Late-Holocene expansion of a south Swedish peatland and its impact on marginal ecosystems: Evidence from dendrochronology,peat stratigraphy and palaeobotanical data

Abstract

In this study, a reconstruction of the long-term development and lateral expansion of a south Swedish peat bog was performed using a multi-proxy approach, including dendrochronology, peat stratigraphy and macrofossil and pollen analyses. By combining mapping of cross-dated subfossil trees with radiocarbon-dated peat sequences, an improved approach to reconstruction of lateral peat expansion was applied. Apart from providing approximate ages of tree burial episodes, the ring-width records offer information on hydrological variations prior to the bog expansion. New bog oak, pine and alder chronologies are presented and their potential as a dating tool for peatland expansion as well as for local to regional environmental interpretations is examined. Our tree-replication records show that increased amounts of bog trees in the central parts can be linked to drier bog-surface conditions, whereas an increase in wood remains in the marginal zone is related to enhanced preservation due to lateral bog expansion. Our reconstructions of the development of the peat deposit and associated changes in the distribution of vegetation communities provide new insight into peatland responses to climate change at the end of the ‘Holocene Thermal Maximum’ (5000–4000 cal. yr BP).

Keywords

alder dendrochronology late Holocene lateral bog expansion macrofossil analysis oak peat stratigraphy pollen analysis South Sweden subfossil trees

Introduction

Peatlands are globally important landscape elements, covering approximately 4,000,000 km² across Eurasia and North America (MacDonald et al., 2006). Improved understanding of forcing mechanisms behind changes in peatland ecosystems is urgent in order to predict future climate change (Intergovernmental Panel on Climate Change (IPCC), 2007), as, for example, carbon stored in peatlands can be made available for exchange with the atmosphere due to environmental changes (Korhola, 1994; MacDonald et al., 2006; Weckström et al., 2010; Yu et al., 2010). Peat deposits and especially raised bogs can provide long-term archives of climate and vegetation dynamics (Aaby, 1976; Barber et al., 1994), and their histories are influenced by a combination of internal and external forcing mechanisms (Robichaud and Bégin, 2009). Variations in effective moisture, related to temperature and/or precipitation, affect the hydrology and vegetation of peatlands. Records of plant communities and peat humification can therefore be used for reconstruction of past changes in bog-surface wetness (Aaby, 1976; Barber et al., 1994; Granlund, 1932; Rundgren, 2008; Van Geel, 1978).

Often peatland studies are based on single stratigraphic sequences from the central parts, offering the thickest peat layers. However, as newly formed minerotrophic fens are considerable sources of CH₄ (MacDonald et al., 2006), marginal deposits also need to be investigated to shed light on the dynamics of lateral peatland expansion and related CH₄ emissions. Studies based on basal peat dates along transects have been performed by, for example, Robichaud and Bégin (2009), Korhola (1994), Korhola et al. (2010) and Weckström et al. (2010), and some of these studies show periods of rapid lateral expansions, with mire fronts advancing several metres per year.

Peat deposits commonly offer excellent preservation conditions for plant material, such as pollen grains, plant macrofossils and wood remains, based on which changes in local vegetation and hence climate can be inferred. Pollen records provide information on local to regional vegetation changes (Bradshaw and Lindbladh, 2005; Huntley and Birks, 1983), whereas plant macrofossil records primarily reflect local vegetation dynamics and offer higher taxonomic precision (Dudová et al., 2013). Periods of relatively warm and dry climatic conditions may sometimes allow trees to establish on peat bogs, and tree horizons buried in peat deposits are clear indications of past episodes of decreased bog-surface wetness (Eckstein et al., 2009; Edvardsson et al., 2012a, 2012b; Leuschner et al., 2002). Pioneer studies describing layers of wood in Scandinavian peat deposits and their relationships to climate change were performed already 150 years ago, as reviewed by Birks and Seppä (2010) and Nielsen and Helama (2012). Since the establishment of dendrochronology in the early 20th century (Douglass, 1921), considerable methodological improvements have taken place, and annually resolved ring-width (RW) records are now frequently used in palaeoclimatology (Fritts, 1976; Hughes et al., 2011; Lindbladh et al., 2013). Studies based on subfossil bog trees from southern Sweden have so far focused on the large quantities of Scots pine (Pinus sylvestris L.) that are frequently encountered in this region (Edvardsson, 2010; Edvardsson et al., 2012a, 2012b; Gunnarson, 1999). However, initial dendrochronological studies of subfossil bog trees were mainly based on oak (Quercus robur L.), and multi-millennial chronologies have been developed in Germany (Leuschner et al., 1987, 2002) and Ireland (Pilcher et al., 1977, 1984). Oak trunks buried in peat deposits also occur in southern Scandinavia, and the material has potential, as shown by extensive Danish bog oak chronologies, which together cover about 5000 years from 6132 bc to ad 430 (Christensen, 2007; Christensen, unpublished data).

In connection with a dendroclimatic study of subfossil bog-pine populations recovered from ombrotrophic peat deposits in southern Sweden (Edvardsson et al., 2012a, 2012b), well-preserved trunks of oak, ash (Fraxinus excelsior L.) and alder (Alnus glutinosa L.) were found at the peat bog Viss Mosse. These deciduous trees probably represent a forest ecosystem that occurred in the marginal parts of the peat deposit and which was buried by lateral expansion of the peat bog in response to a transition to colder and moister climatic conditions at the end of the ‘Holocene Thermal Maximum’ (HTM). Here, we present tree-ring records based on oak and alder in combination with new and updated chronologies based on the pine population that grew in the central part of the bog during the HTM. Peat stratigraphic records, including pollen and plant macrofossil data, are used to provide supportive information on the peatland development and to place the subfossil wood material in the context of long-term vegetation dynamics. The aim of the study is to provide a well-dated, spatial and temporal visualization of the late-Holocene lateral expansion of a south Swedish peat bog, to assess the impacts of climate change on bog-tree species and population dynamics and to evaluate the potential of the new bog-oak material for extended studies. The reconstruction of the lateral expansion of Viss Mosse may also contribute to other research areas, such as the role of peatlands in the global carbon cycle and regional climate variability in southern Scandinavia.

Material and methods

Site description and fieldwork

Viss Mosse (55°51′N; 13°49′E) is a south Swedish raised bog located on the bedrock ridge Linderödsåsen (approximately 173 m a.s.l.; Figure 1). The bog is about 2 km² and subject to extensive peat mining since the early 20th century. During fieldwork campaigns in 2010, 2011 and 2012, cross sections from 40 oaks, 1 ash and 29 alders were collected with a chainsaw. In total, 43 of these trees were found in situ, whereas the remaining trees were moved from their primary growth position to enable further peat cutting at the bog surface (Figure 2). Here, the expression in situ is used in a broader sense and includes rooted stumps, and also trunks from fallen trees. In addition to the deciduous trees, 172 pine trees were sampled. Most of these were described by Edvardsson et al. (2012b). The growth positions of in situ trees were obtained with a handheld global positioning system (GPS) and marked on a map during fieldwork (Figure 1). To enable reconstruction of the timing of tree establishment and die-off phases, information about the presence of pith, bark and sampling distance from the roots was recorded for each sample.

Figure 1.

(a) The location of the peat bog Viss Mosse, southern Sweden. (b) Locations of sampled trees, with dots representing in situ trees and circles representing trees removed from their original growth positions. Till hummocks protruding through the peat deposit are shown with dashed black lines. The square represents the area enlarged to the left, the grey border shows the area used for peat mining and the shaded area represents total extension of the bog. (c) Close-up of the eastern part of the bog and the till hummock is shown in Figure 7. Locations and directions of in situ tree trunks (arrowheads point towards the top of the trees) and chronology codes show where the different groups of trees were found. The shading shows the depth of the remaining organic deposits and the white dots indicate coring sites for peat sequences (PS1, PS2 and PS3). The transect shown in Figure 7 is indicated by the white lines in (b) and (c).

Figure 2.

(a) In situ trunks of alder (dashed lines) and oak (white lines) found at the excavated peat surface. The arrow shows the position of PS3. (b) The longest peat sequence (PS1) was collected from a remnant peat bank following more extensive peat mining of the surrounding peat. The alder trunk in the ditch shows the stratigraphic level where the majority of oaks and alders were found. (c) Deposits of stumps and trunks removed from their original growth positions, and these trees were mainly used to improve the quality of the RW records.

Peat sequences were collected adjacent to the sampled trunks with a Russian peat sampler (7 cm in diameter and 1 m in length) at three locations from the eastern part where most of the deciduous trees were found (Figure 1c). Distances between the coring points and their relative altitudes were measured with a levelling instrument. To get an impression of the total thickness of the organic layers, the distance from the present-day bog surface to the mineral soil was measured with a probe along transects.

Development of RW records

In order to avoid drying out and deformation of the soft deciduous tree samples, all the wood samples were stored at 4°C after fieldwork and thereafter frozen during surface preparation. RW series of individual radii were created based on measurement of annual rings with a precision of 0.01 mm, using a LINTAB measuring table connected to a stereomicroscope, and a computer, using the TSAP (Rinn, 1996) and CATRAS (Aniol, 1983) software. In order to identify wedging rings and possible measuring errors, at least two radii were measured for each sample. Two statistical tests were applied to evaluate similarities between individual RW series: coefficient of parallel run (Eckstein and Bauch, 1969) and Student’s t-test (Baillie and Pilcher, 1973). These statistical analyses are commonly used in dendrochronology in combination with visual comparisons when assessing the significance of correspondence between two RW series, and this approach is generally referred to as cross-dating. First, the radii from individual samples were cross-dated, and if matching, they were averaged into RW series of individual trees. Subsequently, the RW series from all trees were cross-dated against each other, and series where cross-dating was satisfactorily established (Wigley et al., 1987) were then averaged into RW records.

In the next step, the cross-dating and measurement quality, as well as the strength of the RW records, were evaluated using the software COFECHA (Holmes, 1983). To minimize the influence of non-climatic variations and trends related to, for example, age, height within the stem and geometry, the RW data were standardized and transformed into dimensionless RW indices (Cook and Kairiukstis, 1990; Fritts, 1976). Since some of the trees showed a negative exponential growth trend with thick growth rings close to the pith and other trees showed narrow rings during both the establishment and the concluding years, a flexible standardization method was chosen. The standardization was performed using Friedman’s variable span smoother (Friedman, 1984) in the software ARSTAN_41d (Cook and Krusic, 2006). To assess the reliability of the RW records, the expressed population signal (EPS) was calculated using ARSTAN, and the limit at which the records were considered to be reliable and well replicated was set to EPS ≥0.85 (Wigley et al., 1984). Finally, cross-dating tests between the Viss Mosse RW records and available dated RW chronologies were made to assign ages to the new material.

Peat stratigraphic analyses

Three peat sequences (PS1, PS2 and PS3) were obtained from fieldwork. Stratigraphic units were identified based on visible differences in peat composition and degree of humification. Thereafter, the sequences were sliced into 2 cm segments with some exceptions when 3 cm slices were taken to avoid crossing boundaries between stratigraphic units. Loss on ignition (LOI) and organic bulk density (OBD; Heiri et al., 2001) were determined on the samples to support the initial classifications of variations in peat humification and stratigraphic units.

Radiocarbon dating

Wood and peat samples were dated by radiocarbon (¹⁴C) to give the RW records approximate calendar ages and to develop age–depth models based peat accumulation records. Each wood sample contained 11 growth rings and was, in general, taken more than 50 years from the pith. To avoid contamination by younger material, roots were removed from the bulk peat samples. The samples were dated using accelerator mass spectrometry (AMS) at the Radiocarbon Dating Laboratory, Lund University, followed by calibration and age–depth modelling using the OxCal 4.1 software (Bronk Ramsey, 2001, 2008). For wood samples with known relative ages and peat samples with known stratigraphic contexts, D or P_Sequence analysis (Bronk Ramsey et al., 2001) was used to improve the accuracy and to tie different radiocarbon dates to more narrow intervals on the IntCal09 radiocarbon calibration dataset (Reimer et al., 2009). The ages used are the calculated mean values (µ) from the R_Date, D_Sequence or P_Sequence probability curves, and the margins of error are the uncertainties at the 95.4% probability level.

In total, 33 wood samples and 13 peat samples were radiocarbon dated. As most of the oak samples cross-date among each other, only two or three samples from each oak record (10 samples in total) were needed to date 29 oak trees. As the alders were difficult to cross-date, 23 radiocarbon dates from 21 individual trunks were obtained. The three peat sequences were sampled close to stratigraphic boundaries and radiocarbon dated to place the wood material in a stratigraphic context and to assess the bog development on a centennial to millennial timescale.

Pollen and plant macrofossil analyses

Samples for pollen and macrofossil analyses were taken from PS1 and PS3 as these two peat sequences in combination were believed to generate the most complete record of the development in the study area, and as they could be connected to the stratigraphic level of the wood horizon. Samples for pollen analysis, 1 cm³ in volume, were taken at 2 cm intervals from PS3 while the upper part of PS1 was sampled at every 5 cm. Pollen sample preparation followed the standard acetolysis method (Berglund and Ralska-Jasiewiczowa, 1986). Lycopodium spores were added to calculate pollen concentrations (Stockmarr, 1971). At least 1000 terrestrial pollen grains (arboreal pollen (AP) + non-arboreal pollen (NAP)) were counted at each level using a microscope. Pollen, spores, algae, charcoal and other microfossils were calculated as percentages of the total terrestrial pollen sum. Local pollen assemblage zones (LPAZs) were determined using binary splitting by the sum-of-squares method with the Psimpoll 4.10 program, and the significance of the determined zones was tested using a broken-stick model (Bennett, 1996).

Seven samples, with a thickness of 1 cm and a volume of 10–15 cm³, were taken for plant macrofossil analysis. Five samples were taken at every 10 cm from PS3, corresponding to the levels at and below the tree trunks, while two samples from PS1 were taken at levels above the trees. Each fresh sample was weighed, and the volume was determined by immersion in a known volume of water. The samples were soaked overnight in 5% NaOH and washed through a 250-µm sieve. Macroscopic plant remains retained on the sieve were stored in a known volume of water, which enabled volume determination of the subsamples in order to quantify the main peat components (Janssens, 1983; Van der Putten et al., 2004). The samples were systematically examined using a stereomicroscope, and seeds, fruits and other non-dominant remains were picked out and counted in the complete samples. The absolute number of each taxon was calibrated for a standard sample volume. Both pollen and macrofossil diagrams were compiled using the Tilia and TGView software (Grimm, 2007). Ages of the pollen and macrofossil samples were calculated from the radiocarbon-based age–depth models.

Results

RW records

Four oak RW records (OR1, OR2, OR3 and OR4) were developed based on 29 trunks. OR1 and OR2 contain two and three samples, respectively, and were radiocarbon dated to 7786–7545 ± 66 and 7138–6847 ± 112 cal. yr BP (Table 1). The 326-year OR3 record was developed from 15 samples (Figure 3), eight of which were found in situ in the eastern or northern parts of the bog on up to 0.6 m of organic deposits. The material was absolutely dated to 1725–1399 bc by means of cross-dating against a Danish bog-oak chronology (t value = 6.7; Glk = 66 (p < 0.01); Christensen, 2007), consistent with a radiocarbon-based age of 3634–3308 ± 63 cal. yr BP. OR4 was constructed from nine in situ trees found on a till hummock in the eastern part of the bog and was dated by radiocarbon to 3231–3035 ± 113 cal. yr BP (Figure 3 and Table 1).

Table 1.

Radiocarbon dating of RW records.

Sample number	Number of years between samples	Lab number	Radiocarbon age (¹⁴C age BP)	Calibrated age cal. yr BP	Modelled age (cal. yr BP/bc)
Oak record 1 (OR1)
TM719		LuS 10254	6945 ± 55	7801 ± 126	7744/5795 ± 66
TM719	100	LuS 10255	6790 ± 55	7630 ± 106	7644/5695 ± 66
Oak record 2 (OR2)
TM692		LuS 10252	6190 ± 60	7100 ± 152	7112/5163 ± 112
TM692	200	LuS 10253	6080 ± 55	6976 ± 181	6909/4960 ± 112
Oak record 3 (OR3)
TM646		LuS 9450	3275 ± 50	3511 ± 119	3525/1576 ± 63
TM623	100	LuS 9451	3205 ± 50	3454 ± 105	3425/1476 ± 63
TM623	100	LuS 9452	3145 ± 50	3360 ± 109	3325/1376 ± 63
Oak record 4 (OR4)
TM645		LuS 9447	3055 ± 50	3231 ± 150	3182/1233 ± 113
TM645	100	LuS 9448	2920 ± 50	3070 ± 172	3082/1133 ± 113
TM655	50	LuS 9449	2815 ± 50	2929 ± 138	3032/1083 ± 113
Alder record 1 (AR1)
TM625		LuS 10185	4050 ± 50	4613 ± 195	4473/2524 ± 38
TM624	28	LuS 9455	3990 ± 50	4537 ± 245	4445/2496 ± 38
TM626	62	LuS 10186	3855 ± 50	4260 ± 159	4383/2434 ± 38
TM624	38	LuS 9456	3865 ± 50	4285 ± 135	4345/2396 ± 38
TM658	10	LuS 10187	3930 ± 50	4351 ± 169	4335/2386 ± 38
TM687	32	LuS 10188	3900 ± 50	4332 ± 177	4303/2354 ± 38
Alder record 2 (AR2)
TM622		LuS 9453	3305 ± 50	3545 ± 138	3463/1514 ± 78
TM622	100	LuS 9454	3070 ± 50	3238 ± 155	3363/1414 ± 78

Figure 3.

RW chronologies of oak (OR3 and OR4), pine (PR4) and alder (AR1). The black curves show detrended RW indices and the grey curves show tree replication. The black lines show tree overlap and the grey extensions indicate approximate numbers of missing years to pith and bark. The dashed line shows the overlap of the dated ash tree.

One RW record (AR1), containing five alders, was constructed and radiocarbon dated to 4541–4293 ± 38 cal. yr BP (Table 1). Due to non-significant cross-matches, no other well-replicated and qualitative alder RW records were obtained. Therefore, 21 individual trunks were dated by radiocarbon (Tables 1 and 2), of which AR2 yielded an approximate age of 3328 ± 78 cal. yr BP of the death of the tree above PS3 (Figures 2a and 5), which is in agreement with the uppermost peat sample from PS3 (3332 ± 114 cal. yr BP).

Table 2.

Radiocarbon dating of alders.

Sample number	Ring number (from inner-most ring)	Lab number	Radiocarbon age (¹⁴C age BP)	Calibrated (cal. yr BP/bc)
TM629	21 ± 5	LuS 10184	3910 ± 50	4336/2387 ± 178
TM494	165 ± 5	LuS 10189	6380 ± 50	7301/5352 ± 123
TM614	145 ± 5	LuS 10190	3190 ± 50	3416/1467 ± 142
TM627	35 ± 5	LuS 10191	4065 ± 50	4647/2668 ± 195
TM628	50 ± 5	LuS 10192	3815 ± 50	4248/2299 ± 163
TM632	125 ± 5	LuS 10193	3940 ± 50	4381/2432 ± 141
TM636	10 ± 5	LuS 10194	4030 ± 55	4585/2636 ± 227
TM643	135 ± 5	LuS 10195	3150 ± 50	3364/1415 ± 108
TM653	25 ± 5	LuS 10196	2990 ± 45	3171/1222 ± 165
TM663	105 ± 5	LuS 10197	3270 ± 45	3501/1552 ± 112
TM674	113 ± 5	LuS 10198	6865 ± 60	7711/5762 ± 119
TM675	70 ± 5	LuS 10199	6840 ± 50	7687/5738 ± 99
TM685	170 ± 5	LuS 10200	3940 ± 55	4356/2407 ± 171
TM686	140 ± 5	LuS 10201	3840 ± 50	4255/2306 ± 160
TM689	90 ± 5	LuS 10202	3150 ± 50	3364/1415 ± 108

Table 3.

Radiocarbon dating of the peat samples.

Depth (cm)	Lab number	Radiocarbon age (¹⁴C age BP)	Calibrated age (cal. yr BP)
Peat sequence 1 (PS1)
183–182	LuS 9693	7520 ± 55	8306 ± 107
167–166	LuS 9694	3635 ± 50	3985 ± 152
162	LuS 10035	3780 ± 50	4189 ± 199
115–114	LuS 9695	2875 ± 50	3034 ± 167
102	LuS 10034	2870 ± 50	3012 ± 150
76	LuS 10033	2480 ± 50	2542 ± 179
15–14	LuS 9696	1885 ± 50	1820 ± 112
Peat sequence 2 (PS2)
43–42	LuS 9697	8125 ± 50	9038 ± 247
32	LuS 10036	5155 ± 50	5873 ± 127
5–4	LuS 9698	3420 ± 50	3697 ± 135
Peat sequence 3 (PS3)
48–47	LuS 9699	7130 ± 55	7942 ± 107
35	LuS 10037	4110 ± 50	4637 ± 188
3–2	LuS 9700	3120 ± 50	3332 ± 114

The majority of the pine material from Viss Mosse (PR1, PR2 and PR3) has previously been described by Edvardsson et al. (2012b). PR1 and PR2 were dated by radiocarbon to 8081–7896 ± 88 and 7415–7186 ± 74 cal. yr BP, respectively, and PR3 was absolutely dated to 5284–4559 bc (7233–6508 yr BP) by cross-dating against German bog-pine chronologies (Edvardsson et al., 2012a, 2012b). PR1 and PR3 have been extended to encompass 7 and 90 trees, respectively, and a new 296-year RW record (PR4) was developed from 11 samples, covering the period 4236–3941 bc (6185–5890 yr BP). This RW record was cross-dated (t value = 5.6; Glk = 65 (p < 0.01)) against an absolutely dated pine record from Hällarydsmossen (Edvardsson et al., 2012a).

Peat stratigraphy and peat accumulation

The most complete peat sequence is 187 cm in length and referred to as PS1 (Figure 4). The two shorter sequences, 46 cm and 60 cm, respectively, are referred to as PS2 and PS3. Eight stratigraphic units were identified in PS1 of which the lowermost three could be correlated with PS2 and PS3 (Figure 4). Rapid increases in peat accumulation were recorded between 5000 and 4000 cal. yr BP in all three peat sequences.

Figure 4.

(a) The three analysed peat sequences (PS1, PS2 and PS3) from Viss Mosse. The arrows show depths where samples for radiocarbon dating were taken and corresponding laboratory numbers (Tables 1 –3). The black dots and squares show where samples for pollen and macrofossil analyses were taken and the line shows the surface of the peat bog after mining. (b) The age–depth model is based on depths and radiocarbon ages from PS1. Periods covered by dated trees are shown below the age–depth model, and the abbreviations indicate the chronology representing the main part of the wood material.

Pollen and plant macrofossil data

In all, 30 pollen samples and 7 plant macrofossil samples from PS1 and PS3 were analysed (Figure 5), and the data were plotted against a common age scale based on a combination of lithostratigraphic correlation and independent age–depth models (Figure 4). Based on terrestrial pollen data, three statistically significant LPAZs (referred to as V1–3) were identified (Figure 5).

Figure 5.

(a) Pollen diagram based on samples from PS1 and PS3. Ages are estimated from the age–depth model (Figure 4). The filled curves represent percentage frequencies and the hollow curves 10× exaggerations of the horizontal scale. Solid lines mark the boundaries of local pollen assemblage zones (V1–3), and the dotted lines indicate levels analysed for macroscopic plant remains. The shaded areas represent periods covered by dated trees, and the abbreviations indicate the chronology representing the main part of the wood material. (b) Record of macroscopic plant remains based on samples from PS1 and PS3. Absolute numbers of each taxon are shown, except for Carex sp. root remains, where the dots refer to presence only. Note different abundance scales for some taxa. Taxa that are represented by curves show the main peat components and summarize in some cases the taxa also represented by bars. For taxa marked with (n), the total numbers of plant remains in each sample are shown, while for the remaining taxa quantifications based on 20 mL subsamples are provided. The horizontal line indicates the boundary between the two lower pollen zones (V1 and V2).

Throughout the lower zone (V1), which covers the period 7500–3300 cal. yr BP, pollen grains from both boreal and nemoral tree taxa are present. Alder was the dominating tree component (20–40%) with a maximum about 4500 cal. yr BP. Pine, hazel (Corylus) and birch (Betula) were subdominant (10%), whereas oak and lime (Tilia) were present in lower abundances in the pollen record. The macrofossil data from the corresponding period are clearly dominated by alder. A shift from alder root remains to alder above-ground remains occurred around 4500 cal. yr BP. Leaf remains of birch are present from c. 4000 cal. yr BP. The amount of charcoal is generally low (<5%) except for distinct peaks corresponding to c. 4600 and 3600 cal. yr BP, respectively. Changes in the vegetation are recorded after the second charcoal peak at c. 3600 cal. yr BP. Ferns decrease in the favour of grasses and herbs.

In the middle zone (V2), which covers the period between 3300 and 2800 cal. yr BP, birch is clearly the dominating tree component with pollen values reaching 30%. Alder is present with values around 20%, whereas pine and oak appear at lower frequencies. Sedges (Cyperaceae) occur with values up to 20%. Macrofossils of alder are present but at lower concentrations than in the previous zone (Figure 5). Besides leaf remains, additional above-ground birch remains such as seeds and female catkins occur after 3300 cal. yr BP. At this time, a clear shift in the main peat components occurs, to high amounts of monocots, Sphagnum section Acutifolia and epidermis of Eriophorium vaginatum.

In the upper zone (V3), which covers the period between 2800 and 1500 cal. yr BP, the dominating tree taxa are birch, alder and hazel (c. 20%), whereas oak and pine are present at frequencies around 5%. Heather (Calluna) shows increasing frequencies. The amount of Sphagnum spores increases throughout the zone and a charcoal peak occurs at c. 2300 cal. yr BP (Figure 5).

Discussion

Peatland development

During the early Holocene, a lake existed in the topographic depression occupied by Viss Mosse, and a previously published stratigraphic sequence from the central part of the bog shows a normal hydroseral succession from a lake to a raised bog (Edvardsson et al., 2012b). The lake developed into a wetland where till hummocks, consisting of boulder-rich till, formed small solid-ground islands where trees and shrubs could establish. During archaeological excavations on till hummocks in the central parts of the bog flint tools have been found, dating to about 9000 cal. yr BP (L. Larsson and A. Sjöström, personal communication, 2012), which shows that these islands were repeatedly visited by humans. In the central parts of the bog, Sphagnum mosses dated to 7700 cal. yr BP provide an approximate age of the transition to a raised bog environment (Edvardsson et al., 2012b).

The oldest dated trees were found in the central part of the bog, while the younger material was found closer to the marginal zone. Given that the wood material is very well preserved, it can be assumed that the peat burial process was relatively fast, which enables reconstruction of the lateral expansion of the peatland based on dating of in situ trees preserved within the peat. The oldest dated pine trees grew under relatively dry and stable conditions on central till hummocks (Edvardsson et al., 2012b) and were overgrown by peat between 8000 and 7500 cal. yr BP (Figures 6 and 7a), whereas pine trees that grew on the bog surface during the mid-Holocene were overgrown by peat between 7000 and 5900 cal. yr BP (Figures 6 and 7a). The pollen record shows that birch, hazel and lime were also present in the area (Figure 5), but apart from a few birch trunks, no remains of these tree taxa were encountered in the peat, which indicates that they grew in the surroundings of the bog. Alders were present in the wet marginal zone of the bog, while oak trees probably grew on till hummocks in the bog or in its surroundings. The pine trees that established between 7400 and 5900 cal. yr BP grew on several metres of peat (Edvardsson et al., 2012b), indicating that most of the Viss Mosse depression was already filled by thick organic layers prior to 5900 cal. yr BP. No in situ trees that died earlier than 5900 cal. yr BP have been detected outside the white dashed line in Figure 7a. It can therefore be assumed that the preservation conditions outside this area were poor and that peat growth mainly occurred vertically prior to 5900 cal. yr BP. The peat stratigraphy together with pollen and macrofossil data show that the easternmost part of the bog was characterized by the seasonal presence of open ponds and that peat accumulation rates were low during the mid-Holocene. Aquatic plants that tolerate brief drought periods (e.g. Rumex aquaticus, Typha, Sparganium and Alisma plantago-aquatica) grew in open water bodies, whereas adjacent waterlogged areas were dominated by sedges (Cyperaceae) and ferns (Polypodiaceae; Figure 7b).

Figure 6.

Temporal distributions of all dated tree remains from Viss Mosse. The dotted curve shows the distribution of pine samples from Hällarydsmossen (Edvardsson et al., 2012a) as discussed in the text. The lines above show the approximate lengths of periods discussed. PR1 and OR2 represent tree populations that grew on till hummocks in the central part of the bog. PR3, PR4 and the Hällarydsmossen pines were growing on the surface of the two raised bogs. AR1 represents a population, which grew in a fen area east of the raised bog, OR3, and several alders also grew in this area. OR4 grew on a till hummock east of the bog. OR3 and OR4 were preserved as the bog expanded laterally and buried these tree populations.

Figure 7.

(a) Lateral expansion of Viss Mosse based on dated in situ trees. The codes show where most of the trees from each chronology were found. The ages show when trees were buried by peat in different parts of the bog. The solid lines show the outermost boundary where the dated trees with corresponding ages were found. The dashed lines show where dated trees were found, but these trees may have been moved from their original growth positions. The white dashed line shows the approximate size of the bog at c. 5900 cal. yr BP as discussed in the text. (b–f) Bog development along the transect shown as a white line in the uppermost panel. The transparent tree taxa (names in italics) represent conditions as inferred from the pollen record, while the coloured tree taxa are based on trunks recovered.

A change from slow vertical to increased lateral peat accumulation was recorded after c. 5000 cal. yr BP, when lateral peat expansion and tree burial towards the east and north started (Figure 7a). Well-preserved in situ trunks show that numerous alders grew in the eastern part of the bog between 4800 and 4300 cal. yr BP. These trees were rapidly buried and preserved when the bog expanded laterally, which generated improved preservation conditions. Some of these alders were unusually old and large (diameter >30 cm, trunks >10 m in length), and in some cases, more than 200 annual growth rings were recorded. The local presence of alders is also supported by the macrofossil data (Figures 5 and 7c). Apart from providing approximate ages of the burial of trees in the peat, the tree-ring data provide valuable information on the conditions in the area before the trees died. Many alders show initially strong growth, but suffered from severe growth conditions during their final 100–120 years, probably due to the rapidly accumulating woody fen peat. The local conditions were probably too wet for tree establishment between 4200 and 3500 cal. yr BP as no wood material from this period has been found (Figure 6), and the relatively rapid accumulation rate of the peat continued (Figure 4). The pollen record shows that the areas surrounding the bog were dominated by mixed broadleaved forests composed of lime, hazel, oak, elm (Ulmus) and ash with the presence of boreal taxa such as birch and pine. The amount of pine and lime clearly decreased from about 5000 cal. yr BP and onwards.

Oak trees dating to the period between 3700 and 3300 cal. yr BP have been found both in the northern and eastern parts of the bog (Figures 1 and 7d). There is also a cross-correlation between these oaks and one ash tree from the same part of the bog (Figure 3). Similar correlations between these two species have previously been demonstrated by Sass-Klaassen and Hanraets (2006), providing evidence of common growth responses of oak and ash under moist conditions. Apart from the oaks and the ash tree, trunks from alder were discovered in the eastern fen area. These trees were buried about 3300 cal. yr BP when peat accumulated in the eastern part of the bog, whereas the oak trees in the northern part were buried by a lateral bog expansion. The pollen data indicate a decrease in alder, presence of ash and hazel and an increase in birch. Layers containing high amounts of charcoal indicate fire events at or close to the sampled area of the peat bog at about 4600, 3600 and 2300 cal. yr BP (Figure 5), although no fire scars have been observed on the sampled trunks of oak and alder. The fire at about 3600 cal. yr BP was followed by a rapid increase in birch pollen frequency. The pollen record also shows decreasing frequencies of oak and alder immediately after the inferred fire. It is unclear whether the more frequently occurring charcoal layers in the upper part of the stratigraphy is due to increased anthropogenic activity in the area. However, the influence of fires in the northern part of the bog can be expected to be negligible because of the wet conditions normally characterizing a fen bog.

The youngest dated oak population (OR4) established on a till hummock between the eastern fen and the raised bog area about 3200 cal. yr BP (Figures 1 and 7e). The sampled trunks were relatively large, up to 17 m long and >40 cm in diameter. These trees grew on a relatively dry and stable substrate and were preserved when the till hummock was overgrown by peat about 3000 cal. yr BP. The average growth rate of these trees was in general higher than that of the neighbouring oaks in the fen area (OR3), 1.28 mm/yr compared with 0.97 mm/yr. Interestingly, the oak trees from the fen peat could be cross-dated against Danish bog oaks (Christensen, 2007), whereas the oak trees on the till hummock could only be dated by radiocarbon. The annual growth variability of the two oak populations may therefore have been limited by different factors, with the trees in the fen area responding more strongly to water-level variations than the trees growing on till.

The pollen record shows an increase in oak between c. 3000 and 2500 cal. yr BP, which likely represents a regional vegetation change, as no trunks or macrofossils younger than 3000 cal. yr. BP were recorded in the peat. About 3000 cal. yr BP, a shift from minerotrophic to ombrotrophic state has been recorded in the eastern fen area, which was transformed into a Carex–Sphagnum dominated raised-bog environment. The pollen record shows a shift towards a more open vegetation with less trees and shrubs (Figure 5). A general acidification took place as the thickness of the peat deposit increased and typical raised-bog taxa such as heather (Calluna) and Sphagnum established (Figures 5 and 7f). In general, aquatic plants and ferns (Polypodiaceae) decreased, while more acid-tolerant taxa such as birch (Betula pubescens) and sedges (Cyperaceae) persisted. The vegetation composition in the surrounding areas shifted as well, from a rich mixed broadleaved forest to an oak- and hazel-dominated forest with reduced amounts of elm, ash and lime. No trees younger than 3000 cal. yr BP have been found, although younger populations may be preserved in the remaining peat deposits to the north and east.

Regional palaeoclimatic implications

The analysed peat succession covers the HTM and the transition to the late Holocene. The HTM was a period of relatively warm and dry conditions, which is dated to c. 8000–4500 cal. yr BP in Fennoscandia (Seppä et al., 2009; Snowball et al., 2004), and offered conditions enabling widespread bog-pine establishment in southern Sweden (Edvardsson et al., 2012b; Figure 6). The HTM was followed by a shift to moister and colder conditions, often referred to as the Neoglacial transition (Nesje et al., 1991; Wanner et al., 2008). In south-central Sweden, this transition phase has been dated to between 4600 and 3400 cal. yr BP (Jessen et al., 2005). Temperature reconstructions show that the mean annual temperature decreased between 4500 and 2000 cal. yr BP (Seppä et al., 2005) and lake-level reconstructions suggest a general lake-level rise (Digerfeldt, 1988). The replication record from Viss Mosse reflects tree-population dynamics related to the shift from the HTM to the late Holocene (Figure 6). During the HTM, a widespread pine establishment took place on the bog surface, consistent with generally high pollen frequencies of pine. However, no trees of corresponding age have been found in the marginal zone of the bog, which is probably a result of poor preservation conditions due to low peat accumulation rates. Following the shift towards wetter conditions, no trees established on the bog surface and trees growing in the marginal zone were preserved because of increasing peat accumulation and lateral peatland expansion (Figures 6 and 7).

Increased lateral expansion of peatlands has been observed in the Northern Hemisphere between 5000 and 3000 cal. yr BP (Korhola, 1994; Korhola et al., 2010; Weckström et al., 2010), consistent with our data from Viss Mosse (Figures 4 and 7a). The significant rise in atmospheric CH₄ concentration over the last 5000 years (Blunier et al., 1995) has sometimes been linked to this large-scale peat expansion (Korhola et al., 2010). It is therefore important to increase our knowledge about peat accumulation patterns, related variations in carbon balance and potential responses to future climate change.

Concluding remarks

As demonstrated by this study, reconstructions of Holocene peatland dynamics, particularly lateral expansion phases, can successfully be based on combinations of dendrochronology and peat stratigraphy. Die-off phases obtained from dated in situ trees provided information on the timing and location of tree burial due to peatland expansion. Our multi-proxy approach improved the reconstruction of the site development and provided a more complete view of spatial and temporal vegetation dynamics. For example, the pollen record indicates the presence of oak and pine during a longer period than represented by the RW chronologies, suggesting the possibility of extending the oak and pine chronologies from Viss Mosse as the peat mining continues. The dendrochronological analyses of the alder material were time consuming and did not generate reliable chronologies. However, the dated trunks provided valuable information on the bog development in the marginal zone. The oak replication and pollen records do not always agree, which indicates that the observed variations in oak replication are related to altered preservation conditions due to the lateral peat expansion. In contrast, the pine replication record is largely consistent with the pollen data as a result of changing growth conditions. This first attempt at using bog oaks as a palaeohydrological proxy in southern Sweden shows some potential, but as no modern analogues exist and the growth limitations are poorly known, further studies based on material from peatlands with different moisture status are needed.

Footnotes

Acknowledgements

Anton Hansson is thanked for fieldwork and laboratory assistance. We are grateful to Lars Larsson and Arne Sjöström for information on the archaeological survey at Viss Mosse. Hanns Hubert Leuschner and Kjeld Christensen are thanked for evaluating bog-tree data, and Ulla Kokfelt and Rickard Åkesson for assistance and discussions in the field. Constructive comments from two anonymous reviewers improved the final presentation.

Funding

This research was funded by the Crafoord Foundation, Stiftelsen Anna och Gunnar Vidfelts fond för biologisk forskning, The Royal Physiographic Society in Lund, Lund University Centre for Studies of Carbon Cycle and Climate Interactions (LUCCI), Biodiversity and Ecosystem services in a Changing Climate (BECC), ESF grant 9031 and Johan Christian Mobergs resestipendiefond.

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