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SUPPLEMENTARY INFORMATION (NGS-2009-04-00374A) 1

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Study sites. All studied forest landscapes, except E, were located in Nature Reserves or 3

National Parks, representing the boreal zones from the Hemiboreal to the Northern 4

Boreal, with best representation of the Middle Boreal zone (zonal terminology sensu1). 5

Most sites are used for vegetation ecological monitoring, which started in 1988, and 6

very detailed information on vegetation-environment relationships are available for sites 7

A, B, C, E, F, G, H, I, J, K and N2,3. There is solid dendroecological evidence for large 8

and historically recurring fires in sites L and M from which >90 % of the soil samples 9

contained charcoal, which indicates that the method employed effectively tracks past 10

fire occurrence. Information on site names, geographical positions, study area sizes, 11

time since last forest fire and references are shown in Table S1. 12

Unsorted morainic soils characterized all sites except M (here the soil was sandy 13

and sorted due to glacifluvial transport and deposition) and boulders and stones in the 14

soil caused variation in soil core depth and amount of soil sampled. Site-specific soil 15

core depths are given in Table S2. The vertical distribution of charcoal in morainic 16

boreal forest soils is not known, but microscopic charcoal is probably present in small 17

amounts also at soil depths below those we have analysed. Consequently, total soil 18

charcoal stocks are likely to exceed our estimates slightly. 19

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Figure S1. An example of a soil core from Site M. This particular sample 22

contained 121 g charcoal, of which the majority occurred in the transition 23

between the organic top soil and the underlying mineral soil. It is typical for 24

boreal forest soils that the macroscopic charcoal occurs in the transition 25

between the organic top soil and the underlying mineral soil. 26

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Table S1 Study site information 27

Site Name

No of

samples Lat Long Altitude (m) Study area (km2)

Time since last fire

(year) Ref

A Paulen 50 58°18’ 7°55’ 150-275 3 >200 2,4

B Solhomfjell 50 58°58’ 8°58 350-480 2 >200 3,4

C Lundsneset 50 59°03’ 11°42’ 120-240 10 120 2,4,5

D Grytdalen 50 59°15’ 8°37’ 475-550 0,5 >500 2,4

E Årum 125 59°22' 9°44' 470-560 4 ≈300 6,7

F Rausjømarka 50 59°49’ 11°02’ 220-300 0,2 ≈200 2,8

G Bringen 50 60°32’ 9°23 600-750 6 Unknown 2

H Otterstadstølen 50 60°49’ 5°45’ 220-350 2 >500 2,4

I Gutulia 50 62°00’ 12°09’ 700-850 4 115 2,9

J Urvatnet 45 63°06 9°48’ 300-400 3 ≈200 2,4,10

K Øyenskavelen 50 64°17’ 10°57’ 220-300 3 Unknown 2,4

L Lillberget 50 64°19’ 19°01’ 315-340 0,7 107 11

M Skataheden 75 64°24’ 19°28’ 305-320 0,4 120 12

N Granneset 50 66°30’ 14°52’ 225-325 0,5 Fire free 2,4

O Imoravvi 50 69°18’ 22°00’ 220-280 0,3 ≈150 13

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Charcoal measurement pilot study. A pilot study was conducted to test the precision 29

of the adopted manual approach. Known amounts of charred black carbon (charcoal 30

ranging in size from 0.2 to 1.0 mm) were added to soil samples composed of a mixture 31

of sand and raw-humus that did not contain black carbon. A subsequent search of the 32

samples using a stereomicroscope resulted in a 92 ± 6 % (mean ± 1 standard error of the 33

mean, n=10) recovery of the added mass in a 2-h search per sample (oxidative 34

approaches based on the use of HNO3 did not track the added amounts with a 35

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corresponding precision). BC measurements are thus realistic, although slight 36

underestimates due to the recovery rate. As it is well known that BC estimates are 37

strongly influenced by the technique applied14-19, the uncomplicated manual technique 38

actually provides a useful complement to more complicated thermal or chemical 39

techniques. 40

Inbuilt Age of Dated Charcoal. The maximum inbuilt age ranges between 500 and 41

800 years, which corresponds to the maximum age of the dominating tree species in the 42

study region (i.e. Norway spruce and Scots pine). Indeed, the oldest age of any spruce 43

tree documented in Norway is 507 years. However, considering that such old trees are 44

rare, the inbuilt age is likely to be considerable less. Furthermore, considering that most 45

fire events in boreal Scandinavia are ground fires20, it is most likely that the charcoal 46

originated from young sub-canopy trees that were consumed by the fire21, thus yielding 47

a small inbuilt age factor. 48

49

50

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51 Table S2 Site-specific soil core depth and content of charcoal 52

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Charcoal (g m-2)

Site No of

samples

Average

soil core

depth

(cm)

Min Max Mean Median SE

A 50 17,0 0 3762 279 62 90

B 50 13,8 0 1667 141 20 45

C 50 14,6 0 1583 155 42 41

D 50 14,6 0 1903 211 21 62

E 125 16,6 0 5137 235 65 57

F 50 9,9 0 1073 104 34 28

G 50 13,9 0 972 123 19 31

H 50 13,2 0 299 9 0 6

I 50 8,4 0 1758 74 0 39

J 45 18,0 0 1032 48 0 24

K 50 21,9 0 19 1 0 <1

L 50 12,0 0 2196 298 81 71

M 75 14,1 0 3744 400 181 66

N 50 10,5 0 0 0 0 -

O 50 nd* 0 228 36 0 8

*no data54

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