in northern

Sweden 10 km east of Abisko (681200N, 191030E), is

situated in the sporadic permafrost zone along the

0 1C isotherm at 351 m above sea level (Fig. 1). A large

portion of the mire consists of a slightly elevated

drained area underlain by permafrost. This part of the

mire is characterized by a hummocky topography with

a plant community structure typical of ombrotrophic

conditions (Fig. 2 and Table 1; cf. also Malmer et al.,

2005). The remaining area of the mire is largely lacking

permafrost, with prevailing wet, fen-like conditions.

Sonesson (1972) calculated the basal radiocarbon age

of the mire to be at least 5070 � 65 14C years BP, but the

deposition of ombrotrophic peat might not have started

later than 800 calibrated 14C years BP (i.e. before 1950),

and did not reach its present range until 300–400 years

ago (Malmer & Wallen, 1996). The depth of the organic

layer on the mire is between 1 and 3 m. A mineral layer

containing a large portion of silt underlies the peat.

Climate

The precipitation at Stordalen mire, located about 10 km

east of the weather station at the Abisko Scientific

Research Station (ANS), is not significantly different

from Abisko. The mean temperature, however, is

slightly lower due to temperature inversions driven

by the nearby large lake Tornetrask. The altitudinal

difference, approximately 40 m, is considered to be

negligible but the lake functions as a cold source during

the growing season (Ryden et al., 1980). Abisko mean

annual air temperature (MAAT) for the period 1913–

2003 is �0.7 1C (Table 2). Abisko is in a rain shadow and

the precipitation received is among the lowest in Scan-

dinavia. The mean annual precipitation for the period

1913–2003 is only 304 mm (Table 2). The winter preci-

pitation is low and falls mostly as snow. The mean snow

depth on the Stordalen mire was continuously mea-

sured during the winter months between 1971 and 1975

(Ryden & Kostov, 1980). The yearly maximum snow

depth for Stordalen mire during this period was 0.12–

0.26 m (Ryden & Kostov, 1980). This is significantly less

than that measured at Abisko (0.50–0.60 m) during the

same years (Kohler et al., 2006). The shallow snow depth

at Stordalen mire is most likely due to the openness of

the site, which promotes snowdrift. This is not the case

for the measurements made at Abisko as these transects

are within the mountain birch forest. It is very likely

that the difference in snow depth between Abisko and

Stordalen is due to wind scouring.

Holmgren & Tjus (1996) document trends in the

Abisko summer temperature since the late 1860s. From

1870 to 1910 only minor changes were observed. After

1910 an overall rise of 1.5 1C took place over three

decades ending in 1940. Since then, until the beginning

of the 1980s, a small decline of 0.5 1C occurred. The

same pattern has been shown for the whole of northern

Sweden (Alexandersson & Eriksson, 1989) and is also

similar to the tendency in the global record (Houghton

et al., 2001). In the long-term trend, autumn and winter

Fig. 1 Field site location, shown as a point in northern most Europe. The overview of the Abisko area showing Stordalen mire with the

nearby road built in the 1980s and the railway.

2354 T . J O H A N S S O N et al.


temperatures showed smaller changes than were ob-

served in the spring and summer months during the

analysed years (Holmgren & Tjus, 1996). Since the 1980s

a trend towards warmer annual temperatures has

emerged similar to or above the warming during the

1920s and 1930s (Climate data from ANS, and the

Swedish Meteorological and Hydrological Institute,

SMHI Nordklim dataset 1.0, 2001, Tuomenvirta et al.,

2001).

Active layer measurements

Continuously measurements of permafrost and the

active layer have been made since 1978 at nine repre-

sentative mires in the area (Christensen et al., 2004 and

Fig. 3). Occasional active layer depth records are avail-

able for Stordalen mire already since 1963 (Sonesson,

1969). Here we have used three larger datasets from

1973 and onwards. The active layer data available from

1973 to 1976 are seasonally weighted averages by vege-

tation type compiled from Ryden & Kostov (1980) and

measured in a 25 point grid with a equidistance of 15 m

laid out on the southern most part of the elevated area, a

full account of the set-up is found in Ryden & Kostov

(1980). The measurements ended at the onset of frost.

The measurements between 1978 and 1994 were col-

lected during the third week in September as transects

but with different number of replicates (n 5 29–100),

length of transects, and measured at different places in

different years but always within the dry elevated part

of the mire. The last 5 years of data were collected either

every 0.5 m along 285 m transects across all site classes

(2001–2002) or at 121 CALM (Circumpolar Active Layer

Monitoring Program; Brown et al., 2000) grid points

Fig. 2 A schematic cross section of the different site classes found on the mire and their permafrost- and hydrology regimes (plant

symbols after Rosswall et al., 1975). The site classes are; Hummock (dry ombrotrophic; I), semiwet (ombro-minerotrophic; II), wet

(ombrotrophic; III), and Tall graminoid (wet minerotrophic; IV).

Table 1 Site classes for the Stordalen mire separated in the colour infrared images and the characteristic plant species of each class

Site class Characteristic plant species

Hummock and Tall shrub I Empetrum hermaphroditum, Betula nana, Rubus chamaemorus, Eriophorum vaginatum,

Dicranum elongatum, Sphagnum fuscum, Hepaticea: Lichens

Semiwet II E. vaginatum, Carex rotundata, S. balticum, Drepanucladus schulzei, Politrichum jensenii

Wet III E. vaginatum, C. rotundata, S. balticum, D. schulzei, P. jensenii,

Tall graminoid IV E. angustifolium, C. rostrata, S. lindbergii, S. Riparium

Water V

Stone pits VI

The roman numerals (I–VI) refer to Fig. 2.



arranged with 75% of the points on elevated parts of the

mire underlain by permafrost. An analysis of variance

(data not shown) and geostatistics (semivariogram, data

not shown) was performed to evaluate sample (n)

needed for a stated precision and to address the ques-

tion of spatial autocorrelation. With a 95% confidence

interval at least 10 measurements is required for a

precision of 0.02 m and the spatial dependence on the

elevated portion of the mire is about 8–10 m, thus

comparable mean values are assumed to have been

obtained between 1973 and 2005. Active layer depths

in 2001–2002 were recorded in late August. These data

were modelled to estimate active layer depths for the

third week in September with the assumption of a linear

relation between active layer depth and summed daily

mean temperatures (accumulated thawing degree days,

DDT) using a variant of Stefans solution (Brown et al.,

2000). Data collected on nine different occasions in 2004

support this approach (99% confidence level, r2 5 0.999,

Po0.001). Using data by Ryden & Kostov (1980) to-

gether with collected data we can extend the active

layer measurement period on Stordalen back to 1973

(Fig. 3).

Aerial images

Two sets of aerial CIR photographs were analysed. The

1970 image was taken August 8 from a height of 1500 m

and the 2000 image on July 29 from a height of 4600 m.

The images were interpreted through a combination of

unsupervised classification (isodata) and manual inter-

pretation. A full description of the handling of the two

images and the contemporary vegetation records to-

gether with a presentation of the maps with the dis-

tribution of the distinguished site classes based on the

vegetation and surface structure of the mire is given in

Malmer et al. (2005). For the 2000 image the site classi-

fication was checked against vegetation records in a

grid system covering 70% of the mire while for the 1970

image the classification was checked against vegetation

records in four plots 50 m� 50 m in size (Sonesson &

Kvillner, 1980).

C flux

In the calculation of the decadal change in the C

exchange we have assumed that fluxes from the differ-

ent site classes did not change between 1970 and 2000.

There are admittedly uncertainties associated with this

assumption as we have very little data. No comparable

net ecosystem exchange (NEE) measurements exist

from the 1970s. At that time only photosynthesis mea-

surements using labelled CO2 was conducted at the site

(e.g. Johansson et al., 1973; Johansson & Linder, 1980)Tab

le2

Cli

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and these are difficult to compare with present day

whole ecosystem flux measurements. As there have

only been small changes in growing season tempera-

tures (Table 2) between 1970 and 2000 the temperature

response on both ecosystem respiration and gross

photosynthesis is assumed to be negligible. An earlier

study dealing with possible interdecadal changes in

CH4 and CO2 fluxes indicated that the CH4 flux over

the growing season had not changed, but that the CO2

flux (ecosystem respiratory release) was significantly

higher 1995 than in 1974 specifically within one vegeta-

tion community (Svensson et al., 1999). It was suggested

that the presented increase was connected to permafrost

disintegration, vegetation composition change, and

changed mineralization pathways (Svensson et al.,

1999). Nevertheless, the measured increase of respira-

tory release imply that there could have occurred

changes in the NEE from the different site classes, but

as only the respiratory part was studied by Svensson

et al. (1999) we here make the conservative assumption

of steady state net ecosystem exchange between 1970

and 2000.

The growing season on the mire is here considered to

be between May and September, which is the average

period with photosynthetic activity during contempor-

ary years and climate. It is also the only time period

during the years when chamber flux measurements are

available.

The terrestrial CO2 flux values used in this study are

hourly averages measured during the period May

through September in the years 2002–2004. The CO2

values are measured as NEE, (i.e. the summed result

of photosynthesis, and autotrophic and heterotrophic

respiration). The respiration values presented are the

nocturnal flux values measured. The data were col-

lected by an automatic nine-chamber system measuring

each chamber every third hour on three different vege-

tation types; hummock (dry), semiwet (ombrotrophic),

and tall graminoids (wet minerotrophic). The chamber

system set-up and components is in detail presented in

Goulden & Crill (1997). The NEE measured by the

automatic system chambers in the tall graminoid sites

are shown to correspond well with the eddy covariance

(EC) tower at the Stordalen site, but the CO2 flux

standard deviations are larger from the EC system than

from the automatic chamber system (Fig. 4). The CH4

fluxes from the terrestrial surfaces are compilations of

midday CH4 fluxes from the growing season (Table 3,

cf. Christensen et al., 2004) made with closed chamber

techniques using permanent aluminium collars with

channels filled with water. Measurements of CO2 and

CH4 fluxes from lakes next to Stordalen are unavailable,

–0.31970 1975 1980 1985 1990

Year

1995 2000 2005 20102.5

2.0

1.5

1.0

0.5

0.0 (°C

)

–0.5

–1.0

–1.5

–2.0

–2.5

–0.2

–0.1

0.0(m)

0.1

0.2

0.3Stordalen AL average 0.58 mAbisko AL average 0.60 m

Year vs. AL anomalies (Stordalen)

Abisko average 8.8°C

Year vs. AL anomalies (Absiko all bog sites) Christensen et al. (2004), updatedYear vs. JJAS degree anomalies

Fig. 3 The anomaly of active layer (AL) depth in metres and the anomaly of the mean air temperature during June–September (JJAS) in

1C used here as a substitute for the accumulated thawing degree days (DDT). The correlation between AL and JJAS is significant, r 5 0.62

(P 5 0.003). The regression between JJAS vs. time, the last 30 years is weak but significant at a 95% level using a 2nd order polynomial fit,

r2 5 0.26 (P 5 0.043). The active layer deepening vs. time is significant at the 99% level, r2 5 0.66 (P 5 0.000). The AL anomalies (all bog

sites) is a compilation from Christensen et al. (2004) showing the mean values from all investigated mires in the area from 1978–2002 and

updated with new data for the years 2003–2005. The AL data for Stordalen mire is a compilation from Ryden and Kostov 1980 (1973–

1976), and data presented in this article.



CO2 data from other lakes around Abisko (Jonsson &

Karlsson, 2003) and CH4 data from Northern Finland,

Wisconsin, USA, and Arctic lakes (Zimov et al., 1997;

Riera et al., 1999; Huttunen et al., 2002) were applied

only to ponds within the mire and not to the adjacent

lake areas (Table 3). The CO2 and CH4-fluxes were

spatially extrapolated using the derived vegetation

maps as C-CO2, C-CH4 and together as CO2-equivalents

using IPCCs 100-year GWP estimate for CH4 (Hought-

on et al., 2001, Chapter 6). At this time horizon CH4 has

a 23 times higher accumulated radiative forcing per unit

mass relative to CO2.

Results

Active layer and distribution of permafrost

Since 1978 annual measurements of end-of-season per-

mafrost thaw have systematically been conducted in

mires around Abisko (Fig. 3, cf. Christensen et al., 2004).

These data show an increasing active layer depth, the

rate of which has been accelerating during the last few

years (Fig. 3). Permafrost has also vanished entirely at

one of the monitored mires. The thickness of the per-

mafrost layer has not been measured at the site, but on

palsas in the vicinity and at other northern Sweden sites

a thickness of more than 4 m has earlier been reported.

(Rapp & Annersten, 1969; Zuidhoff & Kolstrup, 2000).

Temperature profiles down to 9 m at one mire in the

Lake Tornetrask area (Knutsson, 1980) suggest a max-

imum permafrost depth of at least 14–17 m.

Permafrost is discontinuously distributed in the

Stordalen mire (Sonesson, 1969; Ryden & Kostov,

1980). During the period 1973–1976 the maximum

annual depth of the active layer reached, at the end

of September, between 0.45 and 0.52 m (mean 0.48 m)

below hummocks and between 0.72 and 1.08 m (mean

0.87 m) in wet depressions (Ryden & Kostov, 1980).

After the IBP period the mire was documented in the

1990s to have significant changes in surface structure

(Malmer & Wallen, 1996; Svensson et al., 1999). During

the third week of September in 2003–2005, the max-

imum depth of active layer in hummocks was between

0.60 and 0.65 m (mean 0.63 m) and between 0.75 and

1.02 m (mean 0.86 m) in the wet depressions. Perhaps,

much more important is that permafrost is now miss-

ing over large areas (1.0 ha) where in the 1970s it

was still present, particularly in the southern part of

the mire.

Disintegration of the permafrost will affect the hy-

drology. Preliminary results from a water track analysis

made by comparing a digital elevation model of the

mire (T. Johansson, unpublished) with the two CIR

images used in the present study indicate changes in

the surface hydrology. The flow of water appears to

have increased from the small lake east of the mire

through the southern part of the mire, across the

extensive fen area to several small outlets to the stream

Fig. 4 Carbon dioxide (CO2) fluxes from the automatic cham-

ber system from three different vegetation classes (Hummock,

wet/semiwet, and Tall graminoid) as growing season averages

with standard deviation. Averages of data collected during

2002–2004. The Tall graminoid site class has data from 2003

and 2004 only. LST, Local Standard Time.



that forms the western limit of the mire. In contrast, the

diffuse flow through the fen area on the north side of

the mire was minimally affected.

Svensson et al. (1999) pointed at a possible connection

between permafrost disintegration and changes in

catchment hydrology caused by road construction work

in the 1980s. However, further analysis indicates that

the road construction to the south of the mire has not

affected the hydrology of the mire (N. Roulet, personal

communication) and, thus, most likely not either the

permafrost distribution. This hypothesis is strength-

ened by the fact that the other monitored mires in the

area, which are located on both sides of the road, also

show active layer deepening (Fig. 3, cf. Christensen

et al., 2004) and subsequent vegetation changes similar

to what has been documented in detail for Stordalen.

Site classes and vegetation types

The interpretation of the CIR images resulted in a

classification of the mire surface structure into five

different site classes (vegetation types) on peaty soil

namely: tall shrub, hummock, semiwet, wet, and tall

graminoid together with open water and rock surface

(Table 1, cf. also Malmer et al., 2005). The tall shrub site

class is characterized by dense stands of Betula nana and

differs from the hummock site class, which is character-

ized by dwarf shrubs and lichens or mosses forming a

dense bottom layer. From the semiwet to the tall gra-

minoid class the vegetation forms a continuous gradient

where the semiwet and wet sites are differentiated by

wetness. The semiwet and wet site classes both have

field layers consisting of sparse shoots of short grami-

noids and a dense moss carpet. In the tall graminoid

sites either Eriophorum angustifolium or Carex rostrata

form dense and tall stands with only sporadic patches

of floating mosses. There was a good agreement be-

tween the interpretation of the CIR images and the

species records for the hummock sites in the 2000 image

(Malmer et al., 2005).

To make use of the distinct site classes for the scaling

of the greenhouse gas exchange we are constrained by

the available gas flux measurements. Hence, for the CO2

fluxes the tall shrub site class has been included in the

hummock site class due to limited flux measurements

from tall shrub sites. The semiwet and wet site classes

are treated together because of plant species similarities.

With respect to the CH4 fluxes, we have combined the

tall shrub class together with the hummock site class

but we have data to differentiate between the wet and

semiwet site classes.

The two vegetation maps from the Stordalen mire

show decadal vegetation changes from 1970 to 2000

(Table 4) the details of which are presented in Malmer

Table 3 Estimates of the over-all net flux (CO2-C and CH4-C) for the growing season of 1970–2000 as area-weighted averages

(g m�2 day�1 and g m�2 gs�1) and total carbon accumulated by site class (kg gs�1)

Site class

Fluxes and estimates

g m�2 day�1 g m�2 gs�1 kg gs�1

Sources2000 1970 2000 1970 2000 1970

CO2-C

Hummock �0.04 �6.7 �559 �623 This article

Semiwet �0.24 �36.5 �508 �548 This article

Wet �0.24 �36.5 �1752 �1623 This article

Tall graminoid �0.64 �97.4 �1910 �1301 This article

Open water* 0.08 14.0 29 20 Jonsson & Karlsson (2003)

Whole mirew �0.18 �0.16 �28.1 �24.3 �4699 �4075

CH4-C

Hummock �0.0004 �0.06 �4.6 �5.1 Christensen et al. (2004) cf. Table 1

Semiwet 0.04 5.5 77 83 Above mentioned

Wet 0.09 13.6 652 604 Above mentioned

Tall graminoid 0.22 33.0 648 441 Above mentioned

Open waterz 0.004 0.6 1.3 1.1 (Zimov et al., 1997; Riera et al.,

1999; Huttunen et al., 2002)

Whole mirew 0.05 0.04 8.2 6.7 1373 1124

*The water fluxes of CO2-C and CH4-C used for scaling are not measured at the Stordalen mire.wThe whole mire values are area-weighted averages except for the total carbon accumulated.zThe CH4-C value used is a median value. gs, growing season 5 153 days.



et al. (2005). Although the general plant community

structure on Stordalen did not change from 1970 to

2000 there are two processes discernible in these

changes. First, the areas of the types of wet sites

dominated by graminoids expanded at the same time

as the hummock sites receded. Second, in the hummock

sites the area of lichens expanded concurrently with a

decrease in the area of evergreen dwarf shrubs and

mosses (Malmer et al., 2005). The area with hummocks

that have disappeared (approximately 1.0 ha or 10% of

the hummock area) have primarily changed into wet

depressions remaining ombrotrophic and covered by

mosses. The largest change has occurred in the southern

part of the mire where a large hummock area has

changed into an area with tall graminoids. The discre-

pancy in the change between hummock and wet site

classes (Table 4) depends on the fact that 0.1 ha classi-

fied as a terrestrial area in 1970 became open water in

2000. Compared with this increase in the area of wet site

classes the increase in the lichen vegetation on the

hummocks might have had very small if any effects

on the trace gas fluxes.

The image from 2000 was taken at the same time of

year (within 1 week) as that in 1970. We have good

reasons to assume the same phenological conditions

exist. Neither the accumulated precipitation nor the

mean temperature differed significantly between the

years in July (68 and 47 mm and 11.4 and 12.2 1C in

1970 and 2000, respectively) although the MAAT and

accumulated precipitation were �0.9 1C/242 mm and

0.2 1C/359 mm, (i.e. cooler and drier and warmer and

wetter than normal), respectively (Table 2). Contempor-

ary vegetation records confirm the interpretation of the

site classes in the images. For example, in the 2000

image the hummock area within a defined part of the

mire was estimated to be 58.4% and 58.0% by the

vegetation records and the image interpretation, respec-

tively (Malmer et al., 2005). According to the 1970

image, hummocks covered 65.0% in the subplots with

species records, (i.e. the same percentage of the area as

the field layer species of the hummocks).

C fluxes

All the daytime values of autochamber CO2 fluxes

reported here are consistent with other daytime mea-

surements published from Stordalen mire using manual

chambers (e.g. Oquist & Svensson, 2002). The hummock

sites had the lowest net accumulation of CO2-C over the

growing season on the mire (Table 3). Although the

measured mean NEE is low for the whole growing

season (0.04 g CO2-C m�2 day�1) it is as high as

0.27 g CO2-C m�2 day�1 during June–July. The low net

uptake is due to large rates of respiration, with maximal

night-time fluxes of 0.96 g CO2-C m�2 day�1 (Fig. 5a).

For atmospheric CH4 flux, the hummock is considered

to be C neutral or a small sink. An average sink function

of 4.1� 10�3 g CH4-C m�2 day�1 has been estimated for

the growing season.

In the vegetation on the Stordalen mire the semiwet

and wet site classes have a lower production of easily

degradable litter (Coulson & Butterfield, 1978; Johnson

& Damman, 1993) than the hummock sites. Poor litter

quality and wet conditions result in a slower turnover

and a lower maximum respiration rate (0.48 g CO2-

C m�2 day�1). During June–July the net uptake of

CO2-C by the hummock and wet/semiwet parts of the

mire are the same (0.27 vs. 0.27 g CO2-C m�2 day�1). The

wet and semiwet systems are sources of CH4 (Table 3).

The tall graminoid vegetation class had the largest total

C net influx rate per unit area on the mire (Table 3).

Probably because of comparatively easily degradable

litter it was associated with a high respiration,

0.96 g CO2-C m�2 day�1 and combined with a substan-

tial CH4 efflux (Table 3).

When the derived area proportions of the different

vegetation classes (Table 4) are used to scale the C-

fluxes to the whole mire, the average fluxes from the

measurements using the chamber system in 2002–2004

indicate a net C accumulation ranging from 10 to

51 g CO2-C m�2 during the growing season (Table 3

and Fig. 5b) giving an average of 28.1 g CO2-C m�2 for

the years around 2000. For the growing season in 2000

Table 4 The area (ha) of each site class distinguished in the interpretation of the CIR images with changes in area from 1970 to 2000

in ha and as percentages

Site class

Hummock

sites: I

Semiwet

sites: II

Wet

sites: III

Tall graminiod

sites: IV

Open water

sites: V

Rock

surfaces: VI

1970 9.2 1.5 4.5 1.3 0.17 0.05

2000 8.3 1.4 4.8 2.0 0.21 0.09

Change 0.9 �0.1 0.4 0.6 0.04 0.04

% �10.2 �7.4 7.9 46.5 22.8 74.5

CIR, colour Infrared.



the scaled mean efflux of CH4-C measured from static

chambers was 0.05 g CH4-C m�2 day�1 (range of 0.012–

0.082 g CH4-C m�2 day�1) or calculated for the whole

growing season 8.2 g CH4-C m�2 (Table 3 and Fig. 6b).

The daytime data are close to those from the summer

2001, (i.e. 0.055 � 0.003 g CH4–C m�2 day�1 as given in

Christensen et al., 2004).

The C flux uncertainty

Uncertainty of the calculated C balance is estimated

using the method of Waddington & Roulet (2000). The

total uncertainty is a function on the individual uncer-

tainties in flux measurements using the standard devia-

tion of each component. Weighting the variance of NEE

Fig. 5 Net carbon dioxide (CO2) flux from the mire in mg C m�2 day�1 for the years 1970 and 2000.

Fig. 6 Net CH4 flux from the mire in mg C m�2 day�1 for the years 1970 and 2000.



and CH4 determines the total uncertainty of the C

balance in relation to their contribution. The total C flux

was 36.3 g C for 2000 and the relative importance of

NEE and CH4 was 77.4% and 22.6%, respectively. The

coefficient of variation ðCV ¼ s=wÞ was calculated to

0.50 (NEE) and 0.39 (CH4). The potential error

ðx dC=dt½ � ¼ ðwNEEÞðCVNEEÞ þ ðwCH4ÞðCVCH4

Þ) of the total

flux was calculated to be 47.5% or 17.6 g C of the total C

sink. The relative importance of especially NEE is

accentuated as no measurements on lateral transport

of DOC and dissolved inorganic C (DIC) has been

conducted.

The total flux uncertainty is simultaneously depen-

dent on the accuracy of the classification of the different

site classes. As the accuracy in site class II (semiwet)

and III (wet) is low, the differentiation between the two

classes is dependent of wetness only. The accuracy of

site class IV (tall graminoid) is low. The main area of site

class IV is outside the area covered by species records

and there are a limited number of points. We have only

analysed the uncertainty in C balance depending on the

accuracy found for site class I (83%). The classification

uncertainty of site class I give a �1.4 ha uncertainty

(17% of 8.3 ha), which is added to, or subtracted from

site classes II, III, or IV weighted by their map contribu-

tion, which is 17.1%, 58.5%, and 24.4% respectively. The

area uncertainty for 2000 site class I is higher than the

estimated change between 1970 and 2000.

Calculating the potential error from these new spatial

data gives a range in total flux 31.2–41.5 g C with the

relative importance of NEE and CH4 of 76.9–78.2% and

23.1–21.8%, respectively with only minor changes in

coefficient of variation (0.49–0.51 and 0.38–0.40). The

potential error range is now estimated to 46.8–48.6% or

15.2–19.4 g C.

Radiative forcing of the short-term fluxes

A GHG budget based on the most commonly used 100-

year horizon where CH4 is a 23 times more powerful

GHG than CO2 was calculated for the 1970 and 2000

situation, and underlines the importance of CH4 emis-

sions on wetland GHG exchange. Despite being an

overall sink of C (Fig. 7a and b) the mire acts as a

source of GHGs, when using the methodology of CO2

equivalent, and has a net positive forcing even during

the growing season when the CO2 uptake is highest.

The mire was a GHG source in terms of CO2 equivalents

to the atmosphere equal to 132.2 g CO2 m�2 (2000) dur-

ing the analysis period. The tall graminoid class has

more than twice the efflux of GHGs as CO2 equivalents

than the second largest emitter (wet, Fig. 8a and b). The

flux increased between 1970 and 2000 by 47.3% (132.2

vs. 89.8 g m�2) rather than decreased by 7.5% due to

CO2 uptake. In Fig. 9 the change is illustrated as the flux

difference between 1970 and 2000. It is evident from

Fig. 9 that the largest GHG difference has occurred in

the southern parts of the mire most influenced by

changing surface hydrology due to loss of elevated

palsa-like area. The present mire function is, and has

been since well before 1970, a net positive forcing onto

the atmosphere independent of time horizon 20 or 100

Fig. 7 Net carbon (C) flux from the mire in mg C m�2 day�1 for 1970 and 2000 (i.e. five corrected against six).



years. Fitting an exponential curve to the average area-

weighted flux calculated as CO2 equivalents at different

time horizons (20, 100, and 500 years) shows the mire

shifts from source to sink at a 163 years horizon in 1970.

The increase in CH4 emission between 1970 and 2000

has pushed the GHG curve almost 30 years later. The

compensation point is now at 191 years (Fig. 10,

cf. Friborg et al., 2003).

Discussion

Climate and permafrost degradation

During the last �20 years the mean annual air tem-

perature at Abisko has often been above 0 1C compared

with the value of �0.7 1C for the long-term mean (1913–

2003). The Abisko area has, during the last 20 years also

experienced more winter precipitation and deeper

snow cover (Kohler et al., 2006). The increase in snow

cover depth is statistically significant when a 15-year

window is applied before the studied periods (Malmer

et al., 2005). Modelling results have shown that changes

in permafrost temperatures can be influenced as much

by temporal variations of snow cover as by changes in

near-surface air temperatures (Stieglitz et al., 2003). If

the enhanced winter precipitation in the Abisko area

has resulted in a deeper snow cover on Stordalen mire

it may contribute to the permafrost degradation. In

particular in expanding depressions where snow can

accumulate, as a deeper snow cover function as an

insulation, which keep the ground warmer as it inhibit

downward propagation of the low temperatures in the

peat. These climatic changes during the last 20 years are

the most plausible reason for the observed degradation

of permafrost on Stordalen mire and elsewhere in the

area.

It has been proposed that there are differences in the

hydrological response to permafrost thawing depen-

dent on ecosystem and its position in the landscape.

In peatlands and low-lying tundra, permafrost thaw

shifts to wetter conditions in contrast to better-drained

boreal and tundra uplands where thaw creates warmer

and drier soil conditions (Camill, 2005). The first part of

the argument is supported both by our results and

accumulating evidences from other sub-Arctic mires

and boreal peatland sites with thawing permafrost in

Europe and North America (Vitt et al., 1994; Camill,

1999; Zuidhoff & Kolstrup, 2000; Jorgenson et al., 2001;

Luoto et al., 2004). Here, the permafrost degradation has

caused the physical foundation of the peatland ecosys-

tem to collapse, in part due to thermokarst erosion of ice

rich soils. A wetter state after permafrost thawing has

also been observed in boreal forests with ice rich soil

and low gradient (Osterkamp et al., 2000). This is in

contrast to changes proposed for tussock and wet sedge

tundra in Arctic Alaska, and upland boreal forests. At

these sites, the soil structure is maintained both with a

deepening of the active layer or a total disappearance of

the permafrost. A degree or two of warming causes the

deepening of the active layer forcing the water table to

Fig. 8 Carbon dioxide (CO2) equivalents in mg m�2 day�1 (i.e. Fig. 7 recalculated using Global Warming Potentials (GWP) at time

horizon 100 years).



follow this downward movement of the permafrost

table leading to a drying of the surface soil as proposed

by Billings et al. (1982), Oechel et al. (1993, 1995), and

Goulden et al. (1998).

The working hypothesis of Billings et al. (1982) and

Oechel et al. (1993, 1995) was that a warmer climate had

increased the active layer thaw depth with a subsequent

lower water table and drier soil. This was put forward

as one explanation for the ecosystem shift from a C sink

to C source at this site based on differences found in

volumetric soil water content in 1971 and 1991 (Oechel

et al., 1995). However, active layer statistics from two

sites at Barrow, Alaska between 1960 and 2000 (inter-

rupted between 1969 and 1991) show the opposite

trend. The mean active layer was deeper in the 1960–

1970 than in early 1990s (Brown et al., 2000). The same

input of energy yielded only about 70% of the thaw

depth achieved in the 1960s. The authors suggest this to

resemble a Markovian process (Brown et al., 2000, i.e. a

stochastic process, which implies that the process is

–4.4– –3.5

GWP100 differences 1970–2000

g CO2 eq m–2 day–1

–3.5– –2.6

–2.6– –1.8

–1.8– –0.9

–0.9–0.0

0.0–0.9

400 m

N

2001000

0.9–1.8

1.8–2.6

2.6–3.5

3.5–4.4

Fig. 9 Visualization of flux changes as the difference in carbon dioxide (CO2) equivalents between 1970 and 2000.

Fig. 10 The greenhouse gas fluxes from the Stordalen mire

calculated as carbon equivalents using Global Warming Potential

(IPCC 2001) radiative forcing values. The three values in differ-

ent time horizons (20, 100, and 500) together with an exponential

fit show that the mire acts as a source until calculating the

radiative forcing at a 191 (163) years horizon.



conditionally independent of the past states given the

present state). This shows how complex the thawing of

permafrost can be; and that it is pivotal to understand

how the hydrology of the area will be affected in order

to make predictions on how northern peatlands and

especially tundra will react to transient warming.

The hydrological regimes are different in peatlands

and tundra due to regional climate and surface mor-

phology. Wetlands occur where drainage is poor owing

to low relief or an impermeable substrate (permafrost,

peat, or sediment; Rouse et al., 1997). Any broad gen-

eralizations of the effects of degrading permafrost in the

vast tundra biome are problematic as local and regional

driving factors (e.g. climate, ice content and, perme-

ability of the underlying material) interact. Neverthe-

less, the ultimate effect of continued warming on high

latitude wetland ecosystems controlled by permafrost

may be their widespread disappearance (Smith et al.,

2005).

Vegetation changes and C fluxes

Vegetation changes in a peatland affect the NEE and

long term C balance in at least two ways: (1) by

changing the net primary production [NPP, i.e. the

summed photosynthetic assimilation (GPP) and auto-

trophic respiration] and (2) by changing the decay

resistance of the annually deposited litter (Malmer

et al., 2005). A change in the NPP will immediately

affect the influx of CO2-C. Its effects on the total efflux

of C, however, is limited to the autotrophic respiration

and will, therefore, be less because the efflux depends

also on the heterotrophic respiration of the litter that

mainly is taking place in the oxic acrotelm. The resi-

dence time for the decaying organic matter may be

about 100 years (Malmer & Wallen, 2004). The decay

resistance of the litter is a main determinant of the

heterotrophic respiration and decay rate (Johnson &

Damman, 1993; Malmer & Wallen, 2004). A change in

the litter quality will, therefore, affect the efflux of C.

However, the immediate effects are in this case small,

while on a scale of several decades or centuries the

effects are very large (Clymo, 1984). In the following

discussion about the vegetation changes from 1970 to

2000 on the Stordalen mire, we have made the assump-

tion that the similar plant community structure of the

specific site classes will have similar rates of C seques-

tration at both occasions. This means that we estimate

the effects of changes from one type of plant commu-

nity or site class to another using the derived area

proportions of the flux sites associated with the differ-

ent vegetation classes presented in Table 4.

Calculating the C fluxes as area weighted averages

over the growing season for the whole mire in 1970

show that the mire gained 24.3 g CO2-C m�2 and lost a

corresponding 6.7 g CH4-C m�2 (Table 4, Figs 5a and b

and 6a and b) over the growing season. The mire on an

average gained 28.1 g CO2-C m�2 and lost 8.2 g CH4-

C m�2 (Table 3) during the growing seasons of 2000–

2003. Calculating CO2 and CH4 exchange separately, the

decadal vegetation change might have increased the net

CO2-C influx to the mire during the growing season by

3.8 g m�2 (15.5%) and the net CH4-C efflux by 1.5 g m�2

(22.2%). The seasonal net C input to the mire (Fig. 7)

increased from 17.6 g C m�2 in 1970 to 19.9 g C m�2 in

2000 [i.e. by 2.3 g m�2 (13%)]. The mire has thus acted as

a net C sink in both growing seasons of 1970 and 2000.

In a calculation based on data including both moss and

above- and belowground vascular plant production

(fully presented in Malmer & Wallen (1996), Olsrud

(2004), and M. Olsrud unpublished) Malmer et al. (2005)

estimated the overall C sequestering in the NPP to 55

and 59 g CO2-C m�2 yr�1 in 1970 and 2000, respectively,

viz. an increase of about 4 g m�2 yr�1 or 7.5%. This

value is in agreement with the results obtained from

the gas flux measurements. Comparing the value of

28.1 g m�2 CO2-C for NEE, derived from the flux data-

sets, with the value for NPP from the year 2000 suggests

that the release of CO2-C through the heterotrophic

respiration during the growing season on an average

ought to be about 31 g C m�2 or about 52% of the NPP.

Although this calculation involves several uncertainties

it is in good accordance with what could be expected

from earlier litter decay studies on the Stordalen mire

(cf. Malmer & Wallen, 1996, 2004).

Most of the release of C through heterotrophic

respiration is taking place near the surface in the oxic

acrotelm (depth about 0.3 m) before the organic matter

becomes included in the catotelm (Malmer & Wallen,

2004), the anoxic, lower part of the peat column that

only contributes a small fraction of the soil respiration

(Clymo, 1984; Trumbore, 2000). Combining the over-all

efflux of CH4-C with the estimated heterotrophic re-

lease of CO2-C gives a value of 39 g C m�2 for the total

heterotrophic C loss during the year 2000 growing

season only. It is a little higher than the over-all decay

losses in the acrotelm and catotelm, 32 g C m�2 yr�1,

obtained as a mean over several decades of the recent

past in a study of the chemical stratigraphy of the litter

and peat on the mire (Malmer & Wallen, 2004) and can

indicate an increased heterotrophic decay loss of C

during the last century, because of a decreasing forma-

tion of recalcitrant moss litter. The changes in the

vegetation from 1970 to around 2000 observed on the

mire also form parts of a secular development that

started well before 1970 (Malmer & Wallen, 1996, 2004).

Although we do not have measurements of the off-

season fluxes (i.e. the winter, spring thaw and autumn



refreeze periods) of CO2 and CH4 it is nevertheless

clearly possible that during the period 1970–2000 the

increased temperature during winter and spring and a

deeper snow-cover (Malmer et al., 2005) have affected

the annual GHG balance. Thus, several studies have

demonstrated that it is likely that a significant part of

the annual heterotrophic respiration, both aerobic and

anaerobic, is taking place during the off-season, but not

accounted for in this study (Moore et al., 1990; Dise,

1992; Windsor et al., 1992; Khalil et al., 1993; Friborg

et al., 1997). Moreover, some studies estimate the winter

fluxes of CO2 and CH4 to be as high as 23% and 22%,

respectively, of the annual balance (e.g. Alm et al., 1999).

Therefore, the off-season GHG fluxes might have in-

creased on the Stordalen mire, too. This would then

give a possible even stronger positive radiative forcing

effect onto the atmosphere.

The development of the CO2 equivalent concept

where primarily meant as a tool to deal with anthro-

pogenic emissions and the interpretation in relation to

ecosystems is highly dependent on the time horizon.

(Frolking et al., 2006). At the same time the difference in

GHG forcing due to the time horizon used indicate the

importance of the history of any peatland in relation to

its impact on climate. If a time perspective of a thou-

sand years is adopted, the integrated climate impact of

GHG exchanges of the mire over the entire period is

strongly affected by the accumulated effect of the peat

storage and, CO2 sink function. Here, we have only

documented the GHG exchanges in the relative short

time horizon of 200 years using commonly applied

GWP100 calculations (Fig. 10).

Conclusions

Our findings document responses in ecosystem func-

tioning to permafrost thawing in sub-Arctic areas. They

suggest that ecosystems such as sub-Arctic peatlands

are vulnerable and respond strongly to changes in

temperature. In particular, this study confirms the link

between recent permafrost degradation and subsequent

vegetation and C flux changes. It is likely that our

findings are generally applicable to the large extent of

peatlands with sporadic or discontinuous permafrost in

the circumpolar region of the high northern latitudes.

Northern peatlands are suggested to become larger

CO2 sinks. At the same time, larger CH4 sources follow-

ing transient warming depend upon how surface hy-

drology will change with further loss of permafrost and

subsequent shifts in the vegetation. Northern peatlands,

in general, provide positive feedbacks on the climate

when applying a GWP with short time horizon due to

their increase in the CH4 emissions. This increased

forcing towards a warmer climate is very likely to

continue in the near future due to further warming.

However, even if sub-Arctic mires are assumed to have

a net warming effect, the forcing from the whole sub-

Arctic due to warming is difficult to project because the

heterogeneous landscape is an amalgamation of differ-

ent ecosystem with, presumably unequal responses to

warming and moisture changes.

Acknowledgements

The European Commission under the 4th framework CONGASproject and 5th framework CARBOMONT project has supportedthe work together by grants from Abisko Scientific ResearchStation to T. J., The Swedish Research Council (VR) to P. M. C.,and T. R. C. and the Danish Natural Science Research Council toT. F. We want to thank the personnel at Abisko ScientificResearch Station (ANS) and in particular the director Prof. TerryV. Callaghan. Kristina Backstrand, Maria Olsrud, MargaretaJohansson Bo Svensson, Mats Oquist, and Lena Strom are allacknowledged for help and contributions to the joint Stordalendatabase this study is building on. We are furthermore thankfulto Jonas Ardo and one anonymous reviewer who made valuablecomments on an earlier version of the manuscript.

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