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19
VI/1/2015
INTERDISCIPLINARIA ARCHAEOLOGICA
NATURAL SCIENCES IN ARCHAEOLOGY
homepage: http://www.iansa.eu
Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
Inke Achterberg
a*
, Andreas Bauerochse
b
, Thomas Giesecke
a
, Alf Metzler
b
, Hanns Hubert Leuschner
a
a
Department for Palynology and Climate Dynamics, University of Goettingen, Untere Karspuele 2, 37073 Göttingen, Germany
b
Lower Saxony State Service of Cultural Heritage, Scharnhorststr. 1, 30175 Hanover, Germany
1. Introduction
The northwest-German lowland changed in the millennia
following the last glacial: from a periglacial wasteland to
a forest-wetland mosaic, a landscape where the expanding
mires eventually covered about one third of the area (Behre
2008; Metzler 2006). While the North Sea successively
claimed the land between Denmark and England, pushing
ground water levels up and maritime conditions further
south-east, people bridged the spreading mires by wooden
trackways, evidently since the early Neolithic (Metzler
2006). Finds of peat-preserved trackways are frequently
reported for northwest Germany (
e.g.
Metzler 2006), and
also from Ireland (Raftery 1996) and southwest England
(Coles and Coles 1992). Whether their construction (Behre
2005; Metzler 2003), or possibly their preservation (Spurk
et al.
2002), might be related to environmental changes and
climatic fuctuations is debated (Bauerochse 2003; Baillie
and Brown 1996). In Ireland, the occurrence of fve “lulls”
in trackway construction activity between the Neolithic and
Modern Age was found to relate rather to cultural changes
than to long-term hydrological variations (Plunkett
et al.
2013). The present study, however, is focused rather on
the precise alignment of individual constructions with the
mostly short (decadal) phases of water table rise in northwest
Germany. Indications for increased humidity in trackway
layers have been repeatedly described – using pollen and
peat analysis (
e.g.
Bauerochse 2003; Leuschner
et al.
2007).
Whether the constructions were actually contemporaneous
to or following such environmental change is investigated
in this study using dendrochronology. This provides
precise dating for both the trackway constructions and
the mire water table rise. The latter is possible due to the
large dendrochronological record of peat-preserved trees,
originating from former mire (and mire-margin) woodland
Volume VI ● Issue 1/2015 ● Pages 19–29
*Corresponding author. E-mail:
iachter@gwdg.de
ARTICLE INFO
Article history:
Received: 8
th
August 2014
Accepted: 19
th
July 2015
Keywords:
wooden trackways
dendrochronology
mire
environmental change
Holocene climate
Neolithic
ABSTRACT
Tree rings provide not only a precise dating tool, but also contain information on environmental
change. The well replicated tree ring record of northwest Germany therefore provides environmental
implications with immanent, absolute and precise dating from 6703 BC to 931 AD. This offers the
opportunity to investigate the environmental context of archaeological fnds, if they, too, are dated by
dendrochronology. We investigated 13 peat-preserved trackways from the Northwest-German lowland
between 4629 BC (Neolithic) and 502 AD (Migration Period) for contemporaneousness with water
table rise in the landscape. Such environmental change is well refected in the clearly notable die-off
phases of trees preserved in the mires. As an environmental proxy, the parameter “
tree die-off rate
a-30
” is introduced: The annual number of tree die-off events is divided by the number of live trees
30 years previously. Overall, the majority of trackway constructions were found to be contemporane-
ous to mire water table rise and mire expansion. Possibly, a period of water table rise was a motivation
for trackway construction.
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
20
in the study area. The tree-ring record consists of 4700 trees,
oak (
Quercus
spec
.
) and pine (
Pinus
sylvestris
), from the
mires of the northwest-German lowland. The chronologies
span from 6703 BC to 931 AD (at various stages described by,
for example, Delorme 1983; Leuschner
et al.
2002; Eckstein
et al.
2011). The peat-preserved trees grew at sites strongly
affected by hydrological change (
e.g.
Schweingruber 1993;
Linderholm 2002; Eckstein 2010). The trees show phases
of woodland establishment, growth and collapse (Eckstein
et al.
2011). These phases show much synchrony across
different sites in the study area (Eckstein
et al.
2011), and
also with the Netherlands, Ireland (Leuschner 2002) and
Southern Sweden (Edvardsson 2011). They therefore qualify
as an indicator for environmental change in the region, which
ought to be mostly climatically driven (Leuschner 2002).
The phases of high tree mortality (die-off phases) have
been identifed to indicate mire expansion and mire water
table rise. This was evident on the basis of upward growing
roots, the composition of peat forming plants and the degree
of decomposition (Leuschner
et al.
2002; 2007; Eckstein
et al.
2009; 2010). Tree die-off phases are a good indicator
for mire water table rise (Leuschner
et al.
2002; 2007;
Eckstein 2009; 2010), whereas tree-ring-width (TRW) has
been found to refect hydrological changes not exclusively
at central European mire sites (Dauskane
et al.
2011). The
meteorological implication of mire water table rise and mire
expansion varies, but here, we focus on the timing of such
landscape-level changes rather than their causes.
This study investigates a possible correlation of trackway
construction with mire water table rise and mire expansion.
In the following, dendrochronological dates for wooden
trackways are evaluated for contemporaneity to die-off
phases of the peat-preserved trees from the area.
2. Material and methods
2.1 The peat-preserved trees of northwest Germany
The tree species regarded in this study only include oak
(
Quercus
spec.) and pine (
Pinus sylvestris
). The woodland
remains were preserved in peat, and mostly exposed by
peat-harvesting. A total of 2090 oak and 2610 pine trees
from northwest-German mires, spanning from 6703 BC
to 931 AD, form the environmental record. These remains
are only tree stumps and stems; no timbers from human
constructions, like the trackways or buildings, are included
in the environmental record.
The chronology of peat-preserved oaks cross-dates well
with the Göttingen chronology of timbers from buildings
and other constructions, which reaches back from the present
(2009 AD) to 610 BC. The main sections of both (oak and pine)
chronologies of peat-preserved tree remains cross-date well
with the Göttingen chronology of riverine oak from central
and northwest Germany, which covers from 7197 BC to 1136
AD. All sections of the peat-preserved pine chronology are
securely cross-dated with the peat-preserved oak chronology.
Figure 1.
Study area in northwest Germany.
Elevations are shaded and sampling sites
indicated.
0 100 km
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
21
Generally, tree die-off refects (a period of) water table
rise in the mires and mire expansion. These are related to
hydrological changes, depending on sites. Ground-water
levels relate to effective precipitation, and – at least in coastal
areas – can be affected by sea level changes (Behre 2008).
Trees from fen sites (minerogenic mires) are affected by
ground water table fuctuations (Leuschner
et al.
1986). In
contrast, trees on raised bog sites (rain-fed mires) should mark
dry phases (Moir 2010). Many of the trees were preserved
at the base of raised bog peat, and therefore mark phases
of raised bog expansion. Raised bogs function like sponges
collecting rain water. Raised bog communities are highly
competitive under nutrient-poor conditions in moist climates
(Van Breemen 1995). Even though raised bog development is
largely associated with high effective precipitation (Overbeck
1975), raised bog development atop fen peat can also be
favoured by decreasing ground water tables (von Bülow
1935; Hughes 2004; Tahvanainen 2011). High precipitation
results in accelerated peat-accumulation, which eventually
disconnects the mire surface from the ground water (Overbeck
1975). Intermediate dry phases can also favour raised bog
development on fen peat, by lowering the ground water table
and thereby cutting the nutrient-supply to the surface (von
Bülow 1935; Hughes 2004; Tahvanainen 2011). The fen-bog
transition is often marked by tree layers, their die-off phases
indicating the beginning of raised bog growth.
The two tree species are represented unevenly at different
sites. Generally, the more competitive oak dominates the
nutrient-richer sites infuenced by ground water. The oak
material from such low-elevation sites displays long and
dispersed die-off phases, caused by ground water level rise
along an elevation gradient (Leuschner 2003). Fen sites at
the transition to raised bog are rather levelled in elevation
and water table. Hydrological changes at these sites cause
distinct die-off phases in both species. The undemanding
pine dominates only at poor sites. The occurrence of subfossil
pines often marks the fen-bog-transition. The record also
contains pines from sandy sites at the base of the peat and
from raised-bog layers. The latter occur only occasionally
and are most indicative of climatic variation (Moir 2010).
Pine die-off phases are typically rather distinct, due to the
more levelled character of the sites and to the sensitivity of
trees growing close to their ecological limit.
Differences in wood preservation between the two species
add to their disparity. The pines are mostly preserved to
bark edge, which enhances the more distinct appearance of
the pine die-off phases. In contrast, many of the oaks are
preserved to the heartwood-sapwood boundary. Estimated
numbers of missing rings were added for the die-off dates,
but this naturally is less precise by comparison.
Tree die-off is a good indicator for the occurrence of
mire water level rise in the study area. However, restrictions
should be considered: frstly, drowning trees can take a few
years to die, and therefore the die-off phase will be slightly
lagging behind the beginning of water table rise. Secondly,
the absence and the end of die-off phases are somewhat less
reliable indicators. This is due to the detection of water level
rise by this method requiring suffciently old trees at affected
sites, which can at times be absent or already dead.
2.2 The trackways
Over 300 peat-preserved trackways have been reported for the
region (Metzler 2006), but precise (
i.e.
dendrochronological)
dates for them are scarce. The dendrochronological
trackway-dates from the study area include dates taken from
literature, as well as age determinations performed in the
dendrochronological laboratories of Göttingen (indicated
in Table 1). The number of dendrochronological dates for
Table 1.
The dendrochronologically-dated trackways of the northwest-German lowland. Dates refer to the last measured ring.
Construction
date no.
Construction dateTrackway IDTimber datesBog AreaPublished
i.a.
14629– 4545 BC
Pr31
4628–4545 BC
1
CampemoorBauerochse
et al.
2012
23798 BC
Pr35
3798 BC
1
CampemoorBauerochse
et al.
2012
33701 BC
Pr34
3701 BC
1
CampemoorBauerochse
et al.
2012
42900–2882 BC
Pr32
2900–2882 BC
1
CampemoorLeuschner
et al.
2007
51357 BC
Wanna
1357 BC
1
b. Wannaunpublished
Ip36 / Pr36
1357 BC
2
IpwegermoorSchmidt 1992
6754–749 BC
Ip12a
754–749 BC
2
IpwegermoorSchmidt 1992
7719–713 BC–682 BC
Le21
Le9
719–718 BC
2
, 683/682 BC
2
Lengener MoorSchmidt 1992
717–714 BC
2
Lengener MoorSchmidt 1992
Ip12
Ip62
716–713 BC
1,2
b. OldenburgSchmidt 1992
b. Oldenburg
8128 BC
Ip42
128 BC1WittemoorMetzler 2006
960–43 BC
Pr6
60–55 BC, 43 BC
53–46 BC
1,2
Großes Moor b.
Diepholz
Fansa, Schneider 1996
10222 AD
Pr4
222 AD
1,2
Großes Moor b.
Diepholz
Fansa, Schneider 1996
11497–502 AD
Uchte
497–502 AD
1
Uchter Moorunpublished
1
– dated by B. Leuschner, DELAG;
2
– dated by B. Schmidt.
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Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
22
peat-preserved wooden trackways is relatively low, due to
the early excavation of the trackways since the 19
th
century,
when dendrochronological dating was not suffciently
established and the long tree-ring-chronologies had not yet
been developed. The trackways that are included vary greatly
in number of dated timbers, preservation state of the wood
and documentation. Therefore, detailed evaluations (
i.e.
the
time-span of usage,
etc.
) were not possible in most cases.
Trackways without certain and precise determination of at
least one felling date were excluded.
Figure 2.
[A] gives the lifespans of trees, sorted by time of death and mire area of origin (several sites taken together). Severe die-off events are indicated by
vertical lines (grey). [B] gives the annual tree die-off (black flled, scale on the left), and the corresponding
die-off rate a-30
(black line, scale on the right).
Both are smoothed by a 15-year running mean. The mean value of the
die-off rate a-30
is indicated by a horizontal line.
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
23
The study includes 12 (+1) trackways (Table 1). The
northwest-German trackway fnds have archaeological
IDs (indicating the location or frst investigator). Two of
the trackways included in this study have no IDs to our
knowledge, and are designated by their place of origin (
i.e.
Wanna
and
Uchte
). In some cases, two trackways excavated
at different times, turned out later to be parts of one trackway,
interrupted by mineral soil islands in the mire. Their sections
have been united for this study, using both original trackway-
IDs (
i.e.
Ip12&Ip62
and
Le9&Le21
). Conversely, one
trackway appeared under dendrochronological investigation
to contain timbers from a predecessor construction (Schmidt
1992). This construction previous to
Ip12
is called
Ip12a
in
the following.
Also, simultaneous constructions occur. This study has
a temporal focus, and therefore
11 construction dates
are
referred to, rather than 12 (+1) trackways (Table 1).
2.2.1 Excluded trackways
Five dendrochronologically-dated trackways were excluded,
because erosion and decay on the outer parts of timbers did
not allow for precise determination of felling dates. The
“dates after which” for these trackways are: Bc34, after
763BC and after 752 BC; Pr3, after c. 665–640 BC; Pr25,
after 218–160 BC; Bc32, after 85 AD and after 120 AD;
Cl01, after 273 AD and after 334 AD.
2.3. Method:
die-off rate a-30
As described in section 1, the peat-preserved trees from the
area display synchronous phases of tree-establishment and
common die-off. The die-off phases in particular are highly
indicative of hydrological change (water table rise and mire
expansion). However, the heterogeneous replication and
overlapping woodland phases in the record do not allow for
a good determination of die-off phases via die-off frequency,
replication or die-off percentage.
Therefore, a measure called
die-off rate a-30
was
developed,
i.e.
annual tree die-off divided by replication
30 years previously. The time lag of 30 years was chosen
according to the common length of tree generations and
die-off phases in the data-set. This reduced the replication-
dependency of die-off peak- height, while retaining the
temporal distribution of the die-off phases (Figure 2).
Oak and pine have been combined in the
die-off rate
a-30
displayed in Figure 2, and are displayed separately in
Figure 3. A separation by species is advantageous, as the
environmental signal of both species is not entirely identical
– due to ecological differences.
The estimated number of rings missing to bark was
closely examined for the die-off dates. A reduced data-set
was tested in which all samples with a standard deviation of
missing rings to bark larger than 3 (eroded samples) were
excluded. This produced no notable alteration of the curve.
Figure 3.
The
die-off rate a-30
is displayed for oaks (black), pines (red), smoothed by a 15-year running mean. The
tree die-off rate a-30
(average of the
other two curves) is shown (grey, flled) with its mean value (grey horizontal line). The trackway timber dates are indicated by vertical lines (blue), with
coloured boxes uniting dates from one fnd. The construction date numbers are given above, in grey for construction dates outside of die-off phases. The
trackway-IDs are given below the date numbers.
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
24
As mentioned above, the vast majority of pines are preserved
to bark edge. The oaks mostly feature sapwood, which allows
for good estimations, and in several cases sapwood rings
could not be measured, but it was still possible to count them.
To show the relation between die-off frequency and the
die-off rate a-30
, dates of all trees are shown and used to
calculate the
die-off rate a-30
in Figure 2. But because
pine is clustered in the record in large numbers, their signal
swallows up that of oak, which had not been sampled in the
same fashion. To retain the environmental signal as best as
possible, the
tree-die-off rate a-30
used in the following
was calculated differently:
die-off rate a-30
was calculated
for each species, and smoothed by a 15-year running mean
(Figure 3). These oak and pine values were averaged to
gain the
tree die-off rate a-30
(Figure 3). Where one species
featured 0-values due to 0 replication, the values of the other
species were used alone. The mean value is calculated from
the
tree die-off rate a-30
over the 5500 years shown.
3. Results
The main result of this study is that the majority of
investigated trackways were found contemporaneous to
tree die-off phases (section 3.2, Figure 3), which indicated
water table rise and mire expansion (see sections 1. and
2.1). The die-off phases are displayed in this study by the
die-off rate a-30
(3.1., Figure 2 and Figure 3). One case of
contemporaneousness of trackway construction and tree
die-off phase (trackway
Pr 31
, 4629–4545 BC) is described
more detailed (3.3., Figure 4).
3.1. Die-off phases
The
tree die-off rate a-30
features over-average values
(regarded as die-off phases) for 40.4% of the 5500 years
regarded. There are two cases of an edge-effect, where a
small number of die-offs are over emphasised by the rate.
This is the case around 3360 BC and around 1645 BC, where
“last survivors” cover the edge of pine-chronology gaps for
some decades (Figures 2 and 3). With this exception, the
die-
off rate a-30
refects the tree distribution very well.
The two tree species show much agreement, but also
differences, in their die-off curves (Figure 3). In general, the
pine die-off phases are shorter and more distinct than those
of the oaks (see Figures 2a and 3a, compare section 2.1.).
In the study period examined, from 4750 BC to 750 AD,
the distribution and appearance of die-off phases varied over
time as well:
The period before c. 3500 BC is well represented by
both tree species. Pine and oak both show distinct and
synchronous die-off phases (Figure 3a). High and distinct
peaks occur approximately every 150–250 years. The die-
off phases are largely related to the initiation of raised bog
growth at the respective sites.
Between c. 3500 BC and c. 1500 BC, the pine record
produces prominent peaks, which mark raised bog expansion
at inland sites, around 2880 BC, 2740 BC, 2280 BC and
2175 BC. The contemporary oaks mainly stem from low
elevation sites, infuenced by groundwater. Their die-off
phases appear more moderate and stretched out, as the trees
die successively along an elevation gradient. An example of
the discrepancy between the species is the pine-only die-off
peak around 2740 BC.
After c. 1500 BC, there are only a few dated pines. The
oak material in this section originates from inland raised
bogs, where they mark the fen-bog transition. Similar to the
frst section (before c. 3500 BC), these die-off phases appear
more distinct. The oak peaks around 700 BC, 550 BC and
around 120 BC appear particularly clear.
3.2 Contemporaneity of trackway construction and die-
off phases of the peat-preserved trees
Nine of the eleven trackway construction dates (81%) clearly
date within die-off phases (Figure 3). Some of them are
represented by more than one trackway. However, at one of
these dates (date 5), the
tree die-off rate a-30
shows values
only slightly above average.
Two trackway construction dates are contemporaneous to
below average
die-off rate a-30
values. One of these, date 2
(3798 BC) dates directly (three years) after a die-off phase,
and therefore appears in the context of environmental change
nonetheless. By contrast, the trackway construction date 10
shows no apparent temporal relation to tree die-off phases;
it dates to a period in which the tree record is represented by
oak only.
3.3 Trackway
Pr31
, documented for 4629–4545 BC
Tree die-off is a sound hydrological indicator, but by no
means the only environmental indicator contained in the
dendrochronological record. In the following, tree-ring-
width (TRW) variability is additionally considered for
one trackway construction date. The Neolithic trackway
Pr31
(date 1, 4629– after 4545 BC; Figure 4) is the oldest
construction in our data. It is represented by a number of
dated timbers, with a range of dates accounting for repeated
work on the trackway (Figure 4a). The contemporaneous
dendrochronological record is well replicated for both tree
species. Pine and oak from several sites display a major die-
off phase at the time (see Figures 3a and 4c). Stratigraphically,
the oaks as well as the pines largely mark the beginning of
raised bog growth at respective sites. Conditions stressful
to the trees are also implied by the contemporaneous series
of growth depressions displayed by the regional TRW-
chronologies (Figure 4b, grey boxes).
Samples of 36 timbers from two excavated sections of
trackway
Pr31
were dated. They display a range of dates,
dispersed over 84 years. A number of samples preserved to
bark edge, accompanied by other samples that date shortly
prior to these, indicate construction and maintenance for
4629 BC, 4614 BC, 4606 BC, 4590 BC, 4557 BC and after
4545 BC (dates regarding last ring, Figure 4a).
The contemporaneous die-off phase appears stretched-out
long with three peaks (Figure 2a). It is refected similarly by
both tree species, with some minor differences (Figure 4c).
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
25
The supposed frst construction (4629 BC) is contemporaneous
to a frst die-off pulse of the die-off phase (Figure 4c).
Maintenance appears to have taken place rather continuously,
as dates are scattered over the following years. The
maintenance activity of 4590 BC displays the most striking
concurrence: in 4590 BC, increased stress is refected by
drastic declines in the oak and pine TRW-chronologies
(Figure 4b). Also, the TRW- mean curve of the trackway
timbers, representing a local signal, displays a sharp decline
in ring width. After 4590 BC, work on the trackway seems
to have come to a halt, as only two, not fully preserved,
timbers date within the following 30 years. This apparent
Figure 4.
[A] shows the life spans of the trackway timbers from
Pr31
. Measured rings in black, missing rings are indicated in grey. Grey arrows indicate
where more rings may be missing. Timbers with sure bark edge are indicated with ‚x‘. Clustering dates, sure felling dates and the last acquired date are
indicated by vertical lines and labelled. [B] shows the TRW-mean curve of the measured trackway timbers, the Northwest-German pine and oak TRW-
chronologies. The year 4590 BC is indicated by a triangle. It is the beginning of the growth depression displayed by the trackway timbers and also the
begin of a growth depression displayed by the pine and oak master chronologies, as well as a apparent maintenance date, indicated by a cluster of trackway
timber dates. [C] gives the
die-off rate a-30
for the trees (average of the 15-year running mean
die-off rate a-30
curves for pine and oak) and its mean value
(horizontal line).
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IANSA 2015 ● VI/1 ● 19–29
Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
26
halt in maintenance is contemporaneous to the main peaks
of the die-off phase (Figure 4c, and also Figure 2a), as well
as two pronounced growth depressions in all three TRW-
chronologies (Figure 4b). Shortly after both tree species
have passed their main die-off peaks, the next maintenance
activity is apparent in 4557 BC. The last peak of the die-
off phase occurs around 4540 BC for pine and oak, around
the same time as the last date acquired from the trackway
(4545 BC with 5±5 missing rings). This makes the time of
documented work on the trackway practically identical with
the duration of the die-off phase. Strikingly, construction, the
halt in maintenance, and abandonment, all appear to coincide
with die-off-peaks. The maintenance activity in 4590 BC is
contemporaneous to a strong negative growth reaction of the
trees and the rise of the die-off-curves toward the main peak.
4. Discussion
4.1 General discussion
This comparison shows a considerable temporal agreement
of trackway construction with tree die-off phases: nine of
the eleven trackway construction dates, corresponding to
ten of the twelve trackways (plus a supposed predecessor
construction), date clearly within tree die-off phases. One
construction date follows a die-off phase by three years.
Indications for wet phases within the context of trackway
fnds have repeatedly been described (
e.g.
Bauerochse
2003; Behre 2005). Whether these wet phases were actually
contemporaneous to the time of construction, is now clarifed
by means of dendrochronological dating for both trackway
constructions and water table rise.
What remains a subject for discussion is the reason for the
observed correlation. There are two possible explanations:
frstly, the correlation might be due to the better preservation
of trackways from wet periods (Spurk
et al.
2002). Secondly,
water table rise and mire expansion might have motivated
trackway construction (Bauerochse 2003; Bauerochse
et al.
2012; Behre 2005; Leuschner
et al.
2007; Metzler 2003).
A preservation-produced correlation of wet phases
and trackways would be expected to depict trackway
constructions preceding water table rise. The data presented
here indicate that the opposite is the case, showing that
the constructions follow a water table rise. This does not
point towards a conservation bias, but rather towards an
environmental change infuencing construction activity.
Conservation conditions in the mires were generally good
over long periods. This is indicated by the thick continuous
layers of weakly decomposed peat and well-preserved
ancient wood, found just below the mire surface. This is
particularly true for nutrient-poor and acid raised bogs.
However, preservation of organic material is generally better
with increasing wetness, and well preserved timbers are
vital for this study. Spurk
et al.
(2002) took the preservation
of ancient trackways for an indication of wet conditions
in itself. Well-preserved constructions are more likely to
be found, excavated and dated. In the study area, over 300
trackways have been described (Metzler 2006), however,
and the relatively small number of dendrochronological
age determinations for them is mostly due to their early
excavation, before dendrochronological dating had been
suffciently established. Decay and erosion on trackway
timbers can often be assigned to the time after mire
drainage. Nonetheless, the impact of varying conservation
conditions cannot be ruled out entirely, at least until more
dated trackways are available for comparison. Should
varying preservation conditions be the cause of the observed
correlation, this restriction should be taken into account
when archaeological wetland-fnds are being interpreted.
Hydrological change at a landscape-level surely
affected the human population: in their land use, mobility,
economics and social stability. Environmental changes and
socio-cultural dynamics are interwoven in multiple ways
(Gronenborn 2006). So there might be indirect connections
between environmental changes and construction activity, as
crop failure can cause people to change their behaviour or
location.
But what of a more direct effect of environmental forcing
on trackway-building activity? The context of increased
surface wetness has repeatedly been suggested for
excavated trackways, mostly based on peat-stratigraphical
and palynological indications (
i.e.
Bauerochse 2003;
Behre 2005; Baillie and Brown 1996; Leuschner
et al.
2002; 2007; Metzler 2003). Several authors see the
motivation for the construction of trackways as being just
such an environmental change (
e.g.
Bauerochse 2003;
Behre 2005; Leuschner
et al.
2007; Metzler 2003).
For the British Isles trackway constructions, clusters have
been observed (
e.g.
Baillie and Brown 1996; Brunning and
Macdermott 2013). Plunkett
et al.
(2013) have compared
these temporal clusters to testate amoebae records from
peat as a proxy for mire surface wetness; they see no clear
relation of trackway clusters in Ireland with hydrological
change. In contrast, in the UK, Brunning and Macdermott
(2013) fnd these clusters to correlate well with phases of
wetter climate, indicated by peaks of ice raft debris in the
Atlantic and atmospheric carbon isotopes. However, in both
cases, clusters which span several centuries are considered
together with long term environmental changes, whereas
the present study focuses on the actual timing of individual
construction activity and its contemporaneousness with
short-term environmental changes. We agree, however,
with Plunkett
et al.
(2013) that motivation for trackway
construction in general is surely not monocausal.
The trackways were constructed very differently, some
are narrow bridges, whereas others are frm wooden roads ft
for wheeled traffc. Mire development should have affected
all of them (Raftery 1996), for example, by the drowning of
older trackways, impassability of previously walkable mires,
or the blockage of formerly dry routes due to paludifcation
and mire expansion.
The previous statement – that the wetlands in question
were generally wet enough to provide good preservation over
several millennia – does not necessarily mean that they were
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Change: a Dendrochronological Study in Northwest-German Mires
27
always too wet to walk on. Of course, passability is relative,
depending on the required frequency and convenience
of passage. However, the observation of numerous short
trackways bridging the lagg (the wet fen at a raised bog
margin) towards raised bogs in the British Isles (Brunning
and Macdermott 2013) supports the view that prehistoric
people did not always require trackways to walk on raised
bogs. To relate the trackway occurrence in raised bog to
humid phases – which might have affected mobility directly
– is therefore obvious.
4.2 Site specifc comparisons
Trackway construction date 1 (4629–4545 BC,
Pr31
) is a
striking example of close temporal agreement between the
environmental parameter and work on the trackway: the frst
construction dates to the frst steep rise of the tree die-off
curve, and the last timber date to the fnal peak of the die-
off phase. Within the period of over 80 years in which the
trackway had been maintained, there is a 30 year activity gap,
which agrees temporally to the main peaks of tree die-off.
A possible explanation for the gap is a temporary fooding
of the trackway or its surroundings, causing intermediate
abandonment. Likewise, the construction, as well as the fnal
abandonment of the trackway, appear to be related to a water
table rise.
Construction date 2 (3798 BC,
Pr35
), dates three years
after
a die-off event, displayed most clearly by pine. At that time,
the northwest-German oak chronology displays a distinct
growth depression, which also commonly corresponds
to increased surface wetness at the mire stands. The
considerable water table rise at this site, which had already
been observed in palynological data (Bauerochse 2003), on
the other hand, probably corresponds to the major die-off
phase which begins some 40 years after the construction of
Pr35, and is contemporary to date 3 (3701 BC, Pr34) instead.
Interestingly, the southwest-England S
weet Track
had been
constructed nine years previous to trackway
Pr35
. For that
site (Somerset Levels, UK), indications for water level rise
were observed between the constructions
of the
Post Track
in 3838 BC and the
Sweet Track
in 3807 BC (Brinnung and
Macdermott 2013). Even though the sweet track is located c.
700 km from the German site, climatic variations can be in
tune. This is apparent in the far-reaching agreement that has
been observed between the oak chronologies of northwest
Germany, the Netherlands and Ireland (Leuschner
et al.
2002), including the regarded time around c. 3800 BC. As
southwest England is located between Ireland and northwest
Germany, a similar development there at the same time seems
likely. Hence, the construction of
Pr35
appears to stand in
context of a preceding mire water table rise, which seems to
have been caused by climatic conditions in the wider region.
Construction date 4 (c. 2900–2883 BC,
Pr32
) is another
example of suggestive contemporaneity. It was found to
have been constructed, as well as abandoned, in the context
of a mire water table rise, uniquely recorded by on-site trees
(Leuschner 2007).
In the case of trackway construction date 5 (1357 BC,
Wanna
and
Ip36
), the contemporary tree-record is oak-
only. The
tree die-off rate a-30
is only slightly over-
average, making this a borderline case. It has a low rate of
replenishment, which can indicate moist periods. The site
being at a low-elevation site near the coast, which had long
been forested, the last trees died off shortly before. A site
just further inland records a contemporaneous fen-bog-
transition at the start of the die-off phase. Both sites point
towards wetness. Most remarkable is the contemporaneous
construction of two trackways in the same year, some 70 km
apart from one another.
Interesting is also a fnd from the adjacent region:
dendrochronological dating of Bronze Age tree-trunk
coffns from the Jutland peninsula (Denmark and N
Germany) revealed that the vast majority (25 of 28 dated
coffns) had been made within only ffty years, between
1391 and 1344 BC (Christensen
et al.
2007). While the
burial custom lasted over a millennium, most of the
thousands of burial mounds in the region did not contain
preserved coffns (Christensen
et al.
2007). Whether more
humid conditions in the mid-14
th
century BC might have
favoured the preservation of the coffns, which were not
peat-embedded, is uncertain.
A water table rise at the time of construction dates 6–7
(date 6, 754–749 BC,
Ip12a
; date 7, 719–713 BC and
682 BC,
Le9&Le21
and
Ip62&Ip12
) is clearly evident
and has been observed at other sites in Europe. Most
prominently, the abandonment of the dwelling at Biskupin
(Poland) has been connected to a water table rise and dated
dendrochronologically to 721 BC (Waszny 1994). Tree
ring records display the event at c. 720 BC in Ireland as
a germination phase, in the Netherlands as a die-off phase
and in the German trees as both a germination and die-
off phase, with simultaneous growth depressions in either
data set (Leuschner
et al.
2002). Furthermore, the Dutch
subfossil oaks from
Diemen
display a gap in germination
events between 880 BC and 670 BC, also attributed to this
wet event (van Geel
et al.
2009).
Moreover, paleo-environmental records from peat and
pollen data record a signifcant wet-shift across Britain and
central-Europe and elsewhere (van Geel
et al.
1996). It has
been ascribed to the time in question,
e.g.
by radiocarbon
dates c. 800–650 BC for Jutland recurrence surfaces
(Barber
et al.
2004). In particular, the pollen records leave
no doubt as to the nature and severity of the event; however,
options of precise alignment are limited, as peat layers are,
by comparison, not well datable, and their formation can be
a very slow and/or time-lagged process (Blackford 2000).
The trackways (754–682 BC) were clearly constructed
during a time of environmental change, which appears
to have been a time of socio-cultural changes as well.
Migration to the coastal marsh and other low elevation sites
(c. 2 m asl) of the northwest-German lowland (Schwartz
1990) and the adjacent northeast Netherlands (
e.g.
Waterbolk 1962) dates to around this period. Connections
with hydrological and coastal changes (van Geel 1996) or
land-use-caused erosion and dune formation on the Geest
plateau (Waterbolk 1962) have been suggested.
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Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental
Change: a Dendrochronological Study in Northwest-German Mires
28
The trackways of date 6–7 are kilometres long,
elaborately built and are broad constructions of massive
oak split-planks, which appear ft for wheeled traffc
and hence might have been part of a long-distance route
connection. The Bronze Age – Iron Age transition was
accompanied by the collapse and establishment of trade-
route systems (Collis 2003), which might have had some
effect on road building activity here. And then again, socio-
cultural dynamics can be linked to environmental change in
turn (
e.g.
Groneborn 2006).
5. Conclusion
The tree die-off phases in the mires of the area are refected
well by the
die-off rate a-30
. This parameter was designed to
show variations within years and decades rather than long-
term changes. The precisely (dendrochronological) dated tree
die-off phases indicate water table rises and mire expansion.
The study found that the majority of dendrochronological
trackway construction dates from the study area are
contemporaneous to die-off phases of peat preserved trees.
If this is not an effect of conservation, the fnding supports
the view that, besides the sociocultural aspects,
the timing of
trackway construction might commonly have been related to
mire expansion and mire water table rises.
Acknowledgments
We thank the German Research society (DFG) for supporting
the study (projects LE 1805/2-1, 2-2 and HA4439/1-1, 1-2).
We thank the peat mining companies and members of ASB-
Humus, Griendsveen and Schwegermoor GmbH for kindly
granting us access to their working areas. Also, we thank
Felix Bittman and the Lower Saxony Institute for historic
Coastal Research (NIhK) for allowing us to use the data of
the wooden trackway at Wanna site and Barbara Leuschner
of the Dendrolabor Göttingen (DELAG) for sharing her
data. We also want to thank Lennart Linde for his valuable
illumination of the archaeological perspective.
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