Fujimoto et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0288-8(2019) 6:41

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Habitat evolution of a peat swamp forest
and belowground carbon sequestration
during the Holocene along the coastal
lowland in Central Sumatra, Indonesia
Kiyoshi Fujimoto1* , Masafumi Miura2,5, Shigeo Kobayashi3 and Herwint Simbolon4

Abstract
Habitat evolution of a peat swamp forest and belowground carbon storage was examined in a coastal lowland
along the eastern coast of Central Sumatra, Indonesia. Boring surveys using a hand-operated peat sampler for 32
sites and radiocarbon dating for 29 samples revealed that the peat swamp forest had begun formation by 6500 cal
BP and expanded rapidly between 6500 and 5000 cal BP and between 4000 and 2000 cal BP to nearly the present
distribution area. The thickness of the peat layer formed under the peat swamp forest reached approximately 4 m,even in the margin of the peat dome, and more than 9 m in the center. Stored carbon per unit volume of the peat
layer was estimated to be between 30 and 44 kg C m−3 in the margin of the peat dome and approximately 26
kg C m−3 in the center, which was less than the general carbon values of the mangrove peat. The relatively higher
amount of stored carbon per unit volume in the margin was possibly caused by an inflow of allochthonous carbon
from flood deposits. Belowground carbon burial rate of the peat layer was calculated as between 31 and 105
g C m−2 years−1 in the peat dome margin and between 27 and 76 g C m−2 years−1 in the center, which wereidentical to mangrove peat. These data suggest that peat swamp forests have played a significant role as places forcarbon sequestration belowground as well as mangrove forest with mangrove peat.
Keywords: Peat swamp forest, Habitat evolution, Belowground carbon sequestration, Holocene, Sumatra

Introduction
Tropical coastal swamp forests, such as mangrove forests,
peat swamp forests, and freshwater swamp forests, areconsidered to be places of significant carbon sequestration
during the Holocene (Fujimoto 2004; Dommain et al.
2011; Page et al. 2011; Alongi 2012). The coastal swamp
forests have generally expanded with the evolution ofcoastal lowland after the postglacial transgression.
The habitat evolution of mangrove forests has been revealed geomorphologically (e.g., Woodroffe 1982; Fujimoto et al. 1996, 1999a), and concrete data on
belowground carbon storage has been accumulating
since the late 1990s (Fujimoto et al. 1999b; Fujimoto
2004; Alongi 2012). Fujimoto (2004) compiled the data
* Correspondence: kfuji@nanzan-u.ac.jp
1
Faculty of Policy Studies, Nanzan University, 18 Yamazatocho, Showa-Ku,
Nagoya 466-8673, Japan
Full list of author information is available at the end of the article

on belowground carbon storage and revealed that the
Rhizophora community, which develops in the low sediment inflow area, can solely create mangrove peat
(Mochida et al. 1999) and has the highest ability to accumulate belowground carbon in mangrove communities.
Freshwater swamp forests generally develop behindthe mangrove forest on deltas with high sediment inflow,such as the Mekong and Chao Phraya deltas, and haveno rich organic sediments except the delta margins awayfrom the river channel (Nguyen et al. 2000; Tanabe et al.
2003; Fujimoto et al. 2011). On the other hand, lowland
peat swamp forests develop on the coastal lowland withlittle sediment inflow on islands, such as Sumatra andBorneo, and they form peat domes that are over 10 mthick (Gastaldo 2010; Dommain et al. 2011).
Sediments and topography of coastal peat swamp forests
were first reported by Polak (1933). Anderson (1964) was
the first to discuss in literature the development of coastal© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

(2019) 6:41

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peat swamps in the tropics using radiocarbon ages. Furukawa and Supiandi (1985) revealed the topography andstratigraphy using a 60-point boring survey along a 70 km
transect on the Batang Hari Lowland in Sumatra. Neuzil
et al. (1993) clarified the inorganic geochemistry of domed
peat in the Siak River Lowland and Bengkalis Island.
Supardi et al. (1993) revealed the geology of the same area
as Neuzil et al. (1993) and reported the depositional agesof peat layer formed under peat swamp forests. Staub andEsterle (1994) and Staub and Gastaldo (2003) revealed thesedimentary structure and peat accumulation processes
based on a large number of cores and radiocarbon ages on
the Rajang River Delta in Sarawak. Neuzil (1997) compiled
the radiocarbon ages obtained from the basal peat of fourpeat swamps in Sumatra and Kalimantan and discussed theinitiation of peat formation and the peat accumulation rate.
Palynological studies were also conducted for Holocene
peat in Malaysia and Indonesia (e.g., Anderson and Muller
1975; Haseldonckx 1977; Morley 1981; Cole et al. 2015).
These studies revealed the outline of evolution for thecoastal lowlands covered by peat swamp forests. However, there are few studies (Staub and Esterle 1994; Staub
and Gastaldo 2003) on the spatial habitat evolution oftropical peat swamp forests in a specific coastal lowland
based on intensive coring and dating. Most previous
studies obtained depositional ages of the peat layer froma few boring cores (e.g., Anderson 1964; Furukawa andSupiandi 1985; Cameron et al. 1989; Staub and Esterle
1993; Supiandi 1988; Neuzil 1997).
In terms of carbon storage, Dommain et al. (2011)
estimated peat dome initiation and peat and carbon
accumulation rates using existing data (e.g., Neuzil 1997;
Shimada et al. 2001; Page et al. 2004) obtained from
Peninsular Malaysia, Kalimantan, and Sumatra. Dommain et al. (2014) calculated stored and released carbon
from the peat swamps of Kalimantan and Sumatrausing the data compiled by Dommain et al. (2011).
Dommain et al. (2015) suggested that mechanisms ofpeat accumulation in tropical forests were possibly different from those of northern peatlands. Cobb et al.
(2017) built a numerical model of waterlogging andpeat accumulation to simulate peat dome geomorphogenesis and carbon fluxes.
However, there are few data (Cameron et al. 1989;
Neuzil 1997; Dommain et al. 2015) on carbon concentrations together with bulk density of peat deposits forthe coastal peat swamp forest, which are essential for
calculating carbon storage.
The aims of this study are to clarify the spatial habitat
evolution of a peat swamp forest on a coastal lowland
along the eastern coast of Central Sumatra, to estimate
carbon accumulation rates related to the geomorphological situation, and to discuss the differences with thevalues of mangrove peat.

Regional settings
This study was carried out on the coastal lowland in
Riau Province, Central Sumatra (Fig. 1). The coastal lowland develops along the Siak, Kampar, and IndragirRivers. Most areas of the coastal lowland of Sumatra,
which are distributed along the eastern coast, were covered by peat swamp forests until recent years. However,the coverage area dropped from 75 to 28% peatlands
between 1990 and 2010 (Miettinen et al. 2011) and to
19% in 2015 (Miettinen et al. 2016) due to development,
mainly for plantation fields. The peat swamp forests in
Riau Province decreased from 30,962 km2 (80% peatlands) in 1990 to 13,879 km2 (36%) in 2010 (Miettinen etal. 2011) and to 9812 km2 (24%) in 2015 (Miettinen et al.
2016) for plantations, such as oil palm, acacia pulp, andcoconut, and agricultural land. Small areas of mangrove
forests are distributed along the coast. Though oilfields
have been developed in the area between the Siak andKampar Rivers, the peat swamp forest in the development area has been protected, except for roads and drillsites.

Fujimoto et al. Progress in Earth and Planetary Science

Methods
Boring survey and sampling

Boring surveys using a hand-operated peat sampler(DIK-105A, Daiki, Japan), which enabled the collectionof undisturbed semicircular columnar cores 50 cm longand 5 cm diameter, excluding large woody debris, wasconducted for 32 sites between 2005 and 2008 (Fig. 1
and Table 1). Eight sites were located along the SiakRiver (SSB-1 to 8), 19 sites were along the Kampar River(SKB-1 to 19), and 3 sites were along the Indragir River(SIB-1 to 3).
SDB-1 was located around the center of the peat dome
between the Siak and Kampar Rivers, approximately 17
km from the Siak River mainstream. SDB-2 was situatedbeside an upland. Most sites were situated along therivers because we had to use a boat for field research,
except along limited roads. However, we conducted theboring survey not only along the mainstream but also
along the tributaries to collect data from as many interior sites as possible. As SDB-1 was situated on theprivate land of PT. Bumi Siak Pusako, an oil development company, we carried out the research with thepermission of the company.
Soil samples for bulk density measurement and carbon
and nitrogen contents analyses were collected at foursites, three of them located beside the Kampar River(SKB-1, 5, and 6), and another one was located aroundthe center of the peat dome between the Siak andKampar Rivers (SDB-1). The soil samples were sequentially collected, except for the top 50 cm horizon of
SKB-5 and the woody horizons of SKB-5 and SDB-1,
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using the peat sampler. The sampled depth of cores foranalysis at SKB-1, 5, and 6 are shown in Table 3. One
sample for each core was collected for analysis at thesesites. At SDB-1, each 50 cm long boring core was cut
into 10 cm sections, and one or two cut cores were used
for analysis for each layer, up to 188 cm deep, 12 cut
cores were used for the analysis of the homogenous peat
layer between 190 and 920 cm deep. The number ofsamples analyzed for each layer is shown in Table 4.

intervals. EC was measured using a portable EC meter(WM-22EP, TOA DKK, Japan) according to the methodby Yokoyama (1993), and TS analysis was conductedusing an elemental analyzer (Flash EA1112, Thermo
Electron Corporation, USA) at the University of Tokyo.
Generally, freshwater, brackish-water, and marine sediments were indicated by EC values less than 0.4 mS/cm,
0.4 to 1.2 mS/cm, and 1.3 to 3.0 mS/cm, respectively(Yokoyama 1993). TS content was also useful in identifying marine sediments. Generally, freshwater sediments
were indicated by a value of less than 0.3%, while marine
sediments were indicated by values over 0.3% (Keith andDigens 1959). We applied these values of combination
to estimate sedimentary environments.
Freshwater peat deposits formed under the peatswamp forest are occasionally underlain by mangrove
peat (Cameron et al. 1989; Dommain et al. 2015). These
two peats can be easily distinguished visually because
the freshwater peat generally contains a large amount ofwoody debris (Cameron et al. 1989, Dommain et al.
2015), though the upper central portion of the peat
dome (Cameron et al. 1989) and tip-up pool deposits
formed after tree falls (Dommain et al. 2015) are more
fibrous and less woody, whereas the mangrove peat
mainly consists of roots (Fujimoto et al. 1999a) with

Fujimoto et al. Progress in Earth and Planetary ScienceFig. 1 Map showing the study area and boring sites

Soil analyses and radiocarbon dating

Dry weight measurements for bulk density calculations
and carbon and nitrogen contents analyses were conducted at the Indonesian Soil Research Institute in
Bogor. Dry weight was measured after 24 h drying at
80 °C, which is an optimal procedure for peat and highlyorganic soils (O'Kelly 2005). Carbon and nitrogen contents were analyzed by the Walklay-Black and Kjeldahl
methods, respectively.
To estimate the sedimentary environment around theboundary between the peat layer and the underlying clay
layer, electrical conductivity (EC), and total sulfur (TS)content analyses were conducted for soil samples obtained from 4 sites along the Kampar River (SKB-1, 3, 5
and 6). The samples were collected at 20 to 30 cm

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Table 1 Description of boring sites
Loc. no.

Latitude (N)/longitude (E)Distance from river mouth (km)Land use

SKB-1~4

N 0° 08′ 39.3″/E 102° 31′ 01.3″

101

Peat swamp forest

SKB-5

N 0° 09′ 31.3″/E 102° 34′ 07.5″

95

Private house

SKB-6

N 0° 15′ 28.7″/E 102° 52′ 45.4″

60

Coconut and pineapple plantation

SKB-7

N 0° 23′ 00.5″/E 103° 06′ 32.6″

30

Mangrove forest

SKB-8

N 0° 21′ 13.5″/E 102° 20′ 58.6″

112 (48)

Peat swamp forest

SKB-9

N 0° 19′ 10.7″/E 102° 24′ 15.0″

107 (49)

Peat swamp forest

SKB-10N 0° 24′ 44.6″/E 102° 42′ 06.8″

73 (39)Peat swamp forest

SKB-11

N 0° 27′ 10.3″/E 102° 01′ 30.7″

147 (59)

Coconut plantation

SKB-12

N 0° 27′ 35.1″/E 101° 56′ 01.7″

158 (66)

Private house

SKB-13

N 0° 27′ 45.6″/E 102° 02′ 09.9″

145 (57)

Private house

SKB-14

N 0° 23′ 15.8″/E 101°41′ 49.1″

183

Private house

SKB-15

N 0° 21′ 21.5″/E 101° 43′ 58.2″

180

Oil palm plantation

SKB-16

N 0° 19′ 36.5″/E 102° 49′ 34.5″

62 (35)Peat swamp forest

SKB-17

N 0° 23′ 20.5″/E 102° 50′ 39.7″

57 (28)Peat swamp forest

SKB-18

N 0° 25′36.2″/E 102° 38′ 26.3″

79 (44)Peat swamp forest

SKB-19N 0° 18′ 40.7″/E 102° 42′ 49.6″

74 (44)Peat swamp forest

SDB-1

N 0° 40′ 23.7″/E 102° 06′ 14.3″(36)Peat swamp forest

SDB-2

N 0° 38′ 17.1″/E 101° 57′ 44.2″(49)

Private house

SIB-1

S 0° 33′ 28.9″/E 102° 59′ 11.3″

72

Private house

SIB-2

S 0° 32′ 27.9″/E 102° 56′ 47.5″

75

Private house

SIB-3

S 0° 28′ 06.0″/E 102° 43′ 32.3″

93

Private house

SSB-1

N 0° 39′ 18.8″/E 101° 38′ 19.4″

86

Secondary forest

SSB-2

N 0° 44′ 21.4″/E 101° 49′ 04.6″

67 (54)

Private house

SSB-3

N 0° 45′ 31.9″/E 102° 03′ 55.8″

53 (32)

Secondary forest

SSB-4

N 0° 53′ 28.1″/E 102° 07′ 32.0″

38 (16)

Pineapple farm

SSB-5

N 0° 46′ 06.3″/E 102° 02′ 27.1″

52 (31)

Grass land

SSB-6

N 0° 56′ 34.3″/E 102° 12′ 56.3″(5)

Oil palm plantation

SSB-7

N 0° 59′ 15.0″/E 102° 08′ 09.0″

27 (10)Oil palm plantation

SSB-8

N 0° 57′ 22.1″/E 102° 14′ 22.5″(2)

Oil palm plantation

Number in parentheses indicates distance from the nearest coast of the Panjang Strait

black bark fragments. To reinforce the visual identification, EC and TS content analyses were applied for thebasal peat at SKB-6.
Radiocarbon dating was conducted for 28 wood fragment
samples and a peat sample at Institute of Accelerator Analysis Ltd. in Japan (Table 2) using the accelerator mass spectrometry (AMS) method; however, four samples (IAA-687,
689, 690, and 691) were analyzed by the radiometricmethod. The radiocarbon ages were calibrated to calendar
year using CALIB 5.0. The calibrated ages in Table 2 show
the minimum and maximum values of 1 σ standard deviation with median possibility in parentheses. Radiocarbon
ages obtained from the basal horizon of peat swamp forest
peat reported by Supardi et al. (1993) were also calibrated
to the calendar year because the ages were indicated by 14C
age only. The calibrated ages were shown in Fig. 4.

Results
Distribution of peat layers and their formative ages

Figures 2 and 3 show the geologic columns of boringsites along the Kampar River and the Siak and IndragirRivers, including the center of peat dome between the
Siak and Kampar Rivers, and the results of radiocarbon
dating using calibrated age. Figure 4 shows the spatial
distribution of the calibrated ages obtained around thebasal horizon of the peat layer, indicating the initiation
age of peat swamp forest or mangrove forest consistingof Rhizophora sp., including the cores that were short ofthe base of the peat layer.
On the inland valley bottom lowlands, an age ofapproximately 7030 cal BP (IAAA-72998) was obtained
from the clay layer with peat (3.20 to 3.23 m depth),
which was just below the peaty clay layer overlain by the(2019) 6:41

Fujimoto et al. Progress in Earth and Planetary Science

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Table 2 Results of radiocarbon dating and calibrated ages
Lab. code

Site
no.

Depth
(cm)

Sample

δ13C

Conventional age(14C BP)

1σ calibrated age(cal BP)Moving average of calibration
curve (years)

IAAA-42316

SKB-1

345–350

Wood frag− 39.90 ± 0.74

2850 ± 40

2880-(2960)-3010

20

IAA-687

SKB-2

320–330

Wood frag− 28.92300 ± 80

2150-(2310)-2430

50

IAAA-42317

SKB-3

370–380

Wood frag− 31.53 ± 0.88

2830 ± 40

2870-(2940)-2980

20

IAAA-42427

SKB-3

430–440Wood frag− 26.91 ± 0.612750 ± 40

2790-(2840)-2870

20

IAA-689

SKB-5

367–397

Wood frag− 26.31670 ± 80

1440-(1580)-1700

20

IAA-690

SKB-5

443–470

Wood frag− 26.02160 ± 80

2060-(2160)-2310

50

IAA-691

SKB-6

330–337

Peat− 27.63690 ± 80

3920-(4030)-4140

50

IAAA-62241

SKB-8

125

Wood frag− 26.25 ± 0.691290 ± 30

1180-(1230)-1280

20

IAAA-62242

SKB-8

362

Wood frag− 25.56 ± 0.702160 ± 30

2120-(2170)-2300

20

IAAA-62243

SKB-9

165

Wood frag− 25.25 ± 0.821360 ± 30

1280-(1290)-1300

20

IAAA-62244

SKB-9

309

Wood frag− 26.67 ± 0.71

2440 ± 30

2360-(2480)-2680

20

IAAA-62245

SKB-10130

Wood frag− 29.16 ± 0.72

2330 ± 30

2340-(2350)-2350

20

IAAA-62246

SKB-10195

Wood frag− 20.11 ± 0.72

3520 ± 30

3720-(3790)-3840

20

IAAA-72997

SKB-11260

Wood frag− 38.78 ± 0.60

5690 ± 40

6410-(6470)-6500

20

IAAA-73002

SKB-15387

Wood frag− 34.00 ± 0.94

5560 ± 40

6310-(6350)-6390

20

IAAA-73003

SKB-17

315–320

Wood frag− 24.02 ± 0.872750 ± 30

2790-(2840)-2870

20

IAAA-73004

SKB-17

340–342

Wood frag− 29.41 ± 0.81

2860 ± 30

2930-(2980)-3050

20

IAAA-73005

SKB-18314–315

Wood frag− 30.16 ± 0.703560 ± 30

3830-(3860)-3900

20

IAAA-73006

SKB-18

380–385

Wood frag− 25.59 ± 0.55

3870 ± 40

4240-(4300)-4400

20

IAAA-52500

SDB-1

180–190

Wood frag− 30.33 ± 0.93

560 ± 40530-(590)-630

20

IAAA-52501

SDB-1

580–590

Wood frag− 27.61 ± 0.78

3990 ± 40

4420-(4470)-4520

20

IAAA-52502

SDB-1

915–920

Wood frag− 24.86 ± 0.745210 ± 40

5930-(5970)-5990

20

IAAA-52503

SIB-2

180–185

Wood frag− 29.21 ± 0.82

1500 ± 30

1350-(1380)-1410

20

IAAA-52504

SIB-2

197–205

Wood frag− 25.76 ± 0.88

1400 ± 40

1290-(1310)-1340

20

IAAA-52505

SIB-3

61–64

Wood frag− 20.64 ± 0.741180 ± 30

1060-(1110)-1160

20

IAAA-52506

SIB-3

146–150

Wood frag− 27.68 ± 0.961200 ± 40

1070-(1130)-1170

20

IAAA-72998

SSB-1

320–323

Wood frag− 29.07 ± 0.616130 ± 40

6950-(7030)-7150

20

IAAA-73000

SSB-4

395–400

Wood frag− 27.24 ± 0.643540 ± 30

3730-(3830)-3880

20

IAAA-73001

SSB-6

262–263

Wood frag− 21.46 ± 0.62

5720 ± 30

6460-(6510)-6550

20

peat layer at SSB-1 in the Siak River basin, and an age ofapproximately 6350 cal BP (IAAA-73002) was obtained
from the middle horizon of the peat layer (3.87 m depth)
at SKB-15 in the Kampar River basin. At SKB-11, which
is located near the mouth of the valley bottom lowlandof the Kampar River, an age of approximately 6470 cal
BP (IAAA-72997) was obtained from the basal horizonof peat layer (2.60 m depth).
Near the center of landward peat dome on the coastal
lowland between the Siak and Kampar Rivers, peat layerover 9.2 m thick was confirmed at SDB-1, and an age of approximately 5970 cal BP (IAAA-52502) was obtained fromthe deepest horizon of the core (9.15 to 9.20 m depth).
On the coastal lowland of the Siak River basin, a peat
layer over 4 m thick was formed 38 km inland from theriver mouth (SSB-4), and an age of approximately 3830

cal BP (IAAA-73000) was obtained from the deepest
horizon (3.95 to 4.00 m depth). At the site locatedapproximately 11 km seaward from SSB-4, an age ofapproximately 6510 cal BP (SSB-6 IAAA-73001) was
obtained from the clay layer overlain by the peat layer(2.62 to 2.63 m depth).
On the coastal lowland of the Kampar River basin, apeat layer less than 4.5 m thick was distributed along theriver, and ages between approximately 2170 and 2980 cal
BP were obtained from the basal horizon of the peat
layer (SKB-1: IAAA-42316, SKB-3: IAAA-42427, SKB-5:
IAA-690, SKB-9: IAAA-62244, SKB-10: IAAA-62245,
and SKB-17: IAAA-73004), though distinctly older ages,
approximately 4030 (IAA-691) and 4300 cal BP (IAAA73006), were obtained from the basal horizon of the peat
layer at SKB-6 and clayey peat layer at SKB-18,

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Fig. 2 Geologic columns of boring sites and the results of radiocarbon dating using calibrated age along the Kampar River

respectively. Sampling depths for the coring sites areshown in Table 2.
The basal peat between 2.5 and 3.4 m depth at SKB-6
was identified as mangrove peat from the main materials, i.e., fibric fine roots with distinctive black barkfragments. The wood sample for IAA-690, which was a
black bark fragment, obtained from SKB-5 is presumed
to be of mangrove origin because of this feature.
Along the Indragir River, relatively younger ages,
between approximately 1130 and 1380 cal BP (SIB-2:
IAAA-52504 and IAAA-52503, and SIB-3: IAAA-52505),

were obtained from the basal horizon of the peat layers,
their thicknesses were less than 2 m.
EC and TS analyses

Figure 5 shows the results of EC and TS content analyses. High values of EC and TS content, indicatingbrackish to marine environments, were measured for theclay layers overlain by peat layers at SKB-1, 5, and 6. At
SKB-1, over 0.5 mS/cm of EC and over 1% of TS content
were obtained from the organic clay layer below 4.0 m
depth. At SKB-5, over 0.4 mS/cm of EC was obtained

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Fig. 3 Geologic columns of boring sites and the results of radiocarbon dating using calibrated age along the Siak and Indragir Rivers and thecenter of peat dome between the Siak and Kampar Rivers

from the organic clay layer between 4.3 and 4.4 m depth,
and over 1% of TS content was obtained from the greenish gray clay layer between 4.5 and 4.8 m depth. At
SKB-6, over 0.5 mS/cm of EC was obtained from theclay layer below 3.5 m depth, and over 0.8% of TS content was obtained from the peat layer below 2.8 m depth,
which was identified as mangrove peat from the mainmaterials, with the clay layers overlain by the peat layer.
Low values of both EC and TS content, indicating
freshwater environments, were obtained from the peat
layer and upper horizon of the organic clay layer at
SKB-1 and 3, less than 0.4 mS/cm and less than 0.3%,
respectively, with the peat layer overlaying the clay
layer at SKB-5 and the peat layer overlaying mangrove
peat at SKB-6.

Belowground carbon storage and its burial rate

Tables 3 and 4 show the bulk density, carbon, and nitrogen concentrations and their masses per square meter atthe boring sites in the margin and center of the peatdome, respectively.

Carbon concentrations of peaty layers at SKB-1, 5, and6 located in the margin of peat dome were between 18
and 43%, which was lower than those at SDB-1 locatedin the center, except for the top 40 cm, those wereapproximately 55%.
At SKB-1 situated in the margin of peat dome, a3.45-m-thick peat layer underlain by clay layer was
deposited. The amount of stored carbon in the 3.45 m
deep peat layer was 102.9 kg C m−2, whereas the entire
5-m-thick deposit, which included a basal clay layer
contained 158 kg C m−2.
SKB-5, situated in the margin of peat dome, was located in a private garden, with a 4.29-m-thick peat layer,
whose top 0.2 m was dried peat and was underlain by aclay layer. Samples for analyses were collected at up to
5.0 m depth, except for the top 0.5 m because non-disturbed samples for bulk density measurement could not
be collected. Stored carbon was calculated at165.2 kg C m−2 in the peat layer between 0.5 and 4.29 m
depth and 184.1 kg C m−2 at up to 5.0 m depth.
SKB-6 that was situated in the margin of the peat
dome was located in a coconut and pineapple field with

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Fig. 4 Spatial distribution of the calibrated ages obtained around the basal horizon of peat layer. Red, orange, yellow, greenish yellow, green, andaqua colors are ages obtained from the deposit of peat swamp forest, indicating over 6000 cal BP, between 5999 and 5000 cal BP, between 4999
and 4000 cal BP, between 3999 and 3000 cal BP, between 2999 and 2000 cal BP, and between 1999 and 1000 cal BP, respectively

a peaty layer deposit of up to 3.37 m depth, though the
top 0.17 m was peaty clay, and between 2.5 and 3.37 m
was mangrove peat. Stored carbon was calculated at
97.0 kg C m−2 at up to 2.5 m depth, 127.8 kg C m−2 at
up to 3.37 m depth, including the mangrove peat layer,and 175.9 kg C m−2 at up to 4.33 m depth, including theunderlying clay layer.
At SDB-1 located in a peat swamp forest near the center of peat dome, a peat layer over 9.2 m deep accumulated. Stored carbon was calculated at 239.5 kg C m−2 at
up to 9.2 m depth.
Belowground carbon burial rates of the peat layers
were calculated to be between 31 and 105 g m−2 years−1
for the sites located in the peat dome margin andbetween 27 and 76 g m−2 years−1 for the site near thecenter of peat dome (Table 5).

Discussion
Habitat evolution of peat swamp forest

The calibrated ages indicating approximately 6350 cal BP
and approximately 6470 cal BP obtained from the middle
and bottom horizons of peat layers, which were inferred

to be peat swamp forest peat from the features, were in
the valley bottom lowland of the Kampar River basin andits mouth (Figs. 2 and 4, SKB-15 and 11 located 180 km
and 147 km inland from the mouth of the Kampar River,respectively); they show that the peat swamp forest in thevalley bottom lowland began to be formed by 6350 cal BP.
The calibrated age indicating approximately 7030 cal
BP obtained from the clay layer with peat just below thepeaty clay layer overlain by peat swamp forest peat at
SSB-1 located approximately 86 km inland from themouth of the Siak River (Figs. 2 and 4) has two possible
interpretations, i.e., there was either a period of mangrove forest or freshwater swamp forest because the clay
layer overlain by the peat swamp forest peat was deposited under blackish or freshwater environments. The
calibrated age indicating approximately 5970 cal BP
obtained from the middle horizon of the peat swamp
forest peat located approximately 36 km inland from thecoastline of the Panjang Strait (Figs. 2 and 4, SDB-1)
shows that the peat swamp forest expanded to near thecenter of present peat dome between the Siak and Kampar Rivers by 6000 cal BP.

(2019) 6:41

Page 9 of 13

The age, approximately 6510 cal BP, obtained from
SSB-6 located near the present coastline of the Panjang
Strait possibly does not indicate the initiation age of thepeat swamp forest but it does show the distribution ofmangrove forest because the age was obtained from theclay layer overlain by the peat swamp forest peat andthe site is situated in a geomorphological environment
with low flooding impact because due to its distancefrom the rivers.
Supardi et al. (1993) reported 26 radiocarbon ages obtained from the peat swamp forest peat along the SiakRiver including 7 samples obtained from the basal horizon. SK-1, 2, 5, 7, 8, 11, and SKLL-4 in Fig. 4 are sites
obtained the basal ages, which were indicated the ages
between 3950 and 5990 cal BP.
The calibrated ages obtained from the Siak River Lowland in this study and Supardi et al. (1993) indicated thatthe peat swamp forest along the Siak River expanded
between 6000 and 5000 cal BP.
On the other hand, the calibrated ages obtained fromthe bottom horizon of peat layers in the Kampar Riverbasin suggest that the peat swamp forest on the coastal
lowland expanded to the present area between 4300 and2000 cal BP (Fig. 4, SKB-1, 3, 8, 9, 10, 17, and 18).
Approximately 4000 cal BP, a Rhizophora dominantmangrove forest was distributed around SKB-6 and was

located approximately 60 km inland from the present
river mouth of the Kampar River. The calibrated age of
IAA-690 obtained from SKB-5 and the EC and TS content values of the sampling horizon (Fig. 5) suggest thatmangrove forests were distributed along the river up to
100 km inland approximately 2160 cal BP.
The calibrated ages obtained from the coastal lowlandof the Indragir River (Fig. 4, SIB-2 and 3) strongly indicate that the formation of this coastal lowland occurred
later than those in the Siak and Kampar River basins.
The ages obtained from the basal horizons of peat layers
in this study area suggested that the peat swamp forests
expanded from the north to south, i.e., from the SiakRiver Lowland to the Indragir River lowland.
In the Asia-Pacific region, relatively high sea levels approximately 6500 cal BP and 4000 cal BP and relatively
low sea levels approximately 5000 cal BP and 2000 cal
BP were reported (Tjia 1996; Sinsakul et al. 1985; Sinsakul 1992; Fujimoto et al. 1996, 1999a). In the Sunda
Shelf, a relatively high sea level approximately 4200 cal
BP was recognized (Geyh et al. 1979; Hanebuth et al.
2011), though the high level approximately 6500 cal BP
has not been determined.
The peat swamp forest initiated habitat formation by
6500 cal BP in the inland valley bottom lowlands. Theydeveloped first in the Siak River Lowland between 6000

Fujimoto et al. Progress in Earth and Planetary ScienceFig. 5 Results of EC and TS content analyses

Fujimoto et al. Progress in Earth and Planetary Science(2019) 6:41

Page 10 of 13

Table 3 Bulk density, carbon, and nitrogen concentrations and their masses per square meter at the boring sites in the margin ofpeat dome
Loc.
no.

Depth
(cm)

Sedimentary
facis

Sampled depth of coresfor analyzing (cm)Bulk density(kg m−3)

Organic C(%)

Organic N(%)

C/N

Thickness forcalculation (cm)Stored C(kg C m−2)Stored N(kg N m−2)

SKB-1

0–32

BOC

0–32

168.0

28.2

1.6

18.2

32

15.1

0.8

32–150

PC

32–50

215.8

39.6

1.1

35.4

18

15.4

0.4

50–100

121.4

18.2

0.4

44.3

100

22.1

0.5

150–200

65.7

25.7

0.5

47.5

50

8.4

0.2

200–250

188.0

18.8

0.4

50.8

50

17.6

0.3

250–300

86.2

25.1

0.5

50.2

50

10.8

0.2

300–345

85.4

35.0

0.6

54.7

45

13.5

0.2

345–350

236.3

24.7

0.5

48.4

5

2.9

0.1

350–400

124.9

16.8

0.4

47.9

50

10.5

0.2

400–424

376.6

15.3

0.3

49.2

24

13.8

0.3

150–250

250–345

345–424

SKB-5

P

OC

424–450

GBC

424–450

204.3

11.6

0.3

41.4

26

6.2

0.1

450–500

BGC

450–500

360.8

12.1

0.3

41.6

50

21.7

0.5

0–500

158.0

4.0

0–20

DP

0–20

–

–

–

–

–

20–50

P

20–50

–

–

–

–

–

50–75

DBP

50–75

160.8

34.1

0.6

58.8

25

13.7

0.2

75–140P

75–100

123.6

36.7

0.8

44.2

25

11.3

0.3

100–140

160.0

35.2

0.6

55.8

40

22.5

0.4

140–208

208–429

SKB-6

LP

LP

P

140–150

123.4

37.3

0.6

59.2

10

4.6

0.1

150–200

88.8

42.4

0.7

57.3

58

21.8

0.4

208–250

93.7

35.8

0.7

49.7

42

14.1

0.3

250–300

135.4

36.7

0.7

51.0

50

24.9

0.5

300–350

131.6

23.4

0.4

58.4

50

15.4

0.3

350–367

145.3

32.8

0.6

56.6

50

23.8

0.4

400–429

137.9

32.5

0.6

59.1

29

13.0

0.2

429–450

DBOC

429–450

162.6

38.3

0.8

46.1

21

13.1

0.3

450–500

GGC

450–500

607.1

1.9

0.1

14.8

50

5.8

0.4

50–500

184.1

3.7

0–17

PC

0–17

207.5

36.9

1.0

37.2

17

13.0

0.3

17–100

P

17–50

129.2

33.9

0.8

42.8

33

14.4

0.3

50–100

74.7

32.5

0.6

55.1

50

12.1

0.2

110–150

137.2

40.0

0.7

58.8

50

27.4

0.5

150–180

109.2

42.7

0.8

52.7

30

14.0

0.3

180–200

83.3

34.7

0.7

53.4

20

5.8

0.1

200–250

98.7

20.9

0.4

49.7

50

10.3

0.2

250–300

88.7

43.9

0.8

57.8

50

19.5

0.3

300–330

84.1

36.3

0.8

44.3

37

11.3

0.3

337–350

185.1

26.2

0.5

51.3

13

6.3

0.1

350–375

243.8

28.4

0.5

52.6

25

17.3

0.3

375–400

347.4

12.6

0.3

48.4

25

10.9

0.2

400–433

370.8

11.1

0.2

46.1

33

13.5

0.3

433–450

647.3

–

–

–

–

450–500

449.4

–

–

–

175.9

3.5

100–180

180–250

250–337FPP

MP

337–375

GCwP

375–433

GC

433–500

GGC

–
0–433

P peat, FP fibric peat, LP loose peat, MP mangrove peat, DP dried peat, DBP dark brown peat, PC peaty clay, BOC brown organic clay, DBOC dark brownorganic clay, OC organic clay, GBC grayish brown clay, BGC brownish gray clay, GC gray clay, GGC greenish gray clay, wP with peat(2019) 6:41

Fujimoto et al. Progress in Earth and Planetary Science

Page 11 of 13

Table 4 Bulk density, carbon, and nitrogen concentrations and masses per square meter at SDB-1 near the center of peat dome
Depth
(cm)

Sedimentary
facis

Number ofsamples

Bulk density(kg m−3)

Organic C(%)

Organic N(%)

C/N

Thickness forcalculation (cm)Stored C(kg C m−2)Stored N(kg N m−2)

0–40

DP

2

45.9 (3.0)

35.9 (0.6)

1.4 (0.1)

26.6 (1.7)

40

3.3

0.1

40–130LP

2

33.0 (9.6)

53.2 (1.7)

1.0 (0.2)

56.3 (9.1)

90

19.3

0.4

130–150DeP

2112.8 (47.0)

56.8 (2.9)

1.0 (0.1)

56.8 (7.2)

20

12.8

0.2

150–170

LP

1

42.8

56.5

0.8

71.5

20

4.8

0.1

170–188

DeP

1

105.8

54.7

1.9

28.2

18

10.4

0.4

188–920P

1246.1 (22.7)

55.9 (6.2)

0.8 (0.1)

68.7 (5.4)

732

188.8

2.8

0–920

239.5

3.9

P peat, DP dried peat, LP loose peat, DeP decomposed peat. Values for the layers with plural samples are mean (standard deviation)and 5000 cal BP and expanded after 4000 cal BP in the
Kampar River and the Indragir River Lowlands (Fig. 4).
Namely, the peat swamp forests in the study area seemto have formed their habitat with the sea-level changes;
the sea-level falls between 6500 and 5000 cal BP and
between 4000 and 2000 cal BP seems to have especiallyaffected the habitat expansion on the coastal lowlands
because of decrease of tidally affected area, though thesea-level fall between 6500 and 5000 cal BP has not beenrecognized in this area.
Carbon sequestration ability of coastal peat swamp forest

Stored carbon per unit volume of the peat layer at
SKB-1, 5, and 6 situated in the margin of the peat dome
were calculated at 29.8, 43.6, and 38.8 kg C m−3, respectively, while at SDB-1 situated in the center, it was 26.0
kg C m−3. These results indicated that stored carbon perunit volume of the peat layer in the center was possibly
less than that of the margin.
Carbon concentrations at SKB-1, 5, and 6 were distinctly lower than those of SDB-1; the bulk densities ofthe former sites were higher than those of later site (Tables 3 and 4), suggesting that the deposits in the peat
dome margin contained a clay fraction deposited byflooding. Some parts of the accumulated carbon in themargin were possibly originated by allochthonousorganic matter brought with flood deposits.

Belowground carbon burial rates of the peat layers ofpeat swamp forests were calculated to be between 31
and 105 g m−2 years−1 in the peat dome margin andbetween 27 and 76 g m−2 years−1 near the center of peat
dome. These values were equivalent to the data fromprevious studies conducted for coastal peat swamps in
the tropics (e.g., Neuzil 1997; Page et al. 2004).
Stored carbon per unit volume and long-term carbon
burial rate of the mangrove peat were estimated to be
between 50 and 65 kg C m−3 and between 24 and 92
g C m−2 years−1, respectively (Fujimoto 2004); though
the short-term burial rate was occasionally calculated tobe higher, for example 300 g C m−2 years−1 by Dommain
et al. (2015). Though the belowground stored carbon perunit volume in peat swamp forest was possibly lower
than that of Rhizophora forest, which is the sole mangrove genus that can create mangrove peat, the longterm carbon burial rate was almost identical. The results
suggested that peat swamp forests have played an equallysignificant role as places for belowground carbon sequestration as Rhizophora forests during the late Holocene.

Conclusions
The peat swamp forest initiated its formation by 6500
cal BP in the inland valley bottom lowlands andexpanded rapidly between 6000 and 5000 cal BP and

Table 5 Belowground carbon burial rates of peat layers
Stored C(kg C m−2)

C burialrate(g m−2 years−1)

3015

103

34

AD370 –AD1740

1370

144

105

BC210–AD370

580

35

61

BC2080–AD2005

4085

127

31

AD1360–AD2006

646

49

76

185–585

BC2520–AD1360

3880

104

27

585–917.5

BC4020–BC2520

1500

86

57

Loc.
no.

Depth
(cm)

Span of accumulation
From–to

Years

SKB-1

0–345

BC1010–AD2005

50–382

1)

382–456.5
SKB-6

0–333.5

SDB-1

0–185

SKB-5

1

The age of the horizon at 50 cm deep at SKB-5 was estimated under assumption that the peat accumulation rate was constant since AD1740

Fujimoto et al. Progress in Earth and Planetary Science(2019) 6:41

between 4000 and 2000 cal BP to nearly the present distribution area. The thickness of the peat layer formed
under the peat swamp forest reached approximately 4 m,even in the margin of the peat dome, and more than 9 m
at the center. Stored carbon per unit volume of the peat
layer was estimated to be between 30 and 44 kg C m−3
in the margin of the peat dome and approximately 26
kg C m−3 in the center, which was less than the generalamount for mangrove peat. Belowground carbon burial
rate of the peat layer was calculated to be between 31
and 105 g m−2 years−1 in the peat dome margin andbetween 27 and 76 g m− 2 years−1 in the center, whichwere identical to rates of the mangrove peat; however,
the accumulated carbon in the margin possibly contained some allochthonous carbon brought in by flooding. These data suggest that peat swamp forests haveplayed a significant role as places for belowgroundcarbon sequestration similar to mangrove forests with
mangrove peat.
Abbreviations
AMS: Accelerator mass spectrometry; EC: Electrical conductivity; TS: Total
sulfur
Acknowledgements
We thank Mr. Wardi and Mr. Oscar, researchers at the Research Center for
Biology, the Indonesian Institute of Sciences, for their assistance in our fieldresearch. We also thank PT. Bumi Siak Pusako for giving us permission to
perform on the company’s land.
Funding
This work was supported by Comprehensive promotion expenses for Global
Environmental Research, Ministry of Environment, Japanese Government(FY2003–2007, S2-2b).
Availability of data and materials
All data generated and analyzed during this study are included in thispublished article.
Authors’ contributions
KF designed the methods of field research and analyses and wrote themanuscript. MM carried out EC and TS content analyses and drew the draftsof figures. SK planed the outline of the project and proposed it to theresearch fund. HS arranged the research permission and soil analyses inIndonesia. All authors joined in the field research. MM and SK read andapproved the final manuscript.
Authors’ information
Dr. Herwint Simbolon, who was a core member of our research team,suddenly passed away in 2011. We offer him our sincerest condolences andthis paper.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Faculty of Policy Studies, Nanzan University, 18 Yamazatocho, Showa-Ku,
Nagoya 466-8673, Japan. 2Graduate School of Frontier Sciences, The
University of Tokyo, Kashiwa, Japan. 3Graduate School of Asian and African
Area Studies, Kyoto University, Kyoto, Japan. 4Research Center for Biology,

Page 12 of 13

The Indonesian Institute of Sciences, Cibinong, Indonesia. 5Present Address:
Kasukabe City Office, Kasukabe, Japan.
Received: 28 August 2018 Accepted: 8 May 2019

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