Subfossil oaks from bogs in NW Europe as a

Subfossil oaks from bogs in NW Europe as a (dendro)archaeological archive
HANNS HUBERT LEUSCHNER, UTE SASS-KLAASSEN
Zusammenfassung: Über 2600 dendrochronologisch datierte Jahrringfolgen subfossiler Eichenstämme aus deutschen, niederländischen und irischen Mooren belegen den Zeitraum zwischen 6000 BC und 1700 AD. Die zeitliche
Verteilung der Lebensspannen dieser Bäume weist im Vergleich innerhalb und zwischen Standorten mehr oder weniger deutliche Übereinstimmungen von Keimungs- und Absterbephasen (GDO-Phasen) auf. Beide Phänomene treten
dabei tendenziell gleichzeitig und nicht zeitlich versetzt auf. Als Möglichkeit zur graphischen Erfassung solcher Phasen wurde daher das durchschnittliche Lebensalter aller Bäume in jedem Kalenderjahr berechnet (mean age-Chronologie). Ungestörtes Wachstum der Bäume/Wälder führt zu einem kontinuierlichen Anstieg des Durchschnittsalters, Verjüngungsphasen (Generationswechsel) bedingen einen Abfall der Kurve. Regionale mean-age-Kurven zeigen
im europäischen Vergleich weitgehende Übereinstimmungen. Demnach ist Populationsdynamik der ehemaligen
Moorwälder überwiegend durch überregionale klimatische Veränderungen mit GDO-Ereignissen in Feuchtphasen
bedingt. Dendrochronologisch datierte Bohlenwege um 720 BC und 130 BC stimmen exakt mit markanten Abfällen
in den mean-age-Chronologien überein – ein Hinweis auf den Einfluss des Klimas auch auf menschliche Aktivitäten.
Abstract: The dendrochronological data set of absolutely dated subfossil oak trunks from Irish, Dutch and German
bogs consists of some 2600 series. They cover the period from 6000 BC to 1700 AD. The distribution of the trees in
time shows distinct changes in the frequency, germination and dying-off. The comparison of germination and dyingoff (GDO) phases within and between bog-oak sites shows that there is a tendency for both phenomena to occur not
in separated phases but synchronously. One way to graphically represent germination and dying-off phases is to calculate the ‘mean age’ of all trees at every calendar year. Where trees are uniformly ageing the mean age chronology
rises; recruitment of juvenile trees and dying-off of old trees causes the chronology to drop. Regional mean-age
chronologies of the bog oaks contain similar elements, sometimes over long periods. This observation indicates common large-scale climate forcing. A strong link could be found between dendrochronologically dated bog trackways
from the period around 720 BC and 130 BC and dendroclimatological results that indicate a change towards wetter
conditions at the same periods.
Keywords: bog oaks; climate; dendrochronology; population dynamics
Introduction
From 1970 onwards, European tree-ring laboratories have studied subfossil oak trunks preserved
in bogs, river gravels and marine/brackish sediments. Tree-ring series of these trees have been
used to compile ultra-long absolutely dated treering chronologies which extend back to 8400 BC
(SPURK et al. 1998, LEUSCHNER 1992; PILCHER et
al. 1984 JANSMA 1996). This paper considers these
oaks that grew under difficult ecological conditions on the surface and margins of the peat.
Fluctuations of the generally high ground-water
table – partly triggered by climate – are assumed
to have a major impact on population dynamics
and tree growth on these sites. Obviously dendrochronologists wish to understand if there is a
connection between climatic conditions and the
distribution of the bog oaks through time. In the
beginning, when only a few sites with subfossil
oak findings were available there was evidence
for synchronous dying-off phases of oaks in different bogs (LEUSCHNER et al. 1987). Such agreements in population dynamic would suggest climate control. And indeed, LEUSCHNER et al. (1987)
found some indication that periods of simultane-
ous dying-off episodes at more than one site were
related to an increasing wetness of the climate.
Overall, however, with information from more
and more sites available, the picture tended to
become blurred. Thus, with increasing information it becomes difficult in all but a few cases to
see climate control; it is obvious that there are
other, often local, factors involved. However, the
huge amount of data that has been collected during the last 20 years in Germany, Ireland and the
Netherlands enables to make comparisons on a
bigger scale. In order to handle the large amounts
of data involved in an inter-regional study some
method has to be devised to integrate the information from different areas.
The new variable ‘mean age’ is introduced
because it combines both germination and dying-off
events in a single parameter. It describes the variation in tree age through time and is used to detect
common abrupt changes in population dynamics
(germination and dying-off) of subfossil oaks from
different regions in NW Europe. Regional tree-ring
chronologies are used to detect (contemporary)
changes in tree growth. This paper provides a general outline of the results, a detailed description can
be found in LEUSCHNER et al. (2002).
Subfossil oaks from bogs in NW Europe as a (dendro)archaeological archive
211
NORTH SEA
IRELAND
ENGLAND
NETHERLD.
GERMANY
Fig. 1 Subfossil oak finds in Germany, The Netherlands and Ireland
-6000
100
-5000
-4000
-3000
-2000
-1000
0
1000
-6000 -5000 -4000 -3000 -2000 -1000
0
1000
GERMANY
50
REPLICATION
Beside such large scale comparisons, in situ
finds of subfossil oaks provide the possibility to
reconstruct local hydrological changes and bog
developments. We demonstrate this potential for
the site Hammah near Stade (LEUSCHNER &
DELORME 1986). For the archaeologists it is interesting if periods where subfossil oaks indicate
past climatic changes are also reflected in the
record of human activities. German trackways
that are dated by dendrochronology prove this
link between the “natural” finds and archeological finds of oaks.
0
50
0
50
“Coastal” subset
IRELAND
NETHERLANDS
0
YEAR BC/AD
Material
Dendrochronologically dated tree-ring series of
2600 subfossil oaks from 197 sites located in
Northern Germany (n = 1561, 101 sites), The
Netherlands (n = 301, 43 sites) and Ireland (n =
741, 51 sites) were used in this study. The tree-ring
series run from 6069 BC (Germany) to AD 1596
(Ireland). The German and Irish series are continuously replicated throughout most of this period,
whereas the smaller collection of Dutch subfossil
oaks shows a gap between 4700 BC and 3500 BC.
Fig. 2 Replication of German (total and subset of low
elevation sites), Irish and Dutch subfossil oaks
The two oldest collections of Dutch subfossil oaks
are dated against the German oak chronology
(JANSMA 1996). All subfossil oak chronologies are
dated against regional chronologies of archaeological and modern oak. The German material is
further categorized into two subsets of “inland”
(high elevation sites >2 m a.s.l and “coastal” (low
elevation sites <2 a.s.l) material. Fig. 1 shows the
locations of European subfossil oaks, Fig. 2 the
temporal replication of the material.
212
Method
The basic approach of the large-scale regional and
temporal comparisons of population dynamics is
the calculation of regional mean-age chronologies. The mean-age value for each given year is calculated as the arithmetic mean of the age of all single trees in this specific year. The value of a group
of trees decreases when the older trees die while
younger ones live on, or when young trees replace
old trees. It increases when no population changes
occur and the existing trees live on. In more general terms, this could be thought of as disturbed
(mean age decreasing) and undisturbed (mean age
increasing) populations. This is a robust way of
conveying a lot of complex information and is useful for comparisons between regions. Mean age is
therefore a tool to grasp the chronologically and
spatially fuzzy distribution of generation changes
in forests on a large scale in an objective way.
Results and discussion
Population dynamics of subfossil oaks
The temporal distribution of the life spans of the
North German subfossil oaks and the corresponding mean-age (MA) chronology is shown in Fig. 3.
It is obvious that germination- as well as dying-off
(GDO) phases on different sites occur more gradual instead of being abrupt changes in the population dynamics. However, the MA chronology
clearly mirrors common changes in population
dynamics at different sites. Even less distinct GDO
phases are clearly reflected as declining phases in
the MA chronology. As Fig. 4a shows, there is evidence for considerable similarity in the German
and Dutch oak population, especially if the German material is reduced to the subset of low-elevation sites. This similarity led to the conclusion
that it is valid to combine the German and Dutch
oaks and to calculate a so-called “continental” MA
chronology. This continental MA chronology is
subsequently compared with the Irish MA chronology (Fig. 4b).
The result is surprising: despite a distance of
about 800 km between the two groups of subfossil oak a remarkable agreement can be seen between
the continental and the Irish MA chronology in
the early part of the record, from about 5500 BC
to 2000 BC. There is not only a good match
between the large scale GDO phases, e.g. between
4000-3900 BC, around 2500 BC, and in 2000 BC,
but also a very good agreement in the high frequency variation between the two MA chronologies. It is very likely that phases with reduced mean
Hanns Hubert Leuschner, Ute Sass-Klaassen
age throughout this early period are the result of
increasing wetness, whereas phases of increasing
mean age may point to relatively dry conditions,
with more young oaks establishing.
After 2000 BC until 700 BC there are only
episodic periods of agreement which may be no
more than random. However, the two continental, the German and the Dutch MA chronology,
show a good agreement until about 1300 BC (Fig. 4a).
The Dutch and part of the German oaks grew on
low-elevation sites whereas the Irish oaks generally grew on higher sites. This suggests that marine
factors, i.e. sea-level changes and/or stewing back
of river systems might have had influenced the
hydrology on the low elevation sites in Germany
and the Netherlands considerably. This could
explain the obvious changes in the population
dynamics of oaks at these low elevated sites.
Population dynamics and human activity
It is obvious from the comparison of the continental and Irish MA chronologies (Fig. 4b) that a
large-scale event took place around 2000 BC that
resulted in different population dynamics of oaks
in continental and Irish mire woodlands. In this
context it is interesting that there is a widespread
indication of climate change in the later third millennium BC (DALFES et al. 1997). Moreover, there
are suggestions that vegetation changes around
and after 2000 BC are not related to changes in
climate but are due to increasing human activity
albeit probably driven by environmental conditions. As an example of the difficulty of separating human and natural factors, the notable drop
in the Irish mean-age chronology at 950 BC can
be taken: BAILLIE & BROWN (1996), who were
exclusively looking at Irish evidence have noted
that this decrease in the frequency of naturally preserved subfossil oaks in Ireland coincides with a
major building phase involving oak trackways and
settlements in different bog areas. Something they
argue may have been due to overall drier conditions between ca. 1000 BC to 880 BC. However,
German subfossil oaks show severe growth depression in the middle of the 10th century BC (LEUSCHNER et al. 2002). This wider context implies an
environmental component, which could involve
wetter conditions, something that could equally
have contributed to the demise of Irish subfossil
oaks at that time.
Indications for another sudden change towards
wetter climatic conditions in the first millennium
BC can be found in dendrochronologically dated
wood from continental archaeological excavations.
Subfossil oaks from bogs in NW Europe as a (dendro)archaeological archive
-6000
-5500
-5000
-4500
-4000
-3500
-3000
213
-2500
-2000
-1500
-1000
- 500
1
500
1000
-2500
-2000
-1500
-1000
- 500
1
500
1000
Sites > 2 m asl
200
150
YEARS
Sites < 2 m asl
MEAN AGE CURVE OF BOG SAMPLES
100
-6000
-5500
-5000
-4500
-4000
-3500
-3000
YEAR BC/AD
Fig. 3 The life spans of subfossil oaks from Germany clustered according to their site provenance. The mean-age
chronologies are given at the bottom. For reference, decrease of mean age (determined optically) is marked by lines
300
200
a
150
100
50
250
MEAN AGE [YR]
The Netherlands /
L. Sax
(low elevation sites)
200
150
Continent / Ireland
b
100
50
0
-6000
COINCIDENT MEAN AGE PHASES
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-4000
-3000
-2000
YEAR BC/AD
-1000
1
1000
Fig. 4 Comparisons of macro-scale mean-age chronologies in common periods between a) German and Dutch and
b) continental (all German and Dutch) and Irish subfossil oaks. Phases of good agreement are marked by green boxes
214
Hanns Hubert Leuschner, Ute Sass-Klaassen
100
200
300
G
500
400
1 2
3
4
600
5
700
6
oak
pre 175 AD
alder
Index ring-width series, growth depressions coloured
“events”
100
Starting with a clear
germination phase (G),
oaks grow together with
the alders in a dry phase.
175 - 300 AD
A rapidly expanding and
rapidly growing raised bog
“suffocates” the oaks and
preserves the trunks. Boggrowth steps (1-6) are
mirrored by growth depressions of “surviving” trees
350 - 700 AD
after 700 AD
Circles mark preserved
pith/sapwood,
arrows mark unknown
number of rings/years
in missing (rotten) wood
200
300
oak trunks and stumps
400
500
600
700
The site is as moist that
almost no oaks grow in the
alder carr. Only one oak with
a strong growth depressions
grows before 175 AD
The last of the oaks, standing
on sandy knolls, are covered
by the rising bog
Sphagnum peat
Fen peat
Mineral soil (sand)
YEAR AD
Fig. 5 Reconstruction of a the growth dynamic of the bog Kehdinger Moor near Hammah. Growth depressions of the
tree-ring series and population dynamics of subfossil oaks hint at a stepwise growth of the raised bog
One example involves two distant sites, an iron-age
settlement in Biskupin, Poland, constructed around
720 BC (WAZNY 1994), which was abandoned most
likely because the conditions became too wet, and
trackways (e.g. 9 Le, 21 Le, 12 Ip) in Northwest
Germany (SCHMIDT 1992; METZLER 1993), which
were constructed between 720 BC and 710 BC. In
the last case it seems to have been a rapid bog
growth which implies that it was the change
towards wetter climatic conditions that influenced
the human construction activities.
Another example relates to trackway constructions in Ireland in 148 BC (BAILLIE & BROWN
1996) and in Germany ca. 180 BC (FANSA &
SCHNEIDER 1990) and 130 BC (FANSA 1992) which
coincide with a major dying-off phase in the German subfossil oaks (DELORME et al. 1981; DELORME
et al. 1983, see Fig. 3).
We are looking foreward to results of the ongoing dendrochronological investigation of pine and
oak samples from trackway 32 Pr in the Campemoor area which are found in context with naturally preserved subfossil pines at the same location. (BAUEROCHSE & METZLER 2001, this vol.).
The average radiocarbon age of ca. 2930 cal BC is
very close to a two-step rapid decrease of the German MA chronology in 2900 BC and 2800 BC,
indicating dying-off phases in subfossil oaks. Here
again it seems possible that large-scale climatic
changes have influenced human activities.
Reconstruction of local changes
in hydrology
According to LEUSCHNER et al. (2002) most changes
in population dynamics of subfossil oaks correspond with contemporary long-term (up to decades)
growth depressions in the affiliated regional treering chronologies. Especially at sites with in situ
finds a combined evaluation of ring-width patterns and population dynamics allow an exact
reconstruction of local hydrological changes.
Such a reconstruction is demonstrated for the
site Hammah in the Kehdinger Moor (LEUSCHNER & DELORME 1996). 32 subfossil oaks were dated by using dendrochronology, most of them
lying on alder-carr peat at the base of raised-bog
peat which covered the trunks. Other oaks were
directly lying on the mineral soil at places where
it exceeded the level of the carr peat. Fig. 5 shows
the tree-ring series of the oaks and the reconstruction of the bog- and forest history of the site.
There is clear evidence that both the sharp germination phase at 175 AD as well as the stepwise dying-off of the oaks between 300 AD and
700 AD correspond with phases of growth depression in the surviving oaks. There is an interesting tendency that especially these oaks which
clearly mark a step of dying-off by showing
growth depressions are the next to die. This can
be interpreted as the effect of an increasing
ground-water level, which induces the horizontal and/or vertical expansion of the bog. This
Subfossil oaks from bogs in NW Europe as a (dendro)archaeological archive
makes that oaks on higher locations and higher
elavated stands are later reached by the expanding peat/bog. Such detailed analysis can lead to
a better understanding of the dynamics of bog
extension, which is mainly triggered by changes
in hydrology and climate.
However, to get a better understanding of this
detailed information more research is needed
about (1) the development, structure and ecology
of different types of former mire woodlands and
(2) aspects of conservation of tree remains. First
attempts have been made by DELORME et al. (1983),
LEUSCHNER et al. (1986), LEUSCHNER et al. (1987)
and by LAGEARD et al. (1995). The latter excavated a mire woodland in Cheshire, Great Britain.
More results will soon be available from two excavations in Ypenburg and Zwolle, the Netherlands.
Oak supporting mire woodlands were very complex ecosystems with many sub-types that occurred
due to differences in geographical location, topography, geology, soil characteristics and hydrology.
However, all these sub-types can be sensitive indicators for changes in hydrology.
Acknowledgements
The research was supported by the “Environment
and Climate Programme” under contract ENV4CT95-0127; by the Netherlands Organisation of
Scientific Research (NWO/GW; 250-51-072 and
NWO/AWL 750.700.04).
We thank Mike Baillie, Queens University,
Belfast, Northern Ireland and Esther Jansma, The
Netherlands Centre for Dendrochronology, RING
foundation for providing tree-ring data.
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Hanns Hubert Leuschner, Ute Sass-Klaassen
Addresses
Dr. Hanns Hubert Leuschner
Albrecht von Haller Institut für
Pflanzenwissenschaften
Abteilung Palynologie und Quartärwissenschaften –
Labor für Dendrochronologie
und Dendroklimatologie –
Von-Siebold-Strasse 3a
37075 Göttingen
Germany
e-mail: [email protected]
Dr. Ute Sass-Klaassen
The Netherlands Centre
for Dendrochronology RING Foundation
Kerkstraat 1
3811 CV Amersfoort
The Netherlands
e-mail: [email protected]