Increase in carbon accumulation in a bor

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Increase in carbon accumulation in a boreal
peatland following a period of wetter climate
and long-term decrease in nitrogen...
Article in New Phytologist · February 2015
DOI: 10.1111/nph.13311 · Source: PubMed

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Research

Increase in carbon accumulation in a boreal peatland following a
period of wetter climate and long-term decrease in nitrogen deposition
Simon Utstøl-Klein1, Rune Halvorsen2 and Mikael Ohlson1
Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, PO Box 5003, NO-1432

As, Norway; 2Department of Botany, Natural History Museum,

1

University of Oslo, PO Box 1172, Blindern, NO-0318 Oslo, Norway

Summary
Author for correspondence:
Mikael Ohlson
Tel: +47 64965757
Email: mikael.ohlson@nmbu.no

Received: 14 October 2014
Accepted: 30 December 2014

New Phytologist (2015)
doi: 10.1111/nph.13311

Key words: bog, hummock, ombrotrophic,
peat growth, pine method, Sphagnum.


Rates of peat growth and carbon (C) accumulation in a Sphagnum-dominated boreal peat-

land in south-east Norway were compared over two time periods each 17 yr long, that is, an
earlier period from 1978 to 1995 and a recent period from 1995 to 2012.

Our research was based on 109 peat cores. By using exactly the same study area and sampling protocols to obtain data for the two time periods, we were able to obtain a clear picture
of the spatio-temporal patterns of peat accumulation.

We show that peat growth and C accumulation were significantly higher in the recent than
in the earlier time period. Interestingly, nitrogen (N) deposition was lower in the recent than
in the earlier time period, while precipitation increased in the recent time period. Temperatures did not show any consistent trends over the time periods.

Although our data do not allow assessment of the relative importance of declining N deposition vs increasing precipitation as drivers of peat accumulation, our results suggest that
peatland C sequestration is not significantly inhibited by N pollution at current precipitation

and N deposition levels.

Introduction
Northern peatlands have accumulated carbon (C) over the Holocene and today they account for between one-quarter and onethird of the global soil C pool and play an important role in the
C cycle (Gorham, 1991; Vitt et al., 2000; Yu, 2012). The C sink
function of northern peatlands is a result of low decomposition
rates rather than high productivity (Malmer & Wallen, 1993),
although the proximate reason why a peatland functions as a sink
for C is that its vegetation fixes more C from the atmosphere
than is lost through outflow of dissolved organic C and emissions
of CO2 and CH4 (Roulet et al., 2007). Typically, as much as
70–90% of the fixed C is lost by the respiration of decomposers
in the aerobic surface layer of the peatland (Ohlson & Økland,
1998; Malmer & Wallen, 1999; Vitt et al., 2000), while the
remaining organic material eventually becomes stored in deeper
and anoxic peat layers where the biological activity is very low
(Clymo, 1984; Vitt et al., 2000; Malmer & Wallen, 2004).
Most northern peatlands are naturally nutrient-poor ecosystems (Aerts et al., 1992) and their vegetation is usually dominated
by Sphagnum mosses (Rydin & Jeglum, 2013), which play a fundamental functional role by contributing a major part of the C
that is stored in peat (Clymo & Hayward, 1982). Sphagnum

mosses are sensitive even to small changes in environmental conditions, such as temperature and supplies of nutrients and water
(Rydin, 1986; Robroek et al., 2009; Granath et al., 2012), and
recent studies indicate that the competitive balance between
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Sphagnum mosses and other groups of plants is altered by ongoing and persistent climate warming (Limpens et al., 2008; Dise,
2009; Jassey et al., 2013; Weedon et al., 2013), which in turn is
thought to cause a decline in peatland C accumulation rates (Belyea & Malmer, 2000; Wu & Roulet, 2014). In the light of the
well-established fact that nitrogen (N) fertilization can reduce the
productivity of Sphagnum mosses (Gunnarsson & Rydin, 2000;
Gunnarsson et al., 2008; Granath et al., 2014), possible interacting effects of climate warming and fertilization by anthropogenic
N deposition have been given particular attention. For example,
in a meta-analysis of results from 107 field experiments, Limpens
et al. (2011) concluded that peatland C accumulation is significantly inhibited by current N deposition loads, although the
mechanism by which this is brought about is not fully understood. However, vascular plant remains decompose more rapidly
than remains of Sphagnum mosses (Limpens & Berendse, 2003;
Dorrepaal et al., 2005; Breeuwer et al., 2008), and one of the
hypotheses put forward to explain the negative relationship
between N input and C accumulation rates emphasizes the

importance of shifts from Sphagnum to vascular plant dominance
in the peatland vegetation (Berendse et al., 2001; Gunnarsson
et al., 2002; Turunen et al., 2004; Wiedermann et al., 2007;
Heijmans et al., 2008). High N deposition rates also lead to an
increase in litter N concentrations and a reduction in polyphenol
concentrations (Gunnarsson & Rydin, 2000; Nordin &
Gunnarsson, 2000; Nordbakken et al., 2003; Bragazza & Freeman, 2007; Wiedermann et al., 2009), which accelerates
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decomposition (Limpens & Berendse, 2003; Dorrepaal et al.,
2005; Bragazza et al., 2006, 2012; Bragazza & Freeman, 2007;
Bragazza & Freeman, 2007). In addition, climatic factors such as
the temperature and rainfall during the growing season interact
with N deposition in determining production and C accumulation by Sphagnum mosses (Gerdol et al., 2007). Accordingly, the

findings of Limpens et al. (2011) indicate that current rates of N
deposition will strongly inhibit peatland C sequestration in large
parts of Europe if the climate becomes warmer and wetter.
Our current understanding of the interacting effects of climate
change and N deposition on C accumulation rates in northern
peatlands is based upon experimental studies conducted over a
few years. Longer term interaction effects over decadal time-scales
have, to the best of our knowledge, not yet been measured, or
estimated, by use of time series of empirical data.
Using vegetation analyses and samples of dated peat cores
collected in 1995, Ohlson & Økland (1998) gave a detailed
documentation of the spatial variation in the rates of peat and
C accumulation over the last century in the boreal bog Søndre
Kisselbergmosen, in southeast Norway. Here, we report the
results from a re-sampling of the same bog using exactly the
same field protocol as that used by Ohlson & Økland (1998).
The re-sampling was carried out 17 yr after the sampling in
1995, and by selecting a subset of data from Ohlson & Økland
(1998) that represents the 17-yr time period before 1995, we
obtained two sets of peat samples that cover the same range of

habitat conditions, but different time periods of equal length,
that is, an earlier period from 1978 to 1995 and a recent period
from 1995 to 2012. The aims of our study were (1) to determine
whether peat and C accumulation rates differed between the earlier
and recent time periods, and if so, how; (2) to explore the relationships of peat accumulation with temperature, precipitation, and N
deposition in the two time periods; and (3) to briefly compare our
results with recent long-term eddy covariance-based measurements
of C exchange in boreal peatlands (i.e. Roulet et al., 2007; Flanagan & Syed, 2011; Peichl et al., 2014). Because of a general
decrease in N deposition in our study area over the last 25 yr (Pedersen et al., 1990; Tørseth & Pedersen, 1994; Tørseth & Semb,
1997; Hole & Tørseth, 2002; Aas et al., 2006; Aas, 2012), we predicted that peat and C accumulation rates would be higher in the
recent time period than in the earlier period.

Materials and Methods
Study site
The study was conducted in the northern part of the peatland
Søndre Kisselbergmosen (59°370 N, 11°400 E), located 285 m
above sea level in the municipality of Marker, in the county of
Østfold, south-east Norway (Fig. 1). This peatland is an asymmetrically raised bog with intact hydrology except for some small,
old ditches in the margin of the eastern part where a road passes
close to the bog. Our study area, which corresponds exactly with

the 400-m2 ‘Plot 1’ of Ohlson & Økland (1998), was located at
the mire expanse end of the mire expanse–mire margin gradient
(Økland et al., 2001). Peat depth is c. 4 m and the area is
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characterized by an irregular pattern of hummocks and hollows,
typical of weakly sloping boreal bogs (Fig. 1). The hummocks
were dominated by Calluna vulgaris (L.) Hull, Sphagnum fuscum
(Schimp.) H. Klinggr., Sphagnum rubellum Wilson and Cladonia
rangiferina (L.) Wigg., while the hollows were dominated by
Sphagnum tenellum (Brid.) Brid. and Sphagnum cuspidatum
Hoffm. Further information about the peatland vegetation of the
study site is given by Ohlson et al. (2001).
The climate of the study area was weakly oceanic (Moen,
1998), with an estimated average annual temperature of 3.7°C
and an annual precipitation of c. 850 mm for the period 1961–
1990 (Økland, 1989).
Peat sampling and peat age estimation
A total of 104 peat cores were collected between 25 August and

16 September 2012. A sharp-edged cylindrical corer (length
50 cm; diameter 10 cm) was used to collect peat cores adjacent to
all specimens of Scots pine (Pinus sylvestris L.) in the study area.
Every peat core was cut at the depth of the adjacent pine’s root
collar, so that each peat sample contained exactly the amount of
peat that had accumulated since the pine established. The year
(time) of pine establishment, that is, pine age, was determined in
the laboratory by using a stereomicroscope to count tree rings at
the root collar of the pines. This is the essence of the so-called
‘pine method’ (Borggreve, 1889), which has previously proved
useful for establishment of age–depth relationships in surface peat
(Ohlson & Dahlberg, 1991; Økland & Ohlson, 1998; Ohlson
et al., 2001; Gunnarsson et al., 2008). Care was taken to avoid
compaction of peat during sampling. Peat cores were dried at
80°C to constant mass and weighed to obtain bulk density.
Comparable subsets of dated peat cores from the earlier and
recent time periods were obtained by only accepting cores satisfying the following three criteria. (1) Only pines rooted on hummock-level peat were included because lower level positions are
outside the long-term tolerance range of Scots pine (Ohlson, 1995,
1999). Hummock level was defined as comprising the uppermost
two levels along the five-level hydrotopographical gradient from

carpet, via lower lawn, upper lawn and lower hummock, to upper
hummock levels (see Ohlson & Økland (1998) for characterization
and definitions). (2) Only peat cores with a dated age from 3 to
17 yr were included. Older peat cores were excluded from the 1995
data set because pines older than 17 yr were absent in 2012 as a
result of the exhaustive sampling of all pines in the study area carried out in 1995. Peat cores younger than 3 yr old were excluded
because the variance of estimated peat accumulation rates increases
with decreasing time period over which the sample integrates
Sphagnum and peat growth (see Økland & Ohlson, 1998). (3)
Only peat cores from sites with a total Sphagnum cover ≥ 50% were
included. This criterion was applied to reduce variability in the
data attributable to local environmental conditions and interactions
between Sphagnum mosses and other plants, which are known to
affect peat growth rates strongly (Ohlson & Økland, 1998; Ohlson
et al., 2001). Moreover, this criterion also improved comparability
between the two time periods because the 50% Sphagnum cover
criterion could be applied in exactly the same way to data collected
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Fig. 1 The study site, Søndre
Kisselbergmosen, is an asymmetrically raised
bog located in south-east Norway, close to
the border with Sweden. The black rectangle
on the aerial photograph shows the position
and approximate size of the 400-m2 study
area (i.e. 20 9 20 m). The photograph is
courtesy of The Norwegian Mapping
Authority (©Kartverket).

in 1995 and 2012. A subset of 35 and 74 peat samples collected in
1995 and 2012, respectively, satisfied all three criteria, giving a
total of 109 peat cores that make up the data set used for all further
analyses to represent peat and C accumulation rates in the earlier
and recent time periods.
Peat growth and C accumulation variables (Table 1) were calculated from peat depth, peat age, sample volume, dry weight, and C
concentrations. Our calculations of peat mass and C accumulation
rates follow Ohlson & Økland (1998) and Gunnarsson et al.
(2008). When calculating the vertical peat growth rates, we did
not compensate for peat compaction processes through physical
consolidation and biological decomposition. The reason for this is
that all 109 peat cores in the data set were ≤ 17 yr old, which is too
short a time span for compaction processes to become significant
(see Økland & Ohlson, 1998). Total C and N concentrations
have previously been determined for the 35 peat cores representing the earlier time period (Ohlson & Økland, 1998). For the peat
cores collected in 2012, a random subset of 15 cores were ground
with a mill and subjected to determination of total C and N concentrations by dry combustion as described by Nelson (1982) and
Bremner (1982), using a LECO EC12 Carbon Content Analyser
(LECO Corp., St Joseph, MI, USA).
Vegetation analyses
For each peat core sampled in 1995 and in 2012, one quadrat of
25 9 25 cm was located with the pine in the centre. In each
quadrat, the percentage cover of all Sphagnum species and (collectively) of each of Cladonia lichens, hepatics and vascular plants
was determined. Each quadrat was characterized as belonging to
the upper or lower hummock level along the hydro-topographical
gradient related to the depth to the ground-water table using criteria based on species composition, as given by Økland (1989);
and to one or more of five dominance types according to Økland
& Ohlson (1998) and Ohlson & Økland (1998): Cr, He, Sf, Sm
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and Sr denote cover > 25% of Cladonia spp., hepatics (all species), S. fuscum, Sphagnum magellanicum Brid., and S. rubellum,
respectively.
Climate and N deposition in the study period
Data from meteorological stations close to the study area
(< 20 km distance) were used to estimate temperature and precipitation differences between the two periods 1978–1995 and
1995–2012. The meteorological data were obtained from the
Norwegian Meteorological Institute (http://www.eklima.no),
originating from the following stations (the time period in which
the station was in operation is given in brackets after the station
name): Høland-Kollerud (1978–1988), Høland-Løken (1988–
1990), Høland-Fosser (1991–2007), and Aurskog II (2007–
2012). Meteorological data are missing for 1988 and 1991. Data
on measured total N deposition levels for the 50 9 50 km grid
cell in which the study area is situated were obtained from
Tørseth & Pedersen (1994), Tørseth & Semb (1997), Hole &
Tørseth (2002), Aas et al. (2006) and Aas (2012).
Statistical analyses
Vertical peat growth rate (cm yr1), bulk density (g dm3) and C
accumulation rate (g m2 yr1), recorded for data collected in
2012 and representing peat accumulation in the recent time
period, were used as response variables in generalized linear models (GLMs; Venables & Ripley, 2002) with identity link and normally distributed errors (ANOVA, ANCOVA, and linear
regressions). Peat age (yr), total Sphagnum cover (%), hummock
zone (factor-type variable with two levels: lower and upper) and
dominance type (factor-type variable with five levels) were
included as predictor variables in first-generation models for each
response variable, obtained in order to assess the importance of,
and eventually partial out, fine-scale environmental variables.
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Table 1 Summary statistics for peat accumulation variables recorded at Søndre Kisselbergmosen for the earlier and recent time-periods
Earlier time period 1978–1995

Recent time period 1995–2012

Variable name

Abbreviation

Unit of measurement

n

Mean

SD

Range

n

Mean

SD

Range

Peat age
Vertical peat growth rate
Mass accumulation rate
Carbon accumulation rate
Bulk density
Nitrogen concentration
Carbon concentration
Carbon-to-nitrogen ratio

YR
PG
MAR
CAR
BD
NC
CC
C/N

Years (yr)
mm yr1
g m2 yr1
g m2 yr1
g dm3
Percentage of dry mass
Percentage of dry mass

35
35
35
35
35
35
35
35

9.5
11
470
230
44.0
0.95
49.0
53

3.8
4.6
220
110
10.9
0.18
0.7
10

4–16
3.8–22.5
180–1160
90–580
29.2–80.1
0.58–1.45
47.7–50.2
34–84

74
74
74
74
74
15
15
15

7.9
15
740
370
51.2
1.28
49.7
41

3.6
6.8
370
180
18.5
0.35
1.2
11

3–16
3.0–36.7
160–2100
80 –1050
18.3–128.4
0.77–1.94
47.8–51.9
26–65

Figures in bold are significantly different at P < 0.05.

Multi-predictor models were constructed by two selection procedures used in parallel to compare ‘nested models’: a forward
selection procedure using the F-ratio test (significance level
a = 0.05) and the Akaike information criterion (AIC). Peat age
was tested as the first variable and included in models whenever
significant in order to avoid responses being confounded by the
effects of the growing pine and/or temporal variation in peat
accumulation rates (Ohlson & Økland, 1998). Thereafter, predictors were added sequentially until no predictor could be added
that explained significant amounts of residual variation or caused
a drop in AIC. (The term ‘nested models’ is used for two models
with different numbers of predictor variables, the more complex
model containing all variables included in the simpler model.)
The same final models were obtained with both selection methods and only results obtained by the forward selection F-tests are
therefore shown. Diagnostic plots (Crawley, 2013) were used for
graphical assessment of whether modelling assumptions were met
and whether models were adequately specified.
Differences in peat accumulation variables between the earlier
and recent time periods (1978–1995 and 1995–2012) were
modelled as the marginal effect of the categorical (factor-type)
predictor sampling period (with two levels), added to the best
first-generation GLM models. Confounding effects of differences
in natural conditions between the two data sets obtained in the
two sampling periods were thus reduced to a minimum. Differences in N concentrations between the two periods were also
modelled by GLM, using peat age as a covariate.
Because Levene’s tests (Levene, 1960) of response variables
(bulk density, C accumulation rate and vertical peat growth rate)
for variance inhomogeneity between sampling years were significant, all response variables were log-transformed before GLM
analyses in order to meet the assumption of variance homogeneity. For all response variables, assumptions of independent errors,
linearity, homogeneity of variances and no multicollinearity
between predictors were met and case-wise diagnostics revealed
no single observations, or subset of observations, with unduly
high influence on modelling results.
Student’s t-tests were used to test for differences in temperature and precipitation between the earlier and recent time periods. All statistical analyses were performed in R, version 2.15.1
(The R Development Core Team, 2010).
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Results
Climate and N deposition in the earlier and recent time
periods
Temperature and precipitation were highly variable over the
entire study period. For example, the average annual temperature
varied from c. 2°C in 2010 to 6°C in 2000, and the average
annual precipitation varied by a factor of 2, from c. 500 mm in
1997 to 1000 mm in 2000. Both the highest and lowest annual
averages for temperature and precipitation occurred in the recent
time period. Interestingly, the precipitation levels during the
growing season showed a significant increasing trend over the
recent time period and a declining trend over the earlier period,
which means that the peat samples that were collected in 1995,
representing the earlier time period, were collected at the end of a
17-yr period that was characterized by decreasing precipitation
during the growing season, while the opposite was the case for
the samples collected in 2012 (Fig. 2a). No clear trends in temperature were observed in any of the time periods, although the
average annual temperature in the recent period was slightly
higher than for the earlier period; 5.3 vs 4.6°C, respectively
(GLM: F1,30 = 3702; P = 0.0639; Fig. 2b).
Nitrogen deposition peaked at 15 kg ha1 yr1 in the middle
of the 1980s and declined rapidly over the next decade. By 2012,
deposition was c. 8 kg ha1 yr1 and the studied peatland thus
received significantly more airborne N in the earlier than in the
recent time period (Fig. 3).
Relationships between peat accumulation rates and
predictor variables for the recent time period 1995–2012
Single-variable GLMs revealed a significant increase in vertical
peat growth rate with increasing total Sphagnum cover, and a significant decrease with increasing peat age (Table 2). No significant relationships between C accumulation rate and any of the
predictor variables were found. However, bulk density was significantly higher in lower than in upper level hummocks, and
decreased with increasing Sphagnum cover (Table 2). The best
multiple-predictor models for vertical peat growth rate and bulk
density both included two significant predictors, of which
Sphagnum cover was one (Table 3).
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(a)

Annual

The mean ( SD) vertical peat growth rate increased by c. 40%
from the earlier to the present time period, from 1.1  0.5 to
1.5  0.7 cm yr1, and the mean C accumulation rate increased
by c. 60%, from 230  110 to 370  180 g m2 yr1 (Fig. 4).
Peat cores from the lower hummock zone had the highest bulk
density, and cores from the recent time period generally had
higher bulk densities than cores from the earlier period. Actually,
the mean bulk density increased by c. 16% from the earlier to the
recent time period, that is, from 44  11 to 51  18 g dm3
(Fig. 5). Mean N concentration, after variation attributable to
peat age had been compensated for, was significantly higher in
the peat cores from the recent time period (1.28  0.35%) than
in the cores from the earlier period (0.95  0.18%) (Fig. 6).

Growing season

(b)

Discussion

Fig. 2 Variation in precipitation (a) and temperature (b) during the study
period. Growing season data (triangles) are for June, July and August, and
the shaded areas represent the earlier time period. Significant trends are
shown by regression lines. Names of the meteorological stations are given
in the Materials and Methods section. Further information about the
meteorological stations is provided by the Norwegian Meteorological
Institute (http://www.eklima.no).

Total N deposition rates (kg ha—1 yr—1)

15

●

14

13

12

11
●

10
●

9

●
●
●

8

●

1978−1982 1983−1987 1988−1992 1992−1996 1997−2001 2002−2006 2007−2011

Period
Fig. 3 Total nitrogen (N) deposition during the time span from 1978 to 2011
at the peatland study site Søndre Kisselbergmosen in south-east Norway.

Increasing peat accumulation and peat N concentration
from the earlier to the recent time period
Vertical peat growth rates and C accumulation rates were significantly higher in the recent time period than in the earlier period.
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Our most important result is the increase in vertical peat growth
and C accumulation from the earlier to the recent 17-yr period.
This result is directly relevant to assessment of future changes in
peatland dynamics and biogeochemistry because Sphagnum
growth and C accumulation in peatlands are determined by interactions between N deposition and climatic factors such as temperature and precipitation (Gerdol et al., 2007; Limpens et al.,
2011; Larmola et al., 2013).
Being observational, our study does not, unfortunately, allow
separation of the relative roles of climate change vs declining N
deposition as drivers of the observed increase in peat growth and
C accumulation. There are, however, no indications in our material that temperature change or variability is a key factor for the
observed increased in peat accumulation, as consistent temperature differences were not observed between the earlier and recent
time periods (temperatures were variable among years in both
time periods and no significant trends were observed; see
Fig. 2b). However, the fact that sets of contemporary decadal
peat growth rate estimates obtained using the pine method are
strongly influenced by peat growth conditions in the years just
before sampling (Ohlson & Økland, 1998; Ohlson et al., 2001)
raises the possibility that lower-than-average precipitation in
1992–1995 and higher-than-average precipitation in 2011 and
2012 (Fig. 2a) may have accentuated the difference in peat
growth and C accumulation rates between the earlier and recent
time periods by bringing about between-period differences in the
mean ground-water table. In this context, it is of particular interest that recent eddy covariance measurements of CO2 fluxes in
peatland ecosystems strongly support the importance of annual
and inter-annual water table fluctuations as drivers of both photosynthesis and respiration (Flanagan & Syed, 2011; Peichl et al.,
2014).
Lower N load and wetter conditions
N deposition at our study site reached the historically highest levels of c. 15 kg ha1 yr1 in the middle of the 1980s, which are
relatively low levels by European standards (Bragazza et al.,
2004). This raises the question of whether, and to what degree,
N deposition has had negative effects on Sphagnum productivity
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Table 2 Summaries of generalized linear models (GLMs) with identity link function and normal error distribution for the influence of single predictor
variables on three peat accumulation variables (see Table 1 for explanation of response variables) recorded at Søndre Kisselbergmosen for the time period
1995–2012
Response variable

Predictor

df

R2

F

P(F)

PG

Age
Sphagnum cover
Factor (zone)
Factor (domT)
Age
Sphagnum cover
Factor (zone)
Factor (domT)
Age
Sphagnum cover
Factor (zone)
Factor (domT)

1
1
1
4
1
1
1
4
1
1
1
4

0.11
0.12
0.03
0.06
0.02
0.01
< 0.01
0.07
0.01
0.09
0.09
0.04

9.0
9.9
2.2
1.2
1.6
0.9
< 0.1
1.2
0.9
7.1
6.9
0.7

0.0038
0.0025
0.1427
0.3311
0.2148
0.3471
0.8248
0.3139
0.3418
0.0097
0.0103
0.5640

CAR

BD

Coefficient

Intercept

AIC

0.062
0.021
0.250

2.013
0.105
1.351

0.074
0.018
0.101

4.265
2.272
3.752

0.569
0.482
11.738

46.666
89.339
59.247

148.75
147.92
155.20
158.56
303.53
304.21
305.08
306.10
645.47
639.48
639.60
649.28

F and P refer to F-tests of the change in deviance between nested models. df, model degrees of freedom (degrees of freedom for the residuals = 73 – df).
Regression coefficients for significant terms are given as treatment contrasts. For the factor-type predictor zone, the intercept provides the estimated mean
for the lower hummock zone, while the coefficient gives the difference between estimated means for the lower and upper hummock zones. Factor-type
predictor domT refers to vegetation dominance type (see the Materials and Methods section). AIC, Akaike information criterion.

Table 3 Statistical summaries of full generalized linear models (GLMs) with identity link function and normal error distribution for the influence of
significant predictor variables on two peat accumulation variables recorded at Søndre Kisselbergmosen for the time-period 1995–2012
Response variable

Predictors

df

R2

F

P(F)

Coefficient predictor 1

Coefficient predictor 2

Intercept

AIC

PG
BD

Age + Sphagnum cover
Sphagnum cover + factor (zone)

2
2

0.20
0.18

8.8
7.5

< 0.001
0.001

0.053
0.476

0.018
11.591

0.523
96.856

142.90
634.17

F and P refer to F-tests of the change in deviance between nested models. df, model degrees of freedom (degrees of freedom for the residuals = 101 – df).
Regression coefficients for significant terms are given as treatment contrasts. See Table 1 for description of response variables. AIC, Akaike information criterion.

and C accumulation in the study area. Setting ecosystem-level
critical N loads for peatlands is challenging (Sheppard et al.,
2014), and we do not know whether the loads on our site have
actually exceeded critical levels and, if so, for how long. However,
Bragazza et al. (2004) suggest a critical N load in Europe of
10 kg ha1 yr1 above which Sphagnum mosses change from
being N-limited to being co-limited by P + K. If this critical level
applies to our study site, the first half of the earlier time period,
1979–1995, was characterized by N loads that were at, or above,
the critical level, while N loads were below this level for the entire
recent time period (see Fig. 4). Interestingly, a 3-yr 15N addition
experiment at Søndre Kisselbergmosen that was performed at the
beginning of the recent time period (1997–1999), and only a few
hundred metres from our present study site, showed that an addition of 40 kg N ha1 yr1 increased the N content in surface peat
at depths of 5 and 10 cm, but not at depths of 20 and 40 cm,
indicating that living Sphagnum mosses and surface peat still had
the capacity to take up large quantities of N and thereby function
as an N filter (Nordbakken et al., 2003). This is solid evidence
that the studied peatland ecosystem was not N saturated in the
earlier time period, and that the loads of deposited N have so far
not forced the system beyond critical thresholds.
Given that our study site was exposed to lower N loads in the
recent than in the earlier time period, it was very unexpected to
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find significantly higher N concentrations in surface peat deposited in the recent period than in that deposited in the earlier
period. We can offer no obvious explanation for this result. However, a possible reason for this apparent paradox may be that a
proportionally larger part of the surface peat representing the
recent time period was made up of living and photosynthetically
active Sphagnum tissue, which is known to have generally higher
N concentrations than the dead tissues below (Damman, 1978;
Johnson & Damman, 1993). A likely reason for why proportionally more living tissue was present in the peat samples representing the recent time period is that the ground-water level was
relatively high in the years before the sampling in 2012, which in
turn may have resulted in Sphagnum hummocks that were lusher
in 2012 compared with the situation in 1995, when the peat
cores for the earlier time period were sampled. A transition to
wetter conditions in our study area is also indicated by the
general decrease in the relative abundance of the upper hummock-level plant species Empetrum nigrum L. and Calluna
vulgaris (personal observation by the corresponding author), and
an increase in Rhynchospora alba (L.) Vahl, which is an indicator
of high ground water in the peatland (see Økland, 1989; Ohlson
& Malmer, 1990). A hydrologically driven positive growth
response in the Sphagnum mosses at the hummock level following
the E. nigrum and C. vulgaris decline may also be an explanation
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Research 7

1.0
2012
0.5
0.0
−0.5
−1.0

Log carbon accumulation rate
(g m—2 yr—1)

2012
0.3

0.0

−0.3

−0.6

(b)

4

7

8

12

16

Peat age (yr)

Fig. 6 Relationship between (log) nitrogen (N) concentration and peat age
in the earlier and recent time periods. Generalized linear model (GLM)
analysis of the total data set showed significant differences in bulk density
between the time periods after the effect of peat age had been accounted
for (GLM: F1,48 = 17.750; P = 0.0001).

6

5

4

8

12

16

Peat age (yr)
Fig. 4 Relationships between vertical peat growth rate and peat age (a)
and between carbon (C) accumulation rate and peat age (b) in the earlier
and recent time periods. Generalized linear model (GLM) analysis of the
total data set (i.e. the earlier and recent time periods taken together;
n = 109) showed that both vertical peat growth and C accumulation
differed significantly between the earlier and recent time periods after the
effect of peat age had been accounted for (F1,106 = 8.786, P = 0.0038; and
F1,107 = 21.389, P < 0.0001, respectively).

Log bulk density (g dm—3)

1995

0.6

1995

Log nitrogen concentration (%)

Log peat growth rate (cm yr—1)

(a)

4.5

4.0

3.5

3.0
Lower hummock

Upper hummock

Hummock zone

Fig. 5 Box plot of peat core bulk density for lower and upper hummock
zones in the earlier (black) and recent (gray) time periods. The box length
indicates the interquartile range (IQR), the bottom of the box the 25th
percentile (first quartile (q1)), the top of the box the 75th percentile (third
quartile (q3)), and the horizontal line within the box the median value. The
lower whisker corresponds to q1 – 1.5 IQR, or to the minimum estimate,
and the upper whisker corresponds to q3 + 1.5 IQR, or to the maximum
estimate. The circles denote values outside the whisker limits. Generalized
linear model (GLM) analysis of the total data set showed significant
differences in bulk density between the time periods after the effect of
hummock zone had been accounted for (F1,106 = 5.126; P = 0.0256).
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for the increase in surface peat bulk density from the earlier to
the recent time period. However, it is important to note that our
peat cores represent surface peat at shallow depths in hummocks
only, and that our results cannot be extrapolated to deeper peat
layers and hollow communities, in which relatively wet conditions are likely to give rise to peat with a low bulk density.
Carbon accumulation rates in boreal peatlands
Our present results in combination with those of Ohlson &
Økland (1998) clearly show that the peat accumulation process
can be remarkably variable across fine spatial scales and that
Sphagnum-dominated hummocks are capable of maintaining
high rates of peat and C accumulation over several decades. As
our results are based on material from a single Sphagnum-dominated peatland in south-east Norway, care must be taken in
extrapolating the results to other peatlands in other parts of the
boreal region. However, recent long-term eddy covariance measurements of net C exchange from other parts of the boreal
region corroborate our finding that northern peatlands have functioned as strong C sinks over the last two decades. For example, a
12-yr record from a minerotrophic poor fen in northern Sweden
shows that the fen was a sink for atmospheric CO2 in each of the
12 study years, with a long-term average net ecosystem exchange
(NEE) of 58 g C m2 yr1 (Peichl et al., 2014). This figure is
slightly higher than that reported for an ombrotrophic bog near
Ottawa in Canada, in which an annual average NEE of
40 g C m2 yr1 was reported over a 6-yr study period (Roulet
et al., 2007), while measurements in a moderately rich forest fen
in western Canada revealed a significantly larger value, that is, a
5-yr NEE average of 189 g C m2 yr1 (Flanagan & Syed,
2011). Because the pine method does not take into account C
losses from the peatland through respiration, our figures for C
accumulation rates (i.e. averages of 230 and 370 g C m2 yr1
for the earlier and recent 17-yr time periods, respectively) are
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8 Research

much larger than those obtained for NEE using eddy covariance
measurements which quantify the net C exchange. Instead, our
figures obtained using the pine method are in line with eddy
covariance measurements of peatland gross ecosystem production
(GEP), which averaged 336 g C m2 yr1 and varied from 203
to 503 g C m2 yr1 in the 12-yr study by Peichl et al. (2014).
We conclude that our results, in combination with those of
other recent studies, suggest that northern peatlands have functioned as strong C sinks over the last few decades and that they
can be expected to serve as sinks for large amounts of C in the
future, thereby maintaining their important role in the global C
cycle.

Acknowledgements
The study was supported by the Norwegian University of Life
Sciences and we thank Barbro Dahlberg for assistance with the
counting of year-rings in the Scots pine samples.

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