Mid-Holocene pine woodland phases and miredevelopment – significance of dendroecological datafrom subfossil trees from northwest Germany

Jan Eckstein, Hanns Hubert Leuschner & Andreas Bauerochse

Keywords

Dendrochronology; Fen–bog transition;

Paleoclimate; Paleoecology; Pinus sylvestris.

Received 5 November 2010

Accepted 26 February 2011

Co-ordinating Editor: Sandor Bartha

Eckstein, J. (corresponding author,

jan.eckstein@biologie.uni-goettingen.de) &

Leuschner, H. H. (hleusch@gwdg.de):

Department of Palynology and Climate

Dynamics, Albrecht-von-Haller-Institute,

University of Goettingen, Untere Karspule 2,

37073 Goettingen, Germany

Bauerochse, A. (andreas.bauerochse@nld.

nidersachsen.de): Lower Saxony State Service

for Cultural Heritage, Hannover,

Scharnhorststraße 1, 30175 Hannover,

Germany.

Abstract

Question: Can investigations of subfossil bog-pine woodlands contribute to the

understanding of mire development, especially the influence of climate

fluctuations on the fen–bog transition?

Location: Lowlands of northwest Germany.

Methods: We investigated pine (Pinus sylvestris L.) tree remains (stumps and

trunks) buried in peat deposits. Dendrochronology was used to date each

sampled tree to calendar years and to reconstruct population dynamics of the

pine woodlands. Ecological changes, especially changes in site hydrology

during the pine woodland phases were inferred from peat stratigraphic analyses

and investigations of stem and root morphology of the tree remains.

Results: The subfossil pine woodlands occurred mostly during the transition

from fen to raised bog conditions within the mire development. The population

dynamics are strikingly wave-like whereas woodland phases of 100 to 250

years duration are separated by much shorter (10–50 years) phases of high

germination and dying-off rates (GDO phases). Such GDO phases are often

synchronous at different sites and are also linked to growth depressions of the

independent regional oak master chronology (LSBOC), indicating a climate

trigger.

Conclusions: The development of raised bogs started about 7000 BC and had a

main phase between 5100 and 3600 BC in northwest Germany. The subfossil

bog-pine woodlands document the transitional phase towards the onset of

raised bog formation, as characterized by initial dry conditions that were

followed by increasing wetness of the sites, whereas this development is at

least partly the result of climate variations.

Introduction

The remains of dense pine woodlands buried in peat are a

common feature of northwest German raised bogs. Such

subfossil pines (Pinus sylvestris L.), in the following re-

ferred to as ‘bog-pines’, usually occur in distinct stump

layers within the peat and they testify to striking environ-

mental changes that affected their mire habitat. Dendro-

chronological studies across Europe have demonstrated a

huge potential of subfossil bog-pines to serve as a high-

resolution archive of ecological changes (Munaut 1966;

Munaut & Casparie 1971; McNally & Doyle 1984; Pilcher

et al. 1995; Chambers et al. 1997; Pukiene 1997; Gunnar-

son 1999; Lageard et al. 1999; Boswijk & Whitehouse

2002; Boswijk 2003). However, these investigations were

generally quite local in extent and tended to include only

a limited number of samples, and any ecological inter-

pretation of their findings thus had to be confined to the

local scale. The only regionally representative investiga-

tions of subfossil pines were carried out by Birks (1975)

and Bridge et al. (1990) for Scotland and by Bennett

(1984) for Britain and Ireland, but these studies did not

Journal of Vegetation Science 22 (2011) 781–794

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apply dendrochronological methods. Dendrochronologi-

cal dating of bog-pine chronologies to calendar years was

achieved in a few later studies carried out in Ireland

(Pilcher et al. 1995) and England (Chambers et al. 1997;

Boswijk 2003).

Most of the raised bogs in the northwest German

lowlands are of the plateau type (‘Plateauhochmoor’,

Eurola 1962; Aletsee 1967; Overbeck 1975). The plateau-

type raised bog is characterized by a more or less plane

raised bog plateau, which is generally treeless. But the

subfossil pines testify to the temporary colonization of the

mires by pine woodlands, mainly during the fen–bog-

transition phase (Godwin 1975; Overbeck 1975). This

crucial part in the developmental history of raised bogs

when oligotrophic communities first developed has pre-

viously received little attention and is not yet fully under-

stood (Hughes et al. 2000). The conventional explanation

for the fen–bog transition is that the growing peat surface

is raised above the groundwater table by gradual peat

accumulation, which over time leads to a progressive shift

from groundwater-dependent fen communities to exclu-

sively rain-fed raised bog communities. Once established,

raised bogs are quite stable ecosystems that represent the

final stage of mire development in areas with sufficient

effective precipitation (Joosten 1993; van Breemen 1995;

Mauquoy & Yeloff 2008). Since the fen–bog transition

process takes a long time, and thus cannot be studied

directly, many aspects are still not fully resolved, in

particular with regard to climatic influences, and also

regarding the duration of the process. Some authors see

the fen–bog transition as a primarily autogenic process of

mire development, which is mostly influenced by local

conditions (Walker 1970; Frenzel 1983; Zobel 1988;

Anderson et al. 2003). Others stress the influence of

allogenic factors such as climate variations. One view is

that peat accumulation intensifies under cool and moist

climate conditions, and that the ombrotrophication pro-

cess should therefore be accelerated in such conditions

(Overbeck 1975; Berglund et al. 1983; Damman 1986;

Rydin & Jeglum 2006). By contrast, some researchers

assume that the transition to ombrotrophic conditions is

actually promoted by relatively dry climate conditions

(Svensson 1988; Almquist-Jacobson & Foster 1995;

Hughes et al. 2000). In their interpretation, a falling water

table due to a drier climate is assumed to result in a

decrease of groundwater influence at the peat surface,

which then facilitates the spread of ombrotrophic species.

Dendrochronological data on bog-pines have the ad-

vantage of exact dating and high temporal resolution over

radiocarbon-dated archives, which usually have dating

uncertainties of � 40 years to � 200 years. Investigations

of subfossil pines in situ make it possible to link exactly

dated dendro-archives with past changes of mire plant

communities documented in the peat stratigraphy. De-

spite the abundance of subfossil pine remains in the peat-

harvesting areas of northwest Germany, systematic inves-

tigations of this promising paleo-archive began only

recently (Bauerochse et al. 2006; Leuschner et al. 2007).

Preliminary results were published in Eckstein et al.

(2009). Eckstein et al. (2010) report a detailed account of

one sample site in particular (Venner Moor). These earlier

articles focus on the paleoclimatic implications and eval-

uate the potential of the bog-pines to infer past environ-

mental changes. Since then, the number of exactly dated

bog-pines has been substantially increased and peat stra-

tigraphy has been investigated at eight additional sites.

The resulting temporally and regionally representative

data set of subfossil pines from northwest Germany

enables us to precisely date past environmental changes

and to shed some light on the driving factors behind these

changes. This article aims to (i) use the substantial data

set of subfossil pine to describe typical characteristics of

tree-ring series, population dynamics and stratigraphical

context; (ii) reconstruct past habitat conditions and

changes during the pine woodland phases of the mires;

and (iii) address the question how the investigation of

subfossil bog-pine woodlands can further improve our

understanding of mire development, especially with re-

gard to the influence of climate fluctuations on the

fen–bog transition.

Methods

Study area

Subfossil bog-pines are unearthed in peat harvesting areas

all over northwest Germany. However, as we were inter-

ested in patterns along a climate gradient ranging from

more oceanic conditions in the west to more continental

conditions in the east, we largely focused our investiga-

tion on four extended peatland areas located along a

west–east axis: Bourtanger Moor, Duemmer area, Totes

Moor and Gifhorner Moor. Two additional areas, further

north, Wiesmoor and Teufelsmoor, were also sampled

(Fig. 1, Table 1).

Current climate can be characterized as sub-oceanic,

with annual mean temperature and precipitation ranging

from 9.4 1C and 800 mm at the westernmost sites to 9.2 1C

and 630 mm at the easternmost sites (stations Lingen and

Wolfsburg; DWD 2008). Because of the climate gradient,

with dry conditions to the east, the Gifhorner Moor is the

easternmost atlantic-type raised bog in north Germany

(Overbeck 1975). Altogether, subfossil bog-pine wood-

lands from 17 raised bogs were sampled. The number of

sample sites varied depending on bog size, with larger

bogs containing multiple sample sites, each separated by a

Mid-Holocene pine woodland phases and mire development Eckstein, J. et al.

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minimum distance of 500 m. We sampled 2397 trees from

36 sites (Table 1).

Field sampling, dendrochronological methods and

radiocarbon dating

We sampled pine stumps and trunks exposed during peat

harvesting. Most of the sampling focused on ex situ speci-

mens from wood heaps banked up along the peat harvest-

ing fields (Fig. 2a). Additional samples were collected in

situ from tree stumps found in ditches and in the surface

layer of the peat harvesting fields (Figs 2b and 3). These

samples were used to infer the stratigraphical position and

patterns of rooting depth of the pine stump layers. Root-

ing depths were measured as maximal vertical penetra-

tion of preserved roots. Stem and root morphologies were

recorded, with a focus on the straightness of stems, mean

growth direction of roots (horizontal versus vertical) and

length of main root. Pine roots only grow in aerated soil

layers. Thus, the rooting depth of pine trees in mires can

be used as an indicator of former water table levels

(Kokkonen 1923; Boggie 1972). Peat stratigraphy was

determined in the field according to Preuss et al. (1991).

Dendrochronological analysis followed standard proce-

dures (e.g. Baillie 1982; Schweingruber 1987; Leuschner

1994). Radial sections of sample disks were prepared for

measurement with surgical and razor blades (Iseli &

Schweingruber 1989). Chalk dust was applied to the

cross-sections to fill the cell lumina and enhance the

visibility of the cell morphology and tree-ring boundaries.

Mostly one radius per sample was measured but in about

10% of the samples two or more radii were measured and

a tree mean curve was prepared to represent the tree.

Tree-ring width was measured to the nearest 1/100 mm

using a semi-automatic measuring stage (type Aniol) and

the program CATRAS (Aniol 1983). Each tree-ring series

was cross-dated with all other series and also with the

developing site chronologies using the statistical approach

Fig. 1. Map of Lower Saxony (grey) in NW Germany indicating the location of sample sites (triangles).

Eckstein, J. et al. Mid-Holocene pine woodland phases and mire development

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of Baillie & Pilcher (1973). The pine site chronologies

were dated with reference to the regional bog-oak chron-

ology LSBOC. The LSBOC is comprised of some 1700 oaks

from mire sites all over northwest Germany (Leuschner et

al. 1985, 1987, 2002), and therefore allows independent

comparison with the bog-pines. A detailed description of

the field sampling methods and of the procedures used for

cross-dating and chronology-building can be found in

Eckstein et al. (2009, 2010).

To calculate the tree-ring index curve of the LSBOC,

the ring width series were transformed logarithmically

and then filtered by dynamically weighted moving

averages (mean length of 100 years) as described in

Leuschner et al. (2002). By doing so, we were able to

remove the endogenous trend of decreasing ring width

with tree age (age trend), while at the same time preser-

ving existing long-term decadal growth variations. In this

standardization process, measured ring widths are trans-

formed into dimensionless tree-ring indices (Fritts 1976).

Conventional radiocarbon dating was performed to

check dendrochronological results and to date the floating

pine chronologies. Altogether, 79 trees were radiocarbon-

dated using subsamples containing 11 years. The analysis

was carried out at the Leibniz Institute for Applied

Geophysics (LIAG), Hannover, Germany. To obtain calen-

dar dates, the raw radiocarbon age has to be corrected

with a calibration curve that accounts for natural varia-

tions of C14- levels in the atmosphere over time. Our

radiocarbon dates as well as cited dates from other studies

were calibrated using OxCal v. 3.10 (Bronk Ramsey

2005). Using a confidence interval of 95%, the calibrated

radiocarbon dates have uncertainties between � 40 years

and � 200 years for methodical reasons. All ages were

expressed either as calendar dates or as years Before

Table 1. List of sampled bogs, specifying the exact geographic location of sample sites as well as total number of samples and number of

dendrochronologically-dated and radiocarbon-dated ( = floating) samples (see also Fig. 1).

Geographic co-ordinates # sample difference = not datable

Peatland area (see Fig. 1) Code Bog name # of sample sites Lat. Lon. Total Dated Floating

Bourtanger Moor GEORG Georgsdorfer Moor 2 52.59 7.03 50 – 41

TWIMO Twister Moor 1 52.63 7.07 30 17 –

RUMO Ruehler Moor 2 52.65 7.10 37 9 3

WIETMA Wietmarschener Moor 3 52.57 7.11 80 34 10

HES Heseper Moor 3 52.64 7.12 83 22 7

DAMO Dalumer Moor 1 52.59 7.15 20 0 15

VEMO Versener Moor 1 52.72 7.16 15 6 –

Wiesmoor Duemmer area WIK Wiesmoor 1 53.35 7.77 41 26 –

VOERDEN Voerdener Moor 1 52.46 8.14 81 65 –

VENMO Venner Moor 1 52.44 8.17 267 227 –

CAMP Campemoor 4 52.47 8.21 291 121 65

VECHMO Vechtaer Moor 3 52.70 8.33 116 105 –

DREM Dreiecksmoor 1 52.72 8.35 20 8 –

Teufelsmoor HUVE Huvehoops Moor 1 53.36 9.09 106 35 –

Totes Moor TOMO Totes Moor 5 52.51 9.37 739 411 45

Gifhorner Moor GIMO Gifhorner Moor 5 52.53 10.63 382 124 51

WEIMO Weisses Moor 1 52.60 10.66 39 31 –

Total 17 36 2397 1241 233

Fig. 2. (a) Wood heap of subfossil pines at Campemoor (CAMP). (b) In situ pine stumps at Venner Moor (VENMO).

Mid-Holocene pine woodland phases and mire development Eckstein, J. et al.

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Present (BP), with Present conventionally defined as AD

1950.

Results

Stratigraphical position of the pine trees

The pine wood remains typically occur in distinct layers or

horizons (stump layers) within the peat stratigraphy.

Accordingly, at 11 sites, we could determine the stratigra-

phical position of the pine stump layer from in situ finds

(Fig. 4). The peat profiles at different sites are remarkably

similar in as far as the subfossil pines tend to co-occur

with the transition from fen to bog peat (Fig. 3) or to occur

directly on the mineral ground covered by raised bog peat

(e.g. at sites VECMO and VOERDEN). Therefore the pine

stump layers can be interpreted as marker layers for the

fen–bog transition. However, occasionally, subfossil pines

did not occur exclusively in this layer, but were also found

in smaller numbers within the raised bog peat layer (Fig.

3). In this case, stumps generally originated from small

trees with fewer than 50 tree rings, and were thus not

suited for dendrochronological investigation.

The peat in which the pine roots and stem bases are

embedded tends to be largely made up of macro-remains

of Eriophorum vaginatum (indicated by ‘E’ in Fig. 4), and

contains only relatively few remains of Sphagnum mosses.

The layer with pine roots and stem bases is always

succeeded by more or less pure Sphagnum peat, which

facilitated preservation of the tree remains (Fig. 4).

The basic peat layers of the stratigraphical records

suggest that mire formation at the pine sites was mostly

initiated by paludification of previously dry ground, and

only occasionally by infilling of lakes (terrestrialization).

Directly beneath the pine stump layers, there is often

swamp woodland peat (indicated by ‘W’ in Fig. 4), with

abundant wood remains of Alnus and/or Betula and

occasionally Quercus, indicating that the pines often re-

placed an alder swamp woodland or a wooded fen.

Root morphology

At many study sites stem and root morphology differ

considerably between successive pine generations. Typi-

cally, the maximum rooting depth is found in the first

pine generation at a site, and rooting depth decreases with

every successive generation. Eventually, the last genera-

tion shows very shallow root systems.

The findings from Vechtaer Moor (VECMO 2) may

serve to illustrate this general observation. There, a pine

woodland existed over a period of 500 years, between

4470 and 3970 BC. Four major germination and/or dying-

off (GDO) phases can be recognized, subdividing the local

pine woodland history into three successive pine genera-

tions (Fig. 5). We define GDO phases as short periods with

a high frequency of germination and/or dying-off events.

The oldest pine generation at Vechtaer Moor consisted of

large trees with a root system spreading evenly in all

directions and extending up to 50-cm deep into the

mineral soil. By contrast, stumps from the third pine

generation at the site were rooted in peat to a depth of

35 cm and had no contact to the mineral soil. These

stumps are also much smaller than stumps from the first

Fig. 3. Exposed peat-face at Totes Moor (TOMO_N) showing subfossil pine stumps at the transition from fen to bog peat and within the raised bog

(Sphagnum) peat. Tree trunks at the upper part of the photograph originate also from the fen–bog transition but were extracted during peat-harvesting

work. The complete peat profile (M726_P1) is illustrated in Fig. 4.

Eckstein, J. et al. Mid-Holocene pine woodland phases and mire development

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tree generation (Fig. 5) and often characterized by bent

stems and shallow, skewed root systems. The pine stumps

from the second generation are mostly intermediate in

character. Stumps from the second and third generation

are excellently preserved to a vertical height of up

to 40 cm above the root plate and have conical stem tips

(Fig. 5).

Temporal distribution and population dynamics

All bog-pines date to the mid-Holocene, in the period

between 7000 and 1200 BC (Fig. 6). Five chronologies

were dated to calendar years, the longest of them covering

a period of 1999 years, from 5606 to 3608 BC. Seven

additional chronologies were radiocarbon-dated and

probably did not overlap with the absolutely dated chron-

ologies (Fig. 6). Their temporal position may vary by � 40

years and � 200 years but the radiocarbon-dated chron-

ologies demonstrate that there is no long-term gap in the

temporal distribution of bog-pines. The main phase of

pine woodlands on peatlands was between 5200 and 3600

BC. The 1241 dated bog-pines reached a mean tree age of

110 years.

The most striking feature of the bog-pine data set is the

phased temporal occurrence of samples (Fig. 6). Popula-

tion dynamics are characterized by woodland phases of

100 to 250 years in length that are separated by much

shorter (10–50 years) GDO phases. Germination and

dying-off phases typically coincide, resulting in a clear

sequence of pine generations (Figs 5 and 6). This temporal

coincidence of germination and dying-off events is such a

general feature that we normally use the combined term

GDO for these short phases. Within the GDO phases we

either observe a temporal overlap of dying-off and germi-

nation (e.g. VECMO at ca. 4060 BC, VENMO and VOER-

DEN at 2400–2200 BC) or, less frequently, a short

temporal gap between a dying-off and the next germina-

tion phase (e.g. WEIMO and TOMO N at ca. 4350 BC).

However, a few sites (e.g. WIK, GIMO_9, TOMO_S) are

characterized by longer GDO phases, resulting in a less

distinct separation of woodland generations. This feature

tends to occur more frequently in the period before 4000

BC, compared to later times (Fig. 6).

Figure 6 illustrates that many GDO phases are synchro-

nized across different northwest German sites. This is

strong evidence for a regional-scale, i.e. climatic, forcing

factor behind the population dynamics of the bog-pines.

Furthermore, the GDO phases of the bog-pines often

coincide with long-term growth depression phases of the

LSBOC. This link is illustrated in Fig. 6, in which major

GDO phases are indicated using reference lines. Twenty-

nine of the major GDO events are covered by the LSBOC,

18 of which are clearly associated with growth depres-

sions and only five with phases of above average growth

of the LSBOC.

Sometimes short GDO phases reveal rapid and severe

changes of ecological conditions. The dying-off phase

around 3980 BC at VECMO 2 (Fig. 5) is paralleled at the

0

100

200

300

Dep

th in

cm

Sphagnum peat(weakly humified)

Eriophorum peat

Sphagnum peat(moderately to strongly humified)

Fen peat

Mud

Moss peat

Sand

Wood remains of and/orAlnus Betula

Eriophorum layer

W

E

Pinus stump layer

Fig. 4. Peat stratigraphical context of subfossil pine stump layers from 11 sample sites. The top of each profile is artificial due to the removal of the

upper peat layers for peat harvesting. Laboratory codes for sample sites: CAMP – Campemoor, GIMO – Gifhorner Moor, TOMO – Totes Moor, VECMO –

Vechtaer Moor, VENMO – Venner Moor, VOERDEN – Voerdener Moor; WIETMA – Wietmarschener Moor.

Mid-Holocene pine woodland phases and mire development Eckstein, J. et al.

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44504500 4400 4350 4300 4250 4200 4150 4100 4050 4000 3950 3900Years BC

64006450 6350 6300 6250 6200 6150 6100 6050 6000 5950 5900 5850Years BP

Year withfire scar

GDO-phases

1st pine generation 2nd pinegeneration

3rd pinegeneration

Typical morphologies ofpine stumps

0

1

2

3

4ev

ents

[yea

r–1]

Lifespan of each subfossilpine tree at VECMO 2

Frequency of germination (dotted)and dying-off (solid) events

Fig. 5. Population dynamics of subfossil trees from Vechtaer Moor (VECMO 2). First box: life span of each tree arranged by dying-off date, grey parts

indicate estimated missing rings to pith and waney edge. Second box: typical pine stump morphologies from the three successive generations in lateral

view. Third box: frequency of germination and dying-off events smoothed by 5-year moving average. Shaded areas indicate major germination and/or

dying-off (GDO) phases.

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VECMO 1 site, which is 500-m distant (Fig. 6). Both sites

represent different historic habitat conditions, with the

trees at VECMO 1 having grown in purely mineral soil,

whereas the trees at VECMO 2 grew on top of a ca. 35-cm

layer of peat, with their roots seemingly not having been

in contact with the mineral soil. Nevertheless, irrespective

of the different site conditions, pine populations at both

sites completely died off beginning in 3990 BC. The

majority of trees died within only 10 years.

We inferred fire events in the ancient pine woodlands

by documenting encapsulated fire scars that can be

dendrochronologically dated to calendar years. At Vech-

taer Moor fire scars indicate several fires, which occurred

only during the first and second pine generation, but

these fires were not associated with the GDO phases

(Fig. 5).

Duration of the pine woodland phases

At five of the most intensively sampled sites we were able

to determine the overall duration of the pine woodland

phase, from the germination of the first pines to the death

1000200030004000500060007000Years BC

3000400050006000700080009000Years BP

Legend

Fig. 6. Life span of subfossil pines, dated to calendar years (horizontal black lines), from NW Germany, clustered according to their site provenance and

sorted by dying-off date. Tree-ring width variations (index curve smoothed by 20-year moving averages) of the regional oak chronology (LSBOC) and

extent of dendrochronologically and radiocarbon-dated pine chronologies are plotted at the bottom. See Table 1 for laboratory codes of sample sites.

Mid-Holocene pine woodland phases and mire development Eckstein, J. et al.

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of the last trees. These woodland phases spanned between

230 and 750 years (Table 2).

For the sites not listed in Table 2, the overall duration of

the pine woodland phase could not be reliably estimated.

Either there is uncertainty as to whether we managed to

sample the full range of pine generations or the samples

may not have originated from a single continuous mire

area. The former condition would likely result in an

underestimation, and the latter condition in an over-

estimation of the length of the pine woodland phase.

Overall, our results suggest an average pine woodland

phase of ca. 300 years, with a tendency towards longer

phases before 3600 BC, and also for the eastern sites

TOMO and GIMO.

Discussion

Population dynamics of the bog-pines

The phased temporal occurrences of samples with distinct

tree generations that are separated by brief GDO phases

are characteristic for subfossil mire-rooting pines (Fig. 6).

This can be compared with subfossil oaks from mire sites

in northwest Germany, in which germination and dying-

off phases often overlap by centuries (Delorme et al. 1981;

Leuschner et al. 1985, 2002). The bog-pine GDO phases

are much shorter and the pine populations are more

clearly delimited. Our results are in line with other

dendrochronological studies of subfossil pines, although

in these previous studies patterns were often less clear,

mostly due to smaller sample sizes (Munaut 1966; Mu-

naut & Casparie 1971; McNally & Doyle 1984; Chambers

et al. 1997; Pukiene 1997; Gunnarson 1999; Lageard et al.

1999; Boswijk & Whitehouse 2002). Wave-like regenera-

tion patterns were also reported for pine populations from

dry sites in northern Sweden (Zackrisson et al. 1995). In

this case, regeneration pulses were the result of successful

establishment in a special type of ground vegetation,

which was favoured by dry and warm climate conditions.

In contrast, the phase-wise population dynamics of bog-

pines are a result of stressful growth conditions and the

frequent occurrence of disturbance events in mire ecosys-

tems. This is demonstrated by the common temporal

coincidence of germination and dying-off phases of the

subfossil pines (Figs 5 and 6).

Moreover, in the case of Scots pine, which can be

characterized as a pioneer species with a high demand

for light (Zoller 1981; Schutt & Stimm 2006), establish-

ment and regeneration strongly depends on gap-creating

canopy disturbances, as has been shown by various

studies of extant pine woodlands growing on various soil

types, with fire playing an important role as a disturbance

agent (Dimbleby 1953; Zackrisson 1977; Bradshaw &

Zackrisson 1990; Agee 1998; Hille & den Ouden 2004).

This dependency of pine regeneration on disturbances

explains why, generally, dying-off events and establishing

events occur practically simultaneously at a given site

(e.g. Fig. 6). Fire can drive population dynamics, as is well

documented for present-day pine woodlands (i.e. Engel-

mark et al. 1994; Agee 1998). At Vechtaer Moor, several

fires did occur in the bog-pine woodland but these were

not associated with the GDO phases (Fig. 5). This is

congruent with the results from Totes Moor and Campe-

moor (Eckstein et al. 2009) as well as from Gifhorner

Moor (unpubl. data). At our study sites, we never found a

coincidence between fires and GDO phases. Therefore, we

conclude that fires frequently occurred in the bog-pine

woodlands of northwest Germany, but had little effect on

the pine population dynamics and were probably of low

severity. Lageard et al. (2000) also document fires of low

to moderate intensity from subfossil Scots pine woodlands

at sites in England, Wales and Ireland. Our findings

suggest that hydrology, with high water levels serving as

the main stress factor, may provide an explanation that

fits the observed population dynamics better than fire or

storm events (q.v. Bauerochse et al. 2008; Eckstein et al.

2009). Such an interpretation is also supported by drai-

nage experiments in extant pine stands at mire sites,

which show that low water tables generally promote pine

tree growth, whereas high water tables tend to be accom-

panied by reduced growth (Kokkonen 1923; Boggie &

Miller 1976; Dang & Lieffers 1989). Generally, even small

hydrological changes can significantly alter habitat condi-

tions in mires. High water levels affect trees in two ways:

(i) inundated roots die and (ii) the depth of the aerated

soil layer that permits root growth decreases, and there-

fore fewer nutrients are available. The facts that the

studied bog-pines only reached an average age of 110

years, which is short for Scots pine, and that the tree-ring

series from these trees are characterized by very high

levels of inter-annual variation in ring width, indicate

poor and highly variable growth conditions. From time to

time, these conditions deteriorated sharply, resulting in

the observed well-defined GDO events (Fig. 6).

The large-scale synchronicity of germination and dy-

ing-off phases in mires across northwest Germany and the

Table 2. Duration of pine woodlands on the mire during the fen–bog

transition at five intensively sampled sites.

Site # of pine

generations

Duration of pine

woodland (years)

Time span in

years BC

CAMP_1 2 320 3050–2720

VENMO 2 230 2310–2080

VOERDEN 2 250 2420–2170

TOMO_N 4 750 4380–3630

VECMO_2 3 500 4470–3970

Eckstein, J. et al. Mid-Holocene pine woodland phases and mire development

Journal of Vegetation Science

Doi: 10.1111/j.1654-1103.2011.01283.x r 2011 International Association for Vegetation Science 789

high levels of correspondence between bog-pine dying-off

phases and growth depressions in the regional bog-oak

tree-ring width chronology (LSBOC) both strongly indi-

cate climatic causes for the observed bog-pine population

dynamics. Growth depressions in the LSBOC are usually

attributed to wet climate conditions (Leuschner et al.

2002), and pines have known sensitivity to hydrological

changes. It can therefore be concluded that in the major-

ity, GDO events are likely to have been triggered by

climate wet shifts (q.v. Leuschner et al. 2007; Eckstein et

al. 2009). These wet shifts can be caused by increasing

precipitation or decreasing evapotranspiration due to

lower temperatures or higher air humidity. Nevertheless,

four of the 29 major GDO events (Fig. 6) occurred during

dry climate phases, as indicated by above average growth

of the bog-oaks. A closer look at these four GDO events

reveals that they exclusively represent germination

events. This means that dying-off events of the bog-pine

populations are clearly linked with wet climate phases,

whereas germination events can be associated with both

wet as well as dry climate phases. The common growth

depression in both the bog-pines as well as the LSBOC

indicates that these wet shifts often started abruptly

(Eckstein et al. 2010) and lasted for ca. 10 to 50 years.

Short but severe dying-off events were observed at VEC-

MO 1 (Fig. 5) and VECMO 2 around 3980 BC and are also

reported from Venner Moor (VENMO) for a short period

around 2168 BC (Eckstein et al. 2010). Since these dying-

off events lasted for only 10 to 20 years and even affected

trees from different historic habitat conditions, we con-

clude that the climate variations triggering these dying-off

phases were short (i.e. a few years to a few decades) and at

the same time were of high severity. The occurrence of

such short-term climate fluctuations and their effect on

vegetation dynamics can only be documented by dendro-

chronological records with their high temporal resolu-

tion. However, the longer-lasting severe wet phases

inferred from bog-pine GDO events around 2850 BC and

2150 BC have parallels in other paleo-records across

Europe (Leuschner et al. 2007; Eckstein et al. 2010).

Bog-pines and the fen–bog transition

In northwest German mires, subfossil bog-pine layers are

strongly associated with the fen–bog transition (Fig. 4). In

agreement with the peat stratigraphical observations, the

results of pollen analyses carried out at the three sites,

CAMP_1, VENMO and TOMO_N, also indicate that the

transition to ombrotrophic conditions coincided with the

formation of the pine stump layer (Bauerochse 2003;

Eckstein et al. 2008, 2010). The concentrated occurrence

of bog-pines in this particular stage of mire history has to

be interpreted in the context of the ecological niche of

Scots pine. Essentially, Scots pine is a pioneer species that

can tolerate a wide range of environmental conditions,

but under favourable conditions is less competitive in

Central Europe than most other tree species, especially

broad-leaved species. Therefore, Scots pine generally is

confined to extremely dry or wet and generally poor

habitats (Zoller 1981; Schutt & Stimm 2006), and this

was already the case in the mid-Holocene (Overbeck

1975; Lang 1994). Fens are by definition influenced by

ground or surface water and are therefore relatively

nutrient-rich. At such sites pine cannot compete with

alder, birch, oak and willow, which are the most common

species in wooded fens. Nutrient availability decreased

when the peat surface became more and more isolated

from the influence of ground- or surface water. In this

setting, pine was able to colonize the now nutrient-poor

mire sites. Since bog-pines in most cases occurred during

the fen–bog transition in mire history, our dendroecolo-

gical results provide information about the character and

the duration of this transitional phase that leads to

ombrotrophy.

The time span in which pine woodlands occurred at the

sites is relatively short compared to the entire history of

the mires. The subfossil pines indicate an average dura-

tion of the complete pine woodland phase of ca. 300

years, with periods ranging between 230 and 750 years

at five well-sampled sites (Table 2). Comparative data are

provided in the macrofossil analyses of Hughes & Barber’s

(2004), which suggested an average duration of the

fen–bog transition of ca. 300 years for bogs in England,

and an exceptionally short phase of ca. 90 years for

Tregaron Bog, in Wales.

Stem and root morphology and the degree of preserva-

tion are useful indicators of past hydrological conditions.

At Vechtaer Moor the deep-reaching roots and the poor

preservation of stems of the first pine generation indicate

relatively dry conditions and slow peat accumulation

during the early stages of the pine woodland phase. In

contrast, the shallow roots systems and excellent stem

preservation of the last pine generation indicate increas-

ingly wet conditions and rapid peat accumulation (Fig. 5).

This seems to be a general sequence at the pine woodland

since similar observations have also been made at Cam-

pemoor (Leuschner et al. 2007) and Venner Moor (Eck-

stein et al. 2009). The observation of dry conditions at

early stages of the pine woodland phase agrees with

findings of Casparie (1972) who documented that pine

repeatedly colonized extensive areas of Bourtanger Moor,

The Netherlands, after desiccation of the upper peat

layers. However, the later stages of the bog-pine wood-

lands were characterized by increasingly wet conditions.

The last pine generation at a site is mostly represented

by excellently preserved stems with conical rotten tips

Mid-Holocene pine woodland phases and mire development Eckstein, J. et al.

790Journal of Vegetation Science

Doi: 10.1111/j.1654-1103.2011.01283.x r 2011 International Association for Vegetation Science

(Fig. 5). This appearance is characteristic for stumps

rapidly overgrown by Sphagnum lawns (Hayen 1960),

indicating preservation under raised bog conditions. The

onset of raised bog growth contributes to the increasingly

wet conditions at the pine sites. Raised bogs are able to

maintain a stable, domed water mound through the

development of catotelmic peat, which has a high water

storage capacity, while at the same time being character-

ized by a low water conductivity (Ingram 1982, 1983).

Although the hydrology of fully-developed raised bog

systems is well studied (Ivanov 1981; Ingram 1982, 1983;

Joosten 1993), it is still largely unknown how such a

raised water mound develops in the first place.

Hughes et al. (2000) and Hughes & Dumayne-Peaty

(2002) have argued that Eriophorum vaginatum plays an

important role in the fen–bog transition and seems to

facilitate the transition to true (Sphagnum-dominated)

raised mire communities, especially under sub-oceanic

conditions. The Eriophorum peat is an ideal substrate for

Sphagnum species and the micro-relief of Eriophorum

vaginatum hummocks provides micro-climatic conditions

that are very favourable for Sphagnum colonization. The

peat within the subfossil pine stump layers always con-

tains Eriophorum vaginatum macro-remains, often at a

high abundance (Fig. 4). We speculate that occurrence of

Eriophorum vaginatum-dominated vegetation facilitated

the spread of peat-forming mosses and the development

towards an independent bog water mound in the pine

woodlands. Once a stable water mound had developed,

the resulting permanently high bog water table prevented

pine regeneration, and the pine woodland phase of the

mire ended with the death of the remaining trees. Scots

pine possibly also facilitates Sphagnum colonization

through soil acidification due to the accumulation of

persistent needle litter (Scholes & Nowicki 1998).

The fen–bog transition at the bog-pine sites seems to be

an irreversible process since all our sites show the transi-

tion from fen to bog and not vice versa. This process may

have started with the first colonization of a mire site by

pines, or even earlier. A climate influence on the initially

dry conditions of the bog-pine sites is not clearly indicated

by our results. Only four germination phases coincide

with dry climate conditions, indicated by above average

tree-ring width indices of the LSBOC (Fig. 6). However,

peat stratigraphical studies from Sweden (Svensson 1988)

and England (Hughes et al. 2000) documented phases of

relatively dry climate conditions as a trigger for the

fen–bog transition. On the other hand, we could argue

that bog-pine woodlands were repeatedly influenced by

phases of wet climate, which resulted in growth depres-

sion and triggered GDO events. Therefore, the increas-

ingly wet conditions during the pine woodland phase

were also partly the result of decadal-scale climate wet

shifts. In fact, it is difficult to disentangle the effects of

climate variations and of bog development, but in this

case the influence of climate wet shifts is unequivocally

proved by the regionally synchronous dying-off phases of

the bog-pine populations and the links to growth depres-

sions of the LSBOC. In summary, the subfossil bog-pine

woodlands document the transitional phase towards the

onset of raised bog formation, as characterized by initial

dry conditions that were followed by increasing wetness

of the sites, whereas this development is at least partly the

result of climate variations. A dry–wet sequence was also

considered as a trigger for the formation of raised bog

growth leading to the death and preservation of oak

woodlands in northwest Germany (Delorme et al. 1981;

Leuschner 1992).

If we assume that bog-pine occurrence generally coin-

cided with the fen–bog transition at individual sites, then

the pine data set indicates spatial and temporal patterns of

raised bog initiation in northwest Germany, for which few

previous comparable data sets exist (Petzelberger et al.

1999). The bog-pine woodlands occurred over a long time

period, between 7000 and 1200 BC, with a peak between

5200 and 3600 BC. This is consistent with other studies.

Casparie & Streefkerk (1992) reported the main phase of

fen–bog transitions in The Netherlands occurred between

5000 and 4000 BC. In northwest Germany, the main

phase for the start of raised bog growth is documented in

palynological records for the Atlantic and Sub-boreal

periods as starting around 6800 BC (Overbeck 1975).

The results of Petzelberger et al. (1999) are similar,

suggesting basal dates of raised bog growth between

7600 and 5300 BC for some raised bogs near the coast in

northwest Germany. The start of extensive raised bog

growth ca. 7000 BC and the subsequent main phase

between 5200 and 3600 BC can be explained by a shift to

wetter climatic conditions in the early and mid-Holocene

due to transgression of the North Sea (Petzelberger et al.

1999), which reached the proximity of the present-day

coastline in about 7000 BC (Behre 2007). Casparie &

Streefkerk (1992) demonstrated that an annual precipita-

tion of at least 700 mm is required for raised bog growth in

The Netherlands and northwest Germany. Probably, this

precipitation threshold was reached for the first time ca.

7000 BC.

Interestingly, there is a tendency towards longer tree

lives and more extended bog-pine woodland phases for

eastern sites (TOMO, GIMO), with a similar tendency for

sites representing the period before 3600 BC (Fig. 6). This

suggests that the fen–bog transitions lasted longer in the

middle of the Holocene, until ca. 3600 BC. Later in the

Holocene, between 3000 and 1200 BC, the transition to

raised -bog conditions occurred more rapidly, and pine

woodland could only establish during short transition

Eckstein, J. et al. Mid-Holocene pine woodland phases and mire development

Journal of Vegetation Science

Doi: 10.1111/j.1654-1103.2011.01283.x r 2011 International Association for Vegetation Science 791

phases. This probably reflects a broad climatic trend

towards cooler and more humid conditions during the

mid-Holocene (at the end of the Holocene Climate Opti-

mum), which has been documented in a range of records

(Schweingruber et al. 1988; Korhola et al. 2000; Davis et

al. 2003; Seppa et al. 2005), including for Germany (Voigt

et al. 2008; Litt et al. 2009).

Acknowledgements

This research is part of a project funded by the German

Research Foundation (LE 1805/2-1). We would like to

thank B. Birkholz for analysis of peat stratigraphy, B.

Leuschner for part of the tree ring measurements, B.

Raufeisen for the drawings and M. Wagner for language

improvements.

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