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Carbon Reservoirs in Peatlands and Forests in the Boreal

Regions of Finland

Kauppi, P.E.

Metla

1997

Kauppi, P.E. et al. 1997. Carbon Reservoirs in Peatlands and Forests in the Boreal Regions

of Finland. Silva Fennica 31(1): 13-25.

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Silvo Fennico 3l(l) reseorch ortirles

I

Carbon Reservoirs in Peatlands andForests in the Boreal Regions of Finland

Pekku E. Kouppi, Moximilion Posch, Pekko Hiinninen, Heleno M. Henlfonen,Anfii lholoinen, Eino loppoloinen, Michoel Ston ond Pekko Tomminen

Kouppi, P.E., Pogdr, il., Hiinninen, P., Henltone4 H.M., lholoine+ A., loppoloinen, [., Slorr, t. &

Tonninen, P.1997. Carbon reservoirs in peatlands and forests in the boreal regions of

Finland. Silva Fennica 3l(l): 13-25.

The carbon reservoir of ecosystems was estimated based on field measurements for

forests and peatlands on an area in Finland covering 263 000 km2 and extending about

900 km across the boreal zone from south to north. More than two thirds of the reservoir

was in peat, and less than ten per cent in trees. Forest ecosystems growing on mineralsoils covering 144 000 km2 contained 10-11 kgCm-2 on an average, including both

vegetation (3.4 kg " rn-z; and soil (uppermostT5 cm;1.2kgC m-2). Mire ecosystemscovering 65 000 km2 contained an average of72kg C m-2 as peat.

For the landscape consisting of peatlands, closed and open forests, and inland water,

excluding arable and builrup land, a reservoir of 24.6 kg C m-2 was observed. Thisincludes the peat, forest soil and tree biomass. This is an underestimate of the true total

reservoir, because there are additional unknown reservoirs in deep soil, lake sediments,

woody debris, and ground vegetation. Geographic distributions of the reservoirs were

described, analysed and discussed. The highest reservoir, 35-40 kg C m-2, was observedin sub regions in central westem and north westem Finland.

Many estimates given for the boreal carbon reservoirs have been higher than those ofours. Either the Finnish environment contains less carbon per unit area than the rest ofthe boreal zone, or the global boreal reservoir has earlier been overestimated. In order to

reduce uncertainties of the global estimates, statistically representative measurementsare needed especially on Russian and Canadian peatlands.

Keywords carbon reservoirs, carbon pools, global carbon cycle, peat reserves, borealforests, biomass carbon, global warming, ecological temperature gradient

Auftors'oddresses Kauppi, Henttonen and lhalainen, Finnish Forest Research tnstitute(FFRI), Unioninkatu 40 A, 00170 Helsinki, Finland; Poscft, National Institute of Public

Health and Environmental Protection (RIVM), P.O. Box l, 3720 BA Bilthoven, the

Netherlands; Hiinninen, Geological Survey of Finland (GSF), P.O. Box 96, FIN-02151

Espoo, Finland; Inppalainen, GSF, Regional Office for Mid-Finland, P.O. Box 1237,FIN-70211 Kuopio, Finland; Starr and Tamminen, FFRI, P.O. Box 18, FIN-01301Vantaa, Finland Fox to Kauppi +358 9 625 308 t-moil pekka.kauppi@metla.fi

Arepted 15 January 1997

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Silvo Fennico 3l (l I reseorch oilides

L Introduction

The temperature increase in response to increas-ing amounts of greenhouse gases in the atmo-sphere is projected to be greatest at high lati-tudes (Mitchell et al. 1995). The carbon reser-voir in the forest soil in the boreal zone has beenestimated at 2148 kg m-', which is 24 timesgreater than the corresponding estimate for midor low latitudes (Dixon et al. 1994). There is anexceptionally large reservoir of carbon in thepart of the globe where a large warming hasbeen projected. In a recent overview, Apps andPrice (1996) have discussed the role of forests,particularly the boreal forests, in the globalcarbon cycle. The global distribution of the bo-real biome has been described in many reports,e.g. Mellillo et al. (1993). They estimated thearea of boreal forests and boreal woodlands at12.2. and 6.3 million km2, respectively. For thesub regions and timber resources, see Kuusela(1ee0).

Gorham (1991) has estimated the total reser-voir ofboreal and subarctic peatlands at 455 Pg(Petagram - 1015 g = gigaton = billion metrictons). This refers to an area of 3.46 million km2,using a mean peat thickness of 2.3 m as a basisof calculation. Post et al. (1982) have estimatedthe carbon reseryoir in the soils of "Boreal for-est-wet" and "Boreal forest-moist" at 133.2 and48.7 Pg, referring to areas of 6.9 and 4.2 millionkm2, respectively. In addition, they have esti-mated202.4 Pg on an arcaof 2.8 million km2 ofglobal wetlands. Apps et al. (1993) have esti-mated the boreal C reservoir to be as high as 709Pg, subdivided in peat (419), forest soil (199),plant detritus (32), and plant biomass (64). Theyrefer to a total area of 12.5 million km2 and apeatland areaof 2.6 million km2. The discrepan-cy between the estimates is partly a result ofdifferent definitions (forest vs. forested peatlandvs. peatland; temperate zone vs. boreal zone vs.subarctic zone). Assuming a target area of 15million km2, including peatlands and forests, acarbon reservoir of 400-700 Pg has been givenfor the boreal environment in the most recentestimates. Most of the reservoir is known toexist as peat.

Raich and Schlesinger (1992), Liideke et al.(1995), and Kirschbaum (1995) have suggested

t4

that, if the climate turns warmer the carbon res-ervoir of boreal forests and peatlands would di-minish, i.e. there would be a positive feedbackfrom the boreal enyironment to an eventual cli-matic warming. However, Townsend et al . (1992)and van Minnen et al. (1995) have suggested justthe opposite. In their view, the carbon reseryoirswould grow in the boreal zone in response to awarming, thus providing a negative feedback.Whether the feedback would be positive or neg-ative is an unresolved question at present.

The boreal landscape consists of a mosaic ofclosed forests, open woodlands, peatlands andlakes. Peatlands occur mostly in landscape de-pressions where edaphic, hydrologic and climat-ic conditions maintain a high water table andallow organic matter to accumulate as peat at arate faster than the rate of oxidation.

This study describes the carbon reservoirs ofrural, non-cultivated, terrestrial ecosystems inthe boreal zone based on a large number of meas-urements. Our data are from Finland where peat-lands, with peat layer ) 30 cm, cover 24.7 Vo ofthe land of the study area. The objective is toestimate the carbon reseryoirs of trees, of thesoil ofclosed forests growing on inorganic soils(here referred to as "forest soil"), and of peat-lands; to analyse the spatial variation of thesereservoirs along the temperature gradient fromnorth to south; and to use this information forassessing the total carbon reservoir of the borealzone and the eventual feedback mechanisms to agreenhouse warming.

Our data represent the landscape (regional)scale. It is possible by using such data to over-come most of the inaccuracies and biases result-ing from extrapolation of measurements taken ata small number of ecosystems. Botkin and Simp-son (1990) have demonstated the importance oflandscape level sampling, based on data meas-ured for the vegetation of the boreal forests ofNorth America; see also Brown et al. (1989).

2 Methods

The northernmost and the south western regionsof Finland fall outside the boreal zone and arenot part of the study area. Within the boreal zone

Kouppi et ol. (orbon Reservoirs in Foresls ond Peollonds in fie Boreol Regions of Finlond

in Finland, an area of 40 000 km2 of builrup andarable land was also excluded. The focus was onthe natural and semi-natural landscape mosaic,an area of 263 000 km2, of which inland waterscovered 31 000 km2. Peatlands cover 65 000km2 of the study area, and forests growing onmineral soils an area of 144 000 km2.

Trees. Stem volume was calculated based onmeasurements of the eighth national inventoryof Finnish forests taken in 1986-1994 from about490 000living trees in 69 000 sample plots. Sys-tematic sampling ensures that the measurementsrepresent all living trees taller than 1.35 m(Salminen 1993). About 80Vo oflhenmbergrow-ing stock is on mineral soils, the rest on peat-lands. The forest inventory has been maintainedsince the 1920s, with the main objective of mon-itoring timber resources. The stem volume isdefined over bark, including the entire stem fromthe stump level to the top.

Stem biomass was calculated from volume,assuming a dry weight density of 420,380, and480 kg mr for Scots pine, Norway spruce andthe deciduous species, respectively (Hakkila1989). Branch, root and foliage biomass wasestimated using conversion coefficients relatingother woody biomass to stem biomass (Kauppiet al. I 995). According to these coefficients, stemaccounts for 5I1I Vo of the total tree biomassdepending on species and age, variables record-ed in the inventory. A carbon concentration of5O Vo was used for all woody biomass (Nurmi1993).

Forest soil. Soil samples were taken in 1986-1989 from 377 stands selected as a sub sampleof the basic network of 3000 permanent sampleplots of the national forest inventory located inclusters at intervals of 16 km x 16 km in south-ern and 24kmx 32 km in northern Finland. Thesamples were taken from the humus layer, ex-cluding the litter layer, and from the mineral soilat four depths: 0-5,5-20,2040 and 60-70 cm.For the humus sample, 10 to 30 sub samples,depending on humus thickness, were taken witha cylinder and combined into a single sample forthe plot. The mineral soil samples for each layerconsisted of a composite of five sub samples,except the 60-70 cm layer, which consisted of asingle sample only. Bulk density and C concen-tration were determined separately for the hu-

mus layer and the mineral soil (Tamminen andStan 1990). The C reservoir of each stand wasdetermined for the uppermost 75 cm, correctedfor bulk density and stone volume (Yto 1952,Tamminen 1991).

Peat.The volume of peat was calculated basedon ca. 900 000 measurements of peat thicknesstaken in the field in 1973-1991in a national peatsuryey, carried out mainly to estimate peat ener-gy reserves (Lappalainen and Hiinninen 1993).Bulk density and C concentration were analysedfrom about 11 000 laboratory samples taken at10-20 cm intervals from peat core profiles. Forshallow peatlands where the organic layer is lessthan 30 cm deep, a depth of 2O cm, a (dry) bulkdensity of 80 kg m-3, and C concentration of 50Vo in dry matter were used (= 8 kg C m-2).

Gradients.In order to analyse the geographicvariation of the reservoirs, the area was dividedinto 74 sub regions each covering 1070-12 300km2. The reservoirs in trees, forest soil and peatwere calculated for each sub region. Observa-tions were plentiful for trees and peatlands butnot for forest soils. Only 1-17 observations wereavailable on forest soils in each sub region, andthe arithmetic mean was used.

The spatial variation of the carbon reservoirswas analysed as a function of the mean annualtemperature, as observed in 196l-1990. Themean daily temperature observations from 136Finnish stations and}O adjacent stations in Swe-den and Norway were used. A kriging methodwas applied to calculate the mean monthly tem-perature at a spatial resolution of 10 km x 10 km,taking into account the effects of altitude, slope,distance from the Baltic, and occurrence oflakeswhen filling in gaps between observational sta-tions (Henttonen 1991). From these data the meanannual temperature was calculated for each subregion.

3 Results

3.1 Carbon Reservoirs

Trees.The C reservoir of living trees in the studyarea was 618 Tg (Teragram = 10t, g). Stemwoodaccounted for 314 Tg. Dividing the reservoir by

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Silvo Fennko 3l (l ) reseqrch odides

the area of the landscape, 263 000 km2, yields anaverage of 2.7 kg C m-2. This refers to the totalarea including inland water. As 80 7o of thegrowing stock is on mineral forest soils coveringI44 000 km2, an average of 3.4 kg C m-2can beestimated for such forested land.

In the boreal forests of Russia the vegetationreservoir has been estimated at 19.6 Pg includ-ing stems, roots and crowns based on forest in-ventory covering 5.22 million km2 of stockedforests (Alexeyev et al. 1995). This would meanan average of 3.75 kg C mr. Assuming 7.6 nrrl-lion km2 as the boreal land area in Russia, in-cluding unstocked land (Apps et al. 1993), thereservoir would correspond to 2.6kg C m-2. Thisestimate includes both above- and below-groundbiomass.

In the North American boreal zone, an above-ground reservoir of 1.9 + 0.4 kg C m-2 has beenreported for trees and shrubs (Botkin and Simp-son 1990). Our Finnish data indicate a reservoirof 2.1 kg C m-2 in tree biomass, above-ground.

Forest soil. The C reservoir of forest soil wasestimated at 1040 Tg in the uppermost 75 cmlayer, corresponding to 3.9 kg C m-2 for the land-scape (the area of 263 000 km2 including landand inland water). Mineral forest soilscover 144 000 km2. An average forest ecosys-tem hence contained 7.2 kg C m-2 in the upper75 cm of the soil. This is within the range report-ed earlier for Finnish soils by Liski and West-man (1995).

Peat.T\e total C reservoir in peat was estimat-ed at 4800 Tg, of which only 140 Tg were in shal-low peatlands. Peat contributed 18.3 kg C m-2 tothe landscape carbon, referring to the land andwater area of 263 000 km2. An average mire eco-system containedT2kg C m-2 in peat, referring tothe area of 65 000 kmz where the layer of peat isat least 30 cm thick. The area-weighted averagedepth ofthe organic layer in Finnish peatlands is1.3 m, estimated earlier from these data (Lappa-lainen and Hiinninen 1993).

The sum of the reservoirs measured (trees +soil + peat) was 618 + 1040 + 4800 = 6458 Tg;or2.7 + 3.9 + 18.3 =24.6kgCma. For the landbase of 232 000 km2, excluding inland water,the average carbon reservoir was 27.8 kg C m,(Table 1).

Toble l. Area, carbon density, carbon reservoir, andthe contribution of landscape elements to the totalcarbon density of the study area.

Landscapeelement

Area Average ReservoirContributioncarbon to lmdscapedensity crbon

l000km2 kgm-2 TgC kg6-z

Closed forestson mineral soil

Peatlands(depth > 30 cm)

Shallow peatlands

Open wooded land

Natural and semi-natural land

Inland water

Natural andsemi-naturalenvironment

Arable & built-upland

Total area

r0.71) 15361) 5.82)

731) 47211) 77.92)

11.11) 2OI1) 0 .82)

n.a, n.a. n.a.

27.8 6458

n.a. n.a. n.a.

24.6r) 64581)

t44

65

1 8

5

232

3 l

263

40

303

r)2)

n.a

Includes both trees and soilReservoir divided by total dea, i.e. by 263 000 km2= not available

3.2 Geographic Gradients

The largest reservoir in the landscape, 3540kg C m-2, was found in central western and innorth western parts of Finland and the smallestreservoir, 20-25 kg C m-2, in the lake region ofsouth eastern Finland (Fig. 1a). In some subregions, more than 9O Vo of the reservoir was inpeat. The relative contribution oftrees and forestsoil increased southwards (Figs. lb,c). However,peat made the largest relative contribution alsoin all southern sub regions except one (Fig. 1d).

The peat storage was relatively small near theGulf of Bothnia (in the west) where new landkeeps emerging from the Baltic because of landuplift (Figs. la,d). A land belt up to 50 km fromthe coast is only 500-1500 years old (Eronen atal. 1995). The peatlands were most common incentral and northern Finland, but deepest in south-ern Finland (Fie.2).

16

Kouppi el ol. (ofion Resenoin in Forssils ond Peotlonds in the Boreol Rqiom of tinlond

0

l(gc/in2a <24| 2+8f 26-24r 2&{)I 30'32tr 32.34I >.34

b

Flg. l. Carbon reservoir ofthe landscape, in colours, and mean annual temperature, inisopleths (a); and the contribution to the reservoir by tees (b), forest soil (c), andpear (d).

% cf totalI < 1 0I 1G20I a)-30E 30-.Otr >rO

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Silvo Fennko 3l (l ) reseorrh oilicles

0

"+% of totalI <20r 20.30n 3040tr 40-50e 50.60tr 60-70I >70

3.3 Co-variation with Temperature

The mean annual temperature ranged from -2.0"C in the north to +4.8 oC in the south (Fig. la).The patterns of variation of the reservoirs acrossthe temperature gradient were not alike. The res-ervoir in ffees increased steeply and consistentlywith increasing temperature (Fig. 3a), the one inmineral soils increased less consistently (Fig.3b), and the peat reservoir decreased steeply overthe southern part of the area, over the tempera-ture range of +1 to +4.5 degrees Celsius (Fig.3c). The peat reservoir was largest in central andnorthern regions that is, in the range of the meanannual temperature of -2 "C to +2 oC, despitethe shallow peat depth characteristic to thoseregions (Fig. a). In other words, the gradient inpeatland frequency ovemrled the impact of peatdepth.

l 8

tig. 2. The frequency ofpeatlands where the peat depth > 30 cm (a); and the area weighted

thickness ofthe peat layer in peatlands (b).

4 Discussion

4.1. Measurement Uncertainty

The accuracy of the estimate for stemwood vol-ume is about t 2 Vo (95 Eo confrdence) for anarea as large as a characteristic sub region (Sal-minen 1993). Additional uncertainty is introducedwhen converting stem volume to carbon in whole-tree biomass. However, the estimate for the car-bon reservoir in trees is sufficiently accurate andprecise, within + 10 per cent even for the small-est sub regions. Also the estimate of the peatreservoir is unbiased and relatively accurate (ca.! lO Vo). A higher uncertainty is associated withforest soils, since the observations are fewer.

Ground vegetation and shrubs, which were ex-cluded, are only 2-5 7o of the tree biomass andunimportant quantitatively as a carbon reservoir

^ ^ , 1" T

cmI <100I 100-190tr 130-170E] 170-200& 200-230E 230-260I >260

Kcuppi el ol. (orbon Reservoirs in Foresls ond Peoflonds in fte Boreol Regions of Finlond

0 1 2 3MEAN TEMPEBATURE, C

0 1 2 3MEAN TEMPERATURE, C

(Miikipaa 1995). Coarse woody debris, whichwas also excluded, has been reported in otherworld regions to contain a reservoir which equals3040 Vo of the biomass in living vegetation(Apps et al. 1993, Alexeyev et al. 1993, Turneret al. 1995). This would infer an additional reser-voir of 200-250 Tg, or 0.7-1.0 kg C m-2, for ourstudy area. However, this is an upper estimate,and an overestimate since most forest stands inFinland have been treated with silvicultural thin-

- 2 0 1 2 3

MEAN TEMPERATURE, C

Fig. 3. Carbon density vs. mean annual temperature inwoody biomass (a), in forest soil (b) and in peat-lands (c). The carbon densities are calculated bydividing the reservoirs by the total area in eachsub region. The total area includes the area ofinland waters.

nings, which has a decreasing impact on theamount of coarse woody debris (Krankina andHarmon 1995). An additional reservoir in Fin-land is the one in stumps and in coarse roots offelled trees. However, the largest unknown andomitted reservoirs are expected to be found indeep layers of the soil (Liski and Westman 1995)and in lake sediments.

In conclusion, the observed reservoir of 24.6kg C p-z is an underestimate of the true total

- 1- 2

(\l

5 ,ol

- 1- 2

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Silvo Fennicq 3l(l) resesrd qnides

reservoir in the Finnish environment, becauseadditional reservoirs exist but were excluded asthe data were lacking. By including ground veg-etation, shrubs, stumps, and coarse woody de-bris, an additional contribution of about 1.0kg C m-2 might be recorded in Finnish condi-tions. This equals one third of the reservoir meas-ured in living trees. A larger reservoir is likely toexist in deep soils. Liski and Westman (1995)measured 1.3-2.4 kg C m-2 between the depth ofI m and the ground water layer in eight foreststands. An even larger reservoir can exist in lakesediments. The environment in the sediment issimilar to that in peatlands, accumulating a mat-tress of organic matter over a long period oftime.

4.2 Impact of Land Use

The study area in Finland can be classified assemi-natural landscape. Arable and urban land,which was excluded, covers 13 Vo and is mainlylocated in southern sub regions on fertile, miner-al soils. The remainingST Vo, which was includ-ed in this study, probably contains a little morecarbon per unit area than the entire landscapewould contain, if undisturbed by land clearance.

Of the included land, only insignificant frag-ments have earlier been cleared for agricultureand then abandoned, although large areas wereused for shifting cultivation in the 17th to thel9th century.

Tree species are indigenous. Logging has beenpractised intensively for more than 100 years es-pecially in southern sub regions, however, in away that the growing stock has not been depleted(Karjalainen and Kellomiiki 1996, Pingoud et al.1996). More than 90 Vo of the area has beenlogged, most often treated with partial cuttings butalso with regeneration cuttings such as clear fell-ing. Wild fires, which occur frequently in otherboreal regions (Stocks atal.1996), have almostentirely been suppressed in Finland in the 20thcentury. An area of 58 000 km2 has been drainedin Finland for forestry purposes mainly since the1950s (Tomppo and Henttonen 1996). Accordingto Laine et al. (1995), drainage has not yet affect-ed the carbon reservoir of the peatlands.

20

- 2 - 1 0 1 2 3 4 5

MEAN TEMPERATUBE, C

Fig. 4. The depth [', A] and the proportion [o, B] ofpeatlands vs. mean annual temperature. Only peat-

lands with the organic layer deeper than 30 cm are

included. Each depth observation represents theaverage depth of peatlands measured in one mu-

nicipality, covering 50-7 500 km2. The munici-pality areas are much smaller in southern than in

northern Finland. The proportion of peatlands in

each sub region is calculated as the peatland area

divided by the total area (incl. the area of inland

water).

4.3 Spatial Patterns of Variation

Trees. Acomparison with studies such as Botkinand Simpson (1990), Alexeyev et al. (1993), andwith Swedish forestry statistics (Statistical Year-book of Forestry 1996) indicates that the aver-age tree biomass in Finland can be slightly lowerthan in the other boreal areas, but not by morethan 10-20 per cent. It is possible that the differ-ence is partly explained by measurement error ordifferences in concepts, except when comparingwith Sweden where the methods of measure-ment are very similar.

The variation of tree biomass within Finland,from I to 4 kg C m-2 between north and south(Fig. 3a), is much greater than the differences inthe average reservoir between the boreal worldregions. A similar north-south gradient exists inSweden (Statistical Yearbook of Forestry 1996).The larse variation of vesetation in north-south

E &2oE s ooEEn m

Kouppi el ol. (orbon Reservoirs in Foresh ond Peollunds in fie Boreol Regions of Finlond

lig. 5. The three phases of land appearance in Finland after the latest glaciation: Retreat of the ice to the north (a);the first (faster) phase ofland uplift (b); and the second (slower) phase ofland uplift (c). Reproduced fromEronen et al. (1995).

direction reflects primarily the variation in cli-mate. For example, the length of the growingseason varies from 120 to 180 days between thenorthern and southern border of the area (AtlasofFinland 1988).

Soil and peat. Post et al. (1982) point out thatthe high spatial variation of soil carbon densitywhich has often been reported, is not only aresult of limited sampling intensity. In their view,

o, \-, "... alarge proportion of this variation may bedue to soil variation, and increased samplingwill do little to reduce it". The variation is attrib-utable to factors such as 1) aspect, 2) topogra-' phy, 3) parent material,4) age of the soil profile,and 5) vegetation. Regarding mineral soils, Liskiand Westman (I996a, b) have analysed this vari-ation in Finnish conditions.

Since the peat reservoir dominates, it is essentialto analyse why peatlands me so common in westerncentral Finland, and why the deepest peatlands areto be found in southern Finland. First, there can beimpacts of the glacialhistory which varies globatlywithin the borealzone (Peltier 1994). In Finland,according to Eronen et al. (1995), the post-glacialchanges of the landscape can be divided into three

phases: 1) fastretreat of the ice shield from southemFinland to north western Finland between 10 500and 9 500 years before present (Fig. 5a); 2) quickexpansion oflandbetween 9500 and7000 years BP(Frg. 5b); and 3) slow expansion ofland since 7000years BP (Fig. 5c).

The soil profiles in southern and south easternFinland are generally somewhat older than theprofiles in the peat forming sub regions in west-ern and northern Finland. In general, the spatialvariation of carbon in the landscape does notcorrelate with the age of the soil profile, exceptin areas near the west coast where the profilesare younger than 500 to 1500 years. In otherwords, a period of 2000 to 4000 years has beensufficiently long in western central Finland forthe accumulation of the large peat reserves. Be-yond that, there is little or no correlation overspace between the age of the soil profile and thetotal reservoir of carbon in peatlands.

Annual precipitation in Finland is 500-700mm, and rather similar in all sub regions. Evapo-ration varies more, from 200 mm in the north to450 mm in the south (Atlas of Finland 1988).The gradient from north to south is even steeper

21

Silvo Fennico 3l (l I reseorrh onicles

for evapotranspiration. The productivity of for-est ecosystems increases from north to southand, hence, also franspiration increases from northto south.

There are hills in the landscape in easternparts of Finland while the western parts are gen-erally quite flat. The pattern ofincreasing reser-voirs towards western Finland appears to corre-late with the topography of terrain. Yet, the to-pography does not explain the gradient in north-south direction. The formation of peat is leastcommon in southernmost and south eastern subregions although those areas are also rather flat.

Post et al. (1982) have reported a decrease insoil carbon with increasing temperature for anyparticular levels ofprecipitation. The Finnish dataare consistent with this view. However, thickestlayers ofpeat can be found in southem sub regions(Fig. 2 b). A hypothesis can be presented that,fustly, the height increment of peat is fastest in thewarmest (southem) peatlands. Secondly, the hightemperature also maintains a high rate of eva-potranspiration. Therefore, the water table tendsto be low in southern Finland, and waterloggecareas are uncoflrmon. A single characteristic inclimate - high average temperature compared tothe more northern regions - creates the conditionsfor two different ecological consequencies ac-cording to this hypothesis: The peat layers arethick, but peatlands are rare (Fig. a).

4.4 Extrapolation to Other Boreal Areas

Peaflands are the main reservoir of carbon in theboreal zone. Regarding extrapolation beyondFinnish borders, the main issue is whether peat-lands are as common in Finland as in other bore-al areas, and whether a typical Finnish peatlandis representative of all boreal peatlands. The rel-ative cover of peatlands in our data is 28.0 and24J 7o referring to land area and total area,respectively. Global boreal peatlands have beenreported to cover 2.5-3.5 million km2 (Gorham1991, Apps et al. 1993). Given the total area ofthe boreal zone, 12-15 million km2 dependingon definitions, the peatlands would cover 18-28Vo of the area. In conclusion, peatlands are ascommon or slightly more cornmon in Finlandthan in other boreal areas.

22

Gorham (1991) has reported that the peat lay-ers are less thick in Fennoscandian mires than inother boreal peatlands. In his statistics, the meandepth of Fennoscandian peatlands is only 1.1 m,while being 2.5, 2.2, and 2.5 m in the borealpeatlands of Russia, Canada and the US, respec-tively. Gorham (1991) writes: "The mean depthofCanada's peatlands is also not securely found-ed, thousands of measurements being taken asrepresentative of millions of hectares withoutany effort at stratified sampling". For Canada,he refers to inventories taken by governmentagencies which "in northern Canada especially,are often either broad-scale or lacking". The dataare even fewer for Russia, where vast peatlandsexist in remote areas.

In a recent overview, Lappalainen (1996) esti-mated thatthe total reservoir of carbon inWorld'speatland would be only 234-252 Pg. This is lessthan has been estimated for boreal peatlandsalone, e.g. by Gorham (1991). Assuming that thebulk density of peat varies in a similar way in allboreal peatlands, the data on peat depth is criti-cal in efforts of improving the accuracy and pre-cision of boreal carbon estimates.

4.5 The Eventual Carbon Feedback

The increment of trees in the boreal region ofFinland, both radial and height increment, is high-er during warm than during cold periods (Miko-la 1950). It is also a fact that forest growth ishigher in fhe southern (mild) than in northern(cold) regions within Finland (e.g. Kauppi andPosch 1985). A hypothesis has been presentedthat nitrogen mineralization increases with in-creasing temperature, and supply of nitrogenwould be critical in the boreal zone in enhancingNet Primary Productivity in mild areas (e.g. Mel-lillo 1993). In addition, the variation from northto south in the length of the growing season hasimpacts on productivity. Regarding responses towarming, let us first consider forests with con-tinuous tree canopy on mineral soils.

In Beuker's (1994) data, growth increased rel-ative to the natural rate of growth, when trees weretaken from northern Finland and transplanted tosouthern Finland into an environment 2-5 cenli-grade warmer than the site from which the seed

Kouppi el ol. (orbon Reservoirs in Foresls ond Peollonds in fie Boreol Regions of Finlond

was collected. He concluded that in areas wherelow temperature is the major limiting factor forincrement, tree growth would benefit from an in-crease in annual mean temperature. Beuker (1994)refers mainly to the responses of trees in the north-ern parts of Finland (see also Karjalainen l996a,b).

In all areas within Finland. wild fires havebeen effectively suppressed for more than 50years. Considering the wilderness forests in thenorthern parts of the boreal zone, fire and othernatural disturbances are an important element ofthe functioning of ecosystems (Stocks etal.1996).Climatic warming would presumably increasethe frequency of wild fires.

The carbon reservoir of boreal vegetation issmall, only 30-45 Pg. This according to Mar-land et al. (1994) equals no more than the cumu-lative global emissions of COz in 1987-1991(= 30 Pg C), or in 1984-1991 (= 45 Pg C).Changes in vegetation biomass in the boreal zone,whether positive or negative, will have only asmall impact on the concentration of CO2 in theatmosphere. As a potential feedback to climaticwarming, it does not matter very much whetherthe boreal trees would accumulate more bio-mass, or whether they would burn in flames.

Then, let us consider mire ecosystems, andthe eventual positive feedback to greenhousewarming that is, a possible net release of COzfrom the peat reservoir into the atmosphere. Evenif earlier estimates can have been too high, thereservoir in boreal peatlands is at least 200 Pg C,almost one order of magnitude larger than thecorresponding reservoir in boreal vegetation.Therefore, the issue of a boreal impact on thefuture trend of CO2 in the atmosphere is mainlyan issue of a possible decrease of the largestboreal reservoir, peat.

Acknowledgements

The authors thank T. R. Carter, M. Heikinheimoand E. Hellsten from the Finnish MeteorologicalInstitute for providing the temperature data; M.Eronen and S. Zoltai for discussions, and P. Al-honen, M. Apps, S. Brown, R. Hudson, J. Laine,J. Liski, J. Pastor, H. Rodhe, and one anonymousreviewer for comments to earlier versions of themanuscript.

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