Environmental change, bog history and human impact around 2900 b.c. in NW Germany–preliminary results from a dendroecological study of a sub-fossil pine woodland at Campemoor, Dümmer Basin
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- Leuschner, H.H., Bauerochse, A. & Metzler, A. Veget Hist Archaeobot (2007) 16: 183. doi:10.1007/s00334-006-0084-4
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This paper presents a detailed dendroecological analysis of remains from a sub-fossil pine forest at the Campemoor in the Dümmer basin, NW Germany and of pine timbers from a contemporaneous Neolithic trackway Pr 32 through the Campemoor. Changes in growth pattern and population dynamics of the pines are discussed in context with the time of construction of the trackway. The findings date to the period around 3000 b.c. Together with palaeobotanical investigations (pollen and macro remains) and the archaeological results (trackway) the dendroecological analysis mirrors environmental changes and the response of people to these changes. In order to test this local development for a possible climate background, ring-width variability and population dynamics of the Campemoor pines and of the overall data set of Lower Saxonian sub-fossil oaks from bogs have been compared. The results of these investigations clearly indicate a common widespread turn from drier to more humid climate conditions as trigger for the transition period, initiating the raised bog growth. It happened in two phases at the beginning of the 3rd millennium, interrupted by a drier period between 2825 and 2770 b.c. Afterwards large areas of former settlement sites within today's Campemoor became inaccessible and were covered by raised bog.
KeywordsPalaeoecologyDendrochronologyArchaeologyClimate changeNeolithicWooden trackway
Three objectives are defined: First, to come up with exact dendro-dates for both the construction elements of the trackway Pr 32 (TRACK) and the stem remains of pines from the surrounding pine forest (CAMP). Up to now no reference chronologies for the dating of sub-fossil pines are available for North Germany. The only available reference chronology for sub-fossil wood is the Lower Saxony bog oak master chronology (LSBOC, Leuschner et al. 1987; Leuschner et al. 2002). The LSBOC is replicated by some 1700 (mainly sub-fossil) oak samples and covers the period from 6069 b.c. to a.d. 931. It is unknown whether tree-ring patterns of sub-fossil pine are cross-datable with those of sub-fossil oaks. Even if both species were growing under comparable environmental conditions in peatlands differences can be expected due to different response of these two species to environmental factors. An additional problem is that tree-ring series of sub-fossil pines are much shorter in comparison to oak, i.e. oak trees growing on peat live longer than pine trees, and pine tends to show more ring anomalies, i.e. missing rings or intra-annual density variations than oak.
Second, the relationship between the age of the trackway and the surrounding pine forest are of major ecological interest within the interdisciplinary Campemoor project, (even if dendro-dating proved to be impossible, see objective 1). Exact information about the annual growth dynamics of the pine trees in the woodland surrounding the trackway, in combination with information on the building date of the trackway, answers the question if the trackway construction was the response of man to environmental changes.
Third, is to check whether changes in growth pattern of the pines in combination with information on the timing of the trackway construction reflect local environmental changes in the Campemoor area or are regional phenomena meaning that they can be linked to environmental shifts documented for other peatland sites. Again, the Lower Saxony bog oak chronology (LSBOC) and results from a former study on population dynamics of the NW European bog oaks (Leuschner et al. 2002) are used as a reference. It became obvious that synchronous changes in growth pattern and population dynamics of sub-fossil bog oaks from different locations in NW Germany and even NW Europe (Ireland, The Netherlands, Germany) indicate that contemporary “stress-events” occurred in former wetland woods. The striking common variability in the medium and long frequency domains of the tree-ring records support the assumption that changes in past climate play a key role as a trigger for environmental changes in wetland woods (Leuschner et al. 2002). In a straight-forward approach growth patterns and population dynamics in the Campemoor pines are compared to those in contemporaneous bog oaks to (i) discriminate between local and regional triggers for changes in growth behavior of the pines and (ii) to study whether shifts in hydrology for example, from drier to wetter climate conditions may influence pine growth in the Campemoor area.
Material and methods
Material and sampling strategy
The sampling strategy followed the intention to gather a collection of stem disks that encompasses samples from trees with different morphology. Only extremely eccentric trunks, trees with less than 20 cm in diameter, and stumps were not sampled in order to maximize the chance of dating the tree-ring sequences. Most stem disks originated from stem heights between 30 and 80 cm. In order to reduce the amount of reaction wood, which obscures the tree-ring pattern some long lobate grown stems had been cut as high as possible up to 2 m above the root plate. Some samples from short trunks were taken directly above the root plates.
In total 137 pine samples were taken: 95 from the sub-fossil pine woodland and 42 from beams that form the superstructure of a 100 m long section of trackway Pr 32.
Sample preparation, measurement of tree-ring width and dating
The dendrochronological investigation followed standard procedures, described for example by Leuschner (1994). Radial sections of the discs were cut and smoothed with surgical blades and razor blades (Iseli and Schweingruber 1989; Leuschner and Schweingruber 1996). Chalk was rubbed on the surface to enhance the contrast and make the tree-ring boundaries clearly visible (Fig. 6). By doing so, even the smallest rings, consisting of just one early-wood and one late-wood cell layer, become visible. In most cases two radii per sample were prepared and the tree-ring width were measured with a precision of 1/100 mm by using a semi-automatic measuring stage (type Aniol) and the CATRAS program (Aniol 1983).
The term “population dynamics” refers to (1) the temporal distribution of the dated sub-fossil tree findings over time and (2) the frequency distribution of germination and dying-off phases (GDO phases) of the trees.
The frequency of germination and dying-off events across the period covered by the dated pine samples were calculated as a number of events per year in a running time window of 25 years. This smoothes extreme peaks of just random clustering of events in the same or neighbouring years.
The exact dates of germination and dying-off of trees can be estimated with varying accuracy, due to the stage of preservation of the samples and due to the stem height from which samples were taken. The following uncertainties are given: (1) unknown number of missing rings in rotten or mechanically removed parts around the pith and under the bark or (2) a small number of missing rings due to sampling height reflecting the number of years the tree needed to grow in height to reach the sampling height.
Ring width variations and indexing
Pronounced medium and long term growth variations have been described as typical features for bog oaks and most likely reflect external changes in environmental site conditions (Sass-Klaassen and Hanraets 2006; Leuschner et al. 2002). They are superimposed on the internal “age trend”, mostly expressed by decreasing ring width with age. A clear separation of these environmental and tree-specific components is almost impossible by the use of standard smoothing methods as described by Cook et al. (1990). The application of advanced standardizing methods such as the regional curve standardization (RCS, Briffa et al. 1996) requires uniform site conditions. They are unsuited for the material originating from different kinds of bogs and from stands of varying forest density. A more suitable approach is given by the application of filtering functions with variable length (Riemer 1994). A variable kernel filter of 61 years, which retains some of the common (externally caused) low frequency variations was applied for indexing of the tree-ring series of the Campemoor pines. Both, the raw-data series and the index chronologies are presented in the figures.
Results and discussion
the stratified position of pines above the trackway (Fig. 2) confines the CAMP2 dating to the period after the construction of TRACK.
Medium-term trends in the CAMP2 chronology match well with similar trends in the LSBOC oak master chronology.
Pollen analysis (see below) indicates the existence of dense pine woodland at the initiation of the raised bog episode that covered the trackway.
The extremely good agreement between CAMP1 and TRACK suggests that the CAMP1 woodland was most probably the source for the trackway timbers.
Population dynamics: Trackway construction as response to environmental changes in the peatlands?
As Fig. 7A shows, the Campemoor woodland consisted of two populations, the older (CAMP1) and the younger pine forest (CAMP2). A temporal overlap is only given by a few trees of these two collections. CAMP1 starts with a germination phase of four trees 3040–3015 b.c. (CG1). In the following an abbreviation system is used to specify single germination and dying-off (GDO) phases. It numbers serially the CAMP (C) and the TRACK (T) Germination (G) and Dying-off (D) phases. Only one intermediate tree germinated at 2980 b.c. before a second germination phase took place between 2945 and 2925 b.c. (CG2). The first dying-off phase (CD1) starts sharply between 2882 and 2872 and continues by a flatter phase of dying-off events from 2870 to 2835 b.c.. CAMP2 comprises the majority of the sub-fossil pine samples. Some pines of this population germinated in the fading dying-off phase CD1, but most of the trees germinated later, between 2845 and 2800 b.c. (CG3). Yet 25 years later the pine woodland started to die off between 2775 and 2730 b.c. (CD2). Only one tree survived a few years longer until 2704 b.c.. As shown by the detailed analysis of the temporal density of germination and dying-off events (Fig. 7B), several maxima in the GDO events can be observed.
Interestingly, the ring-width variations of the Campemoor pines show a striking common variability with the GDO phases of the two pine populations (Fig. 8). Typically, long- and medium-term changes in ring width start and end abruptly. Obviously these event phases coincide with the start or with the end of GDO events (see arrows in Fig. 8A). At a finer scale this holds true even for the maxima of GDO peaks and decadal periods of extreme growth depressions. This strong agreement between the tree-ring record and GDO events can be best recognized in Fig. 8B, which shows the index ring-width series (red coloured growth depressions, also triangles in Fig. 8C).
From Fig. 7 it appears that the temporal location of the TRACK samples is strongly related to two GDO events in CAMP1: (1) the germination phase TG1 is synchronized with CG2 with a tendency of TG1 to cluster about 10 years earlier than CG2. This slight temporal delay together with the observation of extremely wide rings in the juvenile phase of the TRACK material (see Fig. 8A and B) can be interpreted as an effect of the tree selection for construction of TRACK. It could well be that these first-germinating trees originate from somewhat higher and dryer sites within the mire. (2) The felling dates, spanning 18 years from 2900 to 2882 b.c. fit to the initial phase of the dying-off of CAMP1 (CD1, Fig. 7) and correlate with two growth depression steps (arrows 4 and 5, Fig. 8A) in the CAMP1 raw-data chronology.
This indicates that the trackway construction has to be seen as a response to changing ecological conditions in the mire. According to Leuschner et al. (2002), both multi-annual growth depressions and GDO phases are reflecting changes towards wetter conditions. Besides the self-evident case of dying-off (e.g. Timmermann 2003), this assumption also makes sense in the case of germination phases: the establishment of a light-demanding tree species such as pine requires clearing of the canopy, which can be caused by the dying-off of older trees during wet phases. This mechanism was also observed in bog pines (Pinus rotundata) growing in bog sites in the Black Forest, southern Germany (Schmid et al. 1995). It is assumed that changes in site hydrology are mainly affecting and controlling nutrient supply in the rootable layer and thus controlling vegetation, i.e. woodland, development in peatlands (Succow and Joosten 2001). The hydrology in peatlands is a complex interaction between precipitation, groundwater and surface water and thus is partly related to changing weather and climate conditions.
Climate as trigger?
The question arises whether changes in growth and population of the Campemoor pines mainly reflect local changes in e.g. site hydrology or whether these changes have a regional dimension, i.e. are they also occurring at other wetland sites in Europe. The latter would mean that a large-scale regional, e.g. climate, factor would be the trigger for the observed centennial, decadal and probably annual changes in tree-growth and population dynamics. In order to answer this question, the population dynamics and ring-width variations of the Campemoor pines are compared with the Lower Saxony bog oak chronology (LSBOC), which comprises tree-ring records of bog oaks from 120 sites in North Germany.
Both records show striking similarity across the whole overlapping period (Fig. 8A and B). The temporary position of the two Campemoor pine horizons is located within two pronounced large-scale growth depressions in the LSBOC chronology, from 2950–2850 b.c. and 2780–2705 b.c., respectively. In detail the comparisons in Figs. 7 and 8 show that almost all of the Campemoor woodland “events”, i.e. striking GDO phases and major ring-width variations are perfectly synchronized with major changes in the trend of the LSBOC chronology (see arrows in Fig. 8a). This backs the assumption that the environmental changes observed in the Campemoor site are regional phenomena that must be triggered by regional, most likely climate changes. According to the above-mentioned interpretation, i.e. growth depressions as indicators of wet phases and growth releases reflecting dry phase in wetland woods, the following stages of development can be distinguished for the Campemoor site. The listing in Table 1 follows the numbering of the arrows in Fig. 8A.
Integration of dendrochronological, palaeobotanical and archaeological results
Environmental history of the Campemoor pine woodland and timing of trackway construction; LSBOC = Lower Saxony Bog Oak Chronology, CAMP1 and CAMP2 = Campemoor pine chronologies
Arrows Fig. 8A
1 and 2
The first pines of CAMP1 grew up in CG1, when two decadal growth depressions can be seen in the LSBOC, which indicate medium-term wet phases with the consequence of local clearing of the canopy (?)
A dense pine woodland (CG2) established during the time when an abrupt long-term growth depression starts in LSBOC. Dying-off of older pines due to a shift to much wetter conditions can be assumed as climatic trigger. This assumption is backed by the formation of wider growth rings in the juvenile phase of the TRACK timbers, indicating optimal light conditions as exist after a clearing of the former vegetation. Perhaps an older woodland generation originating from CG1 is represented only by the numerous findings of stemless pine roots, which were not sampled
A continuous decrease of LSBOC indicates increasing wet conditions. This is when trackway Pr 32 was constructed. The pronounced downward trend in the CAMP1 and CAMP2 chronology between 2898 b.c. and 2879 b.c. (see arrow 5) could be related to an increase of the water level at the Campemoor site possibly due to felling of pine trees leading to a decrease of transpiration.
The peak of CD1 and the corresponding extreme growth depressions of CAMP1, CAMP2 and LSBOC indicate extremely wet conditions during this period. This period could also mark the end of the use of the trackway
6 and 7
During this period, when most likely a shift towards drier conditions took place (growth release in LSBOC) the majority of CAMP2 population grows up. In detail, the germination events cluster in correspondence with drier and wetter sub-phases, i.e. two growth releases (6, 7) phases and a growth reduction phase in CAMP1, CAMP2 and LSBOC (see the black triangles just before 2800 b.c. in Fig. 8C)
Simultaneous with an abrupt growth reduction in LSBOC indicating wetter conditions the pines of the CAMP2 population start to die-off (CD2). The severity of this event is indicated by the fact that almost all pines of CAMP2 along a 2-km long transect, uniformly show this abrupt growth change
9 and 10
Abrupt growth releases point to drier conditions. This is when CD2 ends, except one pine which survived until the next abrupt growth release of LSBOC
From the early Holocene period onwards the Dümmer basin was one of the large wetland areas of Northwest Germany, characterized by a heterogeneous mosaic of different vegetation types with sedge swamps, carrs, and pools. The landscape was riddled by slightly elevated, non-paludified sites and covered by deciduous trees. These sites were, at least partly, used as pasture (Bauerochse 2003). In the second half of the 4th millennium b.c. this landscape became more and more dominated by species indicating less humidity, as the pollen diagram shows (Fig. 9, above 80 cm). This change, which is most likely climate-forced, is mirrored by the transgression and regression of a pool, which continuously existed over hundreds of years north of the excavation site. During the first half of the 4th millennium b.c. this pool expanded close to the excavation site, while during the second half of the 4th millennium the decrease of pollen of floating leaf plants indicates that the southern part of the pool retreated. The profile shows a change from open water to sedge swamp. As a result of the decreasing moisture Betula, Salix and Frangula alnus spread on the mire. Associated with the lowering of the water table they were increasingly displaced by Pinus sylvestris. At the transition from the 4th to the 3rd millennium b.c., most of Campemoor showed a pine-dominated more or less closed forest (see density of pine remains in Fig. 3) with interspersed birches and a grass-dominated understorey. At this time, large areas of the peatland must have been easily accessible for ancient people.
At the beginning of the 3rd millennium the diagram indicates a strong environmental change. The appearance and spread of peat mosses (Sphagnum) indicate the appearance and transgression of a raised bog vegetation (Fig. 9, above 60 cm). This process led to disease and dying-off of the pines, which is indicated by the decrease of pine pollen and the occurrence of pine stomata, due to a massive loss of needles. In addition there is a clear indication given by the root system of pine sample 04 (Fig. 4) that this environmental change was connected with increasing water-logging at the site. This tree, found about 200 m west of the excavation site, is one of the oldest pines from the CAMP1 woodland. It germinated at about 3020 b.c. within the first CAMP CG1 phase and died off in 2875 b.c. during the CG2 phase. The root system of sample 04 spans about 80 cm, reaching from a relatively deep heart root to adventive roots at the upper side of the stump. The middle and upper roots are formed in layers and are mostly bent upward. According to Kokkonen (1923) and Köstler et al. (1968), the depth of pine roots in mires is sharply controlled by the groundwater level. Even if a certain sinking of the tree into the soft peat is taken into consideration, the morphology of the root system indicates that the growth of the tree started on deeply aerated substrate and ended in a mire with a water level close to the surface - probably a raised bog.
The exact temporal position of the fen-bog transition cannot be deduced from the palaeobotanical and dendrochronological results. As described by Hughes (2003) and Hughes and Barber (2004), both increasing humidity and an interim dry period can trigger the process. In case of the Campemoor site we assume, that the fen-bog transition occurs during the two wet phases 2879–2850 b.c. and 2772–2706 b.c.. It may have been reinforced by the intermediate dry phase. Such a wet-dry-wet sequence is also considered as trigger for the formation of raised bogs suffocating and conserving bog oak horizons at 130 b.c. (Delorme et al. 1981) and at 700 b.c. (Leuschner 1992). Based on this information, the oscillation of the Sphagnum curve above 60 cm (Fig. 9) can probably be interpreted as a mirror of the two phases of the establishment of the peat mosses on the mire. However, the simultaneous patterns of ring-width variations and population dynamics representing pines from a 2-km long transect proves that the ecological and hydrological changes did not occur as a consequence of gradually expanding raised bog but instead were triggered by large-scale climatic pulses.
Within this progressively wetter period after 2944 b.c. (see descending course of LSBOC in Fig. 8A) the wooden trackway Pr 32 was built. It is the youngest of six trackways known from the Campemoor area, built from 4750 to 2850 b.c.. From the technical point of view this trackway is of a surprisingly high standard. It was constructed of 4–6 parallel longitudinal rows of pine and - sporadically - birch trunks with a superstructure of about 2–2.5 m width. The spaces between the trunks are often filled with pine and birch branches and bark. Like the substructure, the track surface is made of selected and carefully assembled pine trunks with diameters of about 15 cm. The course of the roadway was very winding in order to encircle trees in the pine carr. Axe marks on several stumps near the trackway indicate that at least some trees from this pine woodland had been chopped down and used for trackway construction. In some cases, even whole trees were simply pushed over and laid across the trackway like the other tree trunks, with their stumps and roots still attached (Fig. 2; Metzler 2003; Bauerochse and Metzler 2001).
Up to now it is not clear for what reasons the trackway was built and how long it was used. But as a large number of artefacts and several settlements testify, there have been intensive human activities in the Dümmer basin during this period. The best known of these settlements are “Huntedorf”, north of lake Dümmer, which has been used very intensively around 2900 b.c. (Reinerth 1939; Kossian 2003; Kossian and Lönne 2003), and “Hüde 1”, at the southern border of the lake (Deichmüller 1968, 1975). It can be assumed, that many further traces of colonization are still covered by peat (Drafehn 2006). This archaeological context, the high technical quality of the trackway and the clustering of trackways at Campemoor suggest that Pr 32 served as a connecting path between settled areas. The stratigraphical and dendrochronological indications for a dramatic shift towards wetter conditions at the Campemoor site prior and after construction of the trackway further suggest that the almost 20-year long construction period and completion of the trackway can be seen as a short attempt to keep an increasingly wet bog passable. The pine stumps grown on the trackway timbers indicate that this attempt finally failed, probably during the period with the strongest growth depressions, in 2880 b.c. and 2835 b.c.. Moreover, the results show that the change of environmental condition happend in time frames spanning only two or three human generations. Hence it becomes obvious in what way environmental changes affected the living condition of ancient people in this area.
Synchronously to the environmental change human impact on the Campemoor area decreased, as proved by anthropogenic indicators in the pollen record (Behre 1981) in Fig. 9. However, the continuous occurrence of small amounts of this pollen gives evidence of man's subsequent presence in the Dümmer basin.
Conclusions and perspective
The results presented in this paper show a strong similarity between the tree-ring records of pine from the Campemoor site and NW European bog oaks. As the NW European record mirrors large-scale environmental changes in wetland ecosystems in Europe it can be concluded that the local bog forest development at the Campemoor site was also triggered by a large-scale environmental, i.e. climate factor. A more detailed interpretation is put on hold until more information is available from ongoing work on root-system evaluations, high resolution pollen- and macro-remain analysis as well as from the archaeological excavations of pine stumps and measurements of stable isotopes in pine tree-ring layers. This is essential to link the dendroecological record with the other palaeorecords and come up with an idea about the effect of changes in the mire ecosystem on human settlement activity.
the wet phase around 2800 b.c. is also documented as a GDO event in the bog-oak material from Lower Saxony and The Netherlands (Leuschner et al. 2002). Some of the preserved bog-oak horizons are located in fen peat–but in two sites the oaks were preserved in the bottom layers of raised-bog peat, indicating that the former oak forest suffocated during the fen/bog transition. Also Behre et al. (1996) report for this period a cluster of radiocarbon dated fen-bog transitions in peat profiles from NW Germany indicating a turn to more humid conditions.
Two dendro-dated bog-pine populations from England (Lageard et al. 1999; Boswijk and Whitehouse 2002) span the period from 2881 to 2559 and 2921 to 2445 b.c., respectively. A number of GDO phases and “stress-events” in the English pines are synchronized with event phases at the Campemoor site.
Blaauw et al. (2004) studied the species composition of high precision radiocarbon dated peat series from a raised bog in the Netherlands. They found one of the wet/cool periods at ca. 2925–2825 b.c. and contextualized it with a major change in solar activity.
Mäkilä (2001) found a period of extreme Sphagnum growth and carbon accumulation in Finnish peat bogs at 5050–4950 cal b.p. (3100–3000 b.c.). Taking into consideration the possible uncertainty of the radiocarbon dating, this event might correspond with the first shift towards wetter conditions as indicated by the Campemoor finds.
Within the record of the depositional frequency of South German riverine sub-fossil oak trunks (Leuschner et al. 2000; Spurk et al. 2002) the Campemoor-period is characterized by a long-term phase of increased deposition.
Macklin et al. (2005) evaluated 506 radiocarbon dated fluvial units in Great Britain and interpreted an increase in the frequency and severity of floods at about 3000 b.c. as a hydroclimatic 'system switch'.
The dendroecological possibilities opened up by the Campemoor findings may be applied to the other four dendrochronologically dated pine horizons in North Germany noted by Bauerochse et al. (2006). The fact that peatlands with pines are known as common stages of mire ecosystems and often found as stump layers in the fen-bog transition zone of peat profiles, provides the chance to use dendroecological reconstructions of peatlands with pines to get a better understanding of past climate changes as well as of climate influence on bog ecosystems.
This publication is dedicated to Hans-Jürgen Beug on the occasion of his 75th birthday.
The dendroecological analysis is part of an ongoing research project, aiming at extended investigations on sub-fossil bog pines in Lower Saxony. It is funded by the German Research Foundation (LE 1805/2-1). The archaeological and palaeobotanical studies have been carried out in the frame of an interdisciplinary research project funded by the German Environmental Foundation (DBU, AZ 21514). The field work at site Campemoor had been greatly supported by the Torfwerk Schwegermoor. The authors especially thank Barbara Leuschner (DELAG, Dendrochronologisches Labor Göttingen), who kindly contributed the major part of the tree-ring measurements and of the intra-site synchronisation work. Special thanks are also due to Ute Sass-Klaassen and Mike Baillie for useful review comments and corrections.