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Dendrochronologia 24 (2007) 53–60

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INVITED REVIEW PAPER

Human time in tree rings

Dieter Eckstein�

University of Hamburg, Department of Wood Science, Division of Wood Biology, Germany

Received 10 March 2006; accepted 7 May 2006

Abstract

‘Human time’ means time in the context of human culture and activities. It is shown how ‘human time’ isbiologically archived in and dendrochronologically extracted from tree rings. For illustration, examples in the latemedieval Hanseatic city of Lubeck are selected.r 2006 Elsevier GmbH. All rights reserved.

Keywords: Dendrochronology; Cultural heritage; Human behavior; Socio-economic environment

Introduction

Time passes continuously and invariably. Only bydividing it into intervals does time become perceptiblefor man. Most trees divide their lifetimes into annualintervals by forming distinct growth rings. In doing so,trees record and archive information about time andenvironment life-long like in an almanac. It is the task ofdendrochronologists to translate this information intoan understandable human language.

A single tree-ring series has, without any doubt, avalue in itself. But by means of high-tech equipment andpowerful computers, it is nowadays easily possible tocollect huge amounts of dendrochronological data andto achieve a novel quality of results with them – that isto say, information about past human cultures andbehavior as well as on the socio-economic environment.

In the following, I will substantiate some aspects ofthis topic within a real context but before that, fewincipient considerations about tree growth may behelpful.

e front matter r 2006 Elsevier GmbH. All rights reserved.

ndro.2006.10.001

of Wood Science, Leuschnerstr. 91, D-21031 Hamburg,

+494073962452; fax: +4940428912835.

ess: d.eckstein@holz.uni-hamburg.de.

Incipient considerations

For trees, the pulse generator of the time is the annualalternation between a growing and a dormant season; inour temperate climate this changeover is normallyforced by temperature. In other areas of our globe, likein the subtropics, this is caused by the seasonallychanging moisture availability. But for trees in everwet tropical rainforests, such a natural external pulsegenerator is sometimes missing so that time has to beartificially introduced in a tree, e.g., by wounding thecambium from time to time. These timestamps can beseen after the tree has been felled and opened.

More difficult than to detect the time is to recognizethe simultaneously recorded non-chronological infor-mation in tree rings, such as information on environ-mental changes. It may be encoded in the width of a treering, or in the latewood and earlywood width, or inwood density and wood structure, or in the chemicalcomposition of the cell wall, and enters a tree through itsleaves and roots because these organs are in closecontact with the environment. In the end, the environ-mental input is transformed into new wood through thecambium and its activity, varying from year to year, siteto site, and tree species to tree species (Fig. 1). It may be

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Fig. 1. Interaction between external and internal influences on

wood formation, and electron-microscopic photo of cell

divisions (arrow heads) in the cambium zone (inset).

D. Eckstein / Dendrochronologia 24 (2007) 53–6054

worth mentioning that the annual character of tree ringswas known and their varying width was interpreted asmeteorological almanac long before the nature of thecambium was identified.

The information about past human cultures, such astimber trade, forest management practices, or prefer-ences in timber selection, is recorded in all living trees,but even more exciting, in all trees which lived at earliertimes and are preserved either underground or on thesea bottom, or were used as construction timber, or inwooden objects of art (Fig. 2), such as paintings onpanels, organs, and other wooden music instruments,furniture, sculptures and the like.

Study object – the late medieval town of Lubeck

and its hinterland

Lubeck was the leading town of the HanseaticLeague, a network of traders and merchants from latemedieval to early modern times around the Baltic Seaincluding England in the west and western Russia in theeast. The UNESCO has declared Lubeck as a WorldCultural Heritage site. Old Lubeck was founded as aSlavic stronghold (Fig. 3), dendrochronologically datedto around 800 AD, at the western edge of the Slavicsettling activities. About 300 years later, the GermanKing Henry the Lion occupied this area and in 1143 ADhis liege, earl Adolf II, founded today’s Lubeck only afew kilometers further south of Old Lubeck. Within thesubsequent 100 years, a settlement evolved with anorderly system of streets and plots of land, five churches,a monastery, a hospital, and a fortification. From foresthistory we know that the country between the North Sea

and the Baltic Sea was densely covered by mixeddeciduous woodlands of mainly beech and oak and thata document from 1188 AD (enacted by the GermanEmperor ‘Barbarossa’) safeguarded the right of thecitizens of Lubeck to cut and use trees (‘Holzungsrecht’)within an area of 1500 km2, but prohibited its utilizationfor ship building and for export. With the subsequentdemographic evolution, the forests were neverthelessincreasingly used for producing fodder for animalhusbandry, timber production for house building, fire-wood for heating homes and for daily cooking. Initially,all secular buildings were made of wood. But after twosevere fires in 1251 and 1276 AD, a law was enactedprohibiting the erection of solely wooden houses. Due tothe fact that the country was poor in natural stones, thefabrication of brick stones became necessary, consumingan additional amount of wood for the generation ofenergy. As early as 1220 AD, a law (‘Sachsenspiegel’)disclosed the principle of sustainability (‘wood landshould remain wood land’). From this year on, a certainextent of forest management could be assumed in theregion. This background information about the avail-ability of and the demand for timber must be kept inmind for the following considerations.

Acquisition of building timber from a market or

from the forest?

During our dendrochronological studies in andaround Lubeck, we were repeatedly confronted by thequestions: Was the building timber selected directlyamong the standing trees in the forest or was there atimber market as early as in the 13th century? Noanswers to these questions were found in the archivesavailable.

Provided that the standing trees were selected in theforest and then felled and transported directly to thebuilding site, all timbers used within one building arepresumably cut in the same year. However, if the treeswere first transported to a storage place and only thensold to a building contractor, timbers from differentfelling campaigns would have been mixed, and differentfelling years should be expected within one and the samecomplex of buildings. We found examples for bothalternatives (Fig. 4). In one of the houses, originally anecclesiastic building, four out of seven timbers of theroof construction had bark preserved and their youngestring was formed in 1317 AD. According to ourassumption the building timber would have beenselected in the forest, cut and brought to the buildingplot and built in without any storage time in-between.The alternative example is a merchant’s house of theGothic brick stone architecture. The 15 dated timberscan be grouped into four felling operations, ranging

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Fig. 2. A selection of ‘trees from former times’: (a) post-medieval woodland; (b) early Iron Age settlement Biskupin, Poland, dated

to 738 BC; (c) carpenters at work on a late-medieval building site (Chroniques de Hainaut I (1448) in: Binding (1993));

(d) Rembrandt painting on an oak panel dated to around 1630 AD; (e) organ in the St. Jacobi church in Lubeck dated to 1500 AD;

(f) oak chest in the monastery of Ebstorf, N. Germany, dated to around 1338 AD.

D. Eckstein / Dendrochronologia 24 (2007) 53–60 55

from June 1284 to the winter 1285/86 AD, which meansover 33 months. These timbers were randomly distrib-uted within the roof construction. In such a case ofmixed felling dates, we assume that the timbers wereprovided from a local timber market.

Around the end of the 13th century, the timbers forthree upper-class houses were identified to have pre-sumably been selected from standing trees in the forest.According to the records, the occupants of these houseswere wealthy merchants and possibly owners of forestland or had good contact to forest owners. Conversely,at about the same time six building contractors,belonging to the lower social classes, apparentlybought the building material at a timber market. Thisreality can be traced dendrochronologically up to the17th century.

Summer or winter felling?

In our dating work in Lubeck, we often found – as inthe last, example that the youngest growth ring underthe bark was not complete. Such trees were felled duringthe summer season, but when and why? In Fig. 5 thedynamics of oak growth during one season is illustratedfor North Germany: The x-axis is the time-scale of onevegetation period and the y-axis shows the accumulatedradial biweekly increments; some phenological keyevents are indicated by arrows. Oak in northernGermany starts its cambial activity around the end ofApril/beginning of May. Further growth is character-ized by different intensities, as shown by the S-shapedline. The earlywood/latewood transition occurs aroundthe end of June/beginning of July. The tree-ring width is

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Fig. 3. Lubeck, Slavic rampart and today’s Old Town (Degn and Muuß, 1965, 1968): (a) circular wall (see arrow) from around

800 AD; (b) section through the wall; (c) Old Town; (d) cloaca made from timbers from around 1252 AD in secondary use.

Fig. 4. Lubeck, house ‘Parade 1’, uniformly dated to 1317 AD (top); house ‘Hundestr. 94’, felling dates between June 1284 and

winter 1285/86 AD; B means bark ring existing.

D. Eckstein / Dendrochronologia 24 (2007) 53–6056

completed between late August and mid-September;the lignification of the cell walls follows with somedelay. On this basis, the cutting time within the growingseason can be fixed rather precisely, at best even to themonth.

Between the 13th and 17th century, approximately25% of the building timbers were cut during the summerseason, although in summer the farm workers were mostlikely occupied with farm work, and felling andtransport of trees were presumably difficult because

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1

2

3

4

51 swelling buds

2 bud-break

3 initiation of phloem formation

4 mature leaves

5 latewood formation

7 partial autumn colouring

of leaves

6 stop of phloem growth

8 total colouring of leaves

date of sampling

28.4.

1.0

2.0

July

3.0

12.5.26.5. 9.6. 23.6. 2.7. 21.7. 5.8. 18.8. 2.9. 15.9.29.9.13.10.

radia

l gro

wth

[m

m]

6

7 8

Fig. 5. Wood formation of oak in northern Germany during

one vegetation season and cross-section through a developing

tree ring in July (insert); onset of earlywood formation around

end of April/beginning of May; end of wood formation

between late August and mid-September.

Fig. 6. Lubeck, median and range of the ages of rafters within

one construction unit in relation to time (50 roof construc-

tions, sum of rafters ¼ 401).

D. Eckstein / Dendrochronologia 24 (2007) 53–60 57

the trees were with foliage and the timber was moist andheavy and more susceptible to attacks by insects andfungi. A possible reason for summer felling could havebeen the need for bark required for tanning animal skinsto produce leather. Bark can be peeled off much easierin summer than in winter. However, bark for tanningwas reportedly obtained from young oaks grown up incoppice forests because it contains a higher content oftannin compared to old oaks. But the constructiontimbers were, of course, not obtained from coppicewoodlands. Whatever reason may have been reallyvalid, oak bark was a much-valued raw material for thenumerous known medieval tanneries in Lubeck.

Timber properties

From various features of the building timbers, such asage, dimensions, or age trend reflected in their tree-ringwidth, the age of the trees used for rafters will be singledout here (Fig. 6).

The median as well as the maximum and minimumage of all dated rafters per construction unit are plottedagainst calendar years resulting in several clusters and inat least one long-lasting gap (1340–1430 AD) of buildingactivities which was partly caused by consequences ofthe Black Death epidemic around 1350 AD. It is strikingthat in the 13th century, 130-year-old oaks were felledwhereas later on the tree age dropped down to 60 beforeit increased again to 120 years in the 16th century.

Between 1300 and 1500 AD, the range of the tree agesaround the median was low compared with the timebefore and after. Up to 1300 and after 1500 AD, slow-grown oaks prevailed, whereas the oaks over theintervening years were fast-grown. Were the forestsover-exploited until 1300 AD due to a high demand for construction timbers after the big fires in 1251 and1276 AD and due to a general economic upswing? Theyoung oaks cut in the mid-15th century germinated after1350 AD and right at the same time as the oaks cutduring the building boom in the second half of the 16thcentury. Because of a depression phase in buildingactivities around 1500 AD, the second group of oakswas conceded a longer lifetime. From such a point ofview, the occurrence of young oaks could be interpretedas a beginning recovery and regeneration of the forestafter heavy devastation and possibly as evidence for asustainable forest management practice.

Timber import or domestic wood?

In a town like Lubeck, a Hanseatic and harbor townwith far-reaching trade connections, one should not besurprised to find imported timber. The first cases ofdendrochronologically proven long-distance timbertransport in Central Europe are reported for Dorestad/The Netherlands and Haithabu/North Germany. Thesesettlements are dated to the 9th century and located atwaterways. However, these timbers were not purpose-fully imported but were transported as barrels andserved as containers for all kinds of goods. At theirdestination they were re-used as enclosures for woodenwells.

The dating and the determination of the origin of oakwood in Lubeck will be illustrated with the Triumphal

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Fig. 7. Lubeck, Cathedral, Triumphal Cross, and Wooden Screen, made by Bernt Notke (1440–1509 AD) and his workshop; figure

of John, open back (bottom row left) and in head-down position (bottom row right) as it was carved from an oak trunk;

bar diagram, showing the dating results for the Triumphal Cross (the results for the Wooden Screen are not shown here)

(photos by U.v. Ulmann).

D. Eckstein / Dendrochronologia 24 (2007) 53–6058

Cross and Screen in the Cathedral, made by BerntNotke and his workshop (Fig. 7). The imposingdimensions of the individual figures (up to 2.30m tall)as well as the large amount of wood raise the question ofhow a wood-working medieval workshop proceeded inthe planning, material acquisition and treatment andwhat technological knowledge presumably was alreadyavailable in the 15th century. An inscription on thecarrying beam revealed that the Triumphal Cross waserected in 1477 AD. During restoration in the 1970s,writing was found on the inside of the sealing boards ofMary and John, stating that the rear opening of thehollow figures had been closed in 1471–1472 AD,respectively. The 170-year-old oak used for sculpturingthe figure of John was felled, according to dendrochro-nology, in the winter 1470/71 AD; because this figurewas sealed in 1472 AD, only 1 year passed betweenfelling and completion of the carving work. Based on thedirection of knot cavities of former branches, it was evenpossible to prove for the two upright figures Mary and

John that they were carved head-down out of the trunks(Fig. 7). The time remaining until 1477 AD wasrequired for drying the wood and painting the surfaces(polychroming).

For the Screen, all plank-shaped wood parts wereradially cut out of the oak trunks. Panels made in thismanner do not warp when the wood moisture changesdepending on changing air humidity. None of theseboards contained sapwood. Some of them originatefrom 200–300 year-old, slow-grown oak trees.

In Fig. 8, the southern coast area of the Baltic Sea,including the location of Lubeck and of five regionaloak chronologies, is mapped. The tree-ring mean curve,made of the tall figures, is compared with these fivereference chronologies; the darker the color of thecircles, the higher the similarity between the tree-ringseries compared. Hence all the wood for figure carvingis of domestic origin. Also the tree-ring mean curve,made of 20 boards of the Screen, is compared with thereference chronologies. On the basis of evidence

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provided, the plank-shaped oak wood of the Screenoriginated from Polish/Baltic forests. Also, the oaktimbers of other 15th century objects in Lubeck, such asthe organ in the St. Jacobi church and the wooden

Fig. 9. Trade routes of Polish/Baltic oak to Western Europe (

(Jakrzewska-Sniezko, 1995); rafting on a river near Dordrecht (bott

Fig. 8. Map showing the location of Lubeck (’) and the

approximate areas of five regional reference chronologies of

oak (O); comparison of the reference chronologies with the

mean curves assembled from the tree-ring series of the five tall

figures (top) and of 20 boards of the Screen (bottom).

screen in the Holy Ghost Hospital, were imported fromthat same area.

Know-how in woodworking

From our study of the Triumphal Cross and Screenwe deduced that fresh, unseasoned domestic oak woodwas used for this late medieval sculpture carving.Suitable-sized trunks of oaks were carefully selectedand utilized optimally with regard to their conical shaftshape and diameter, including the entire sapwood,because timber was a high-valued resource and shortraw material. The plank-shaped wood for the Screenoriginated from up to 300 years old, slowly andhomogeneously grown oaks. The boards were withoutbranches and radially cut; the easily perishable sapwoodhad been removed, very likely just after cleaving,indicating a well-developed experience in wood treat-ment. The planks were obtained from a distant source,because the domestic oak trees were presumably tooknotty and crooked as well as of wider and irregulartree-ring width for producing the desired plank quality.The oak trees were felled in Polish/Baltic forests,cleaved, floated down the rivers to the harbors, suchas Gdansk, Riga, or Konigsberg, where the boards wereloaded onto sea-going ships and brought to the tradingcenters in western Europe (Fig. 9).

Conclusions

The contribution of dendrochronology to the enlight-enment of our past, beyond pure dating, has briefly been

left); the Gdansk harbor (top, right) in the 18th century

om, right) in 1810 AD (Scheifele, 1996).

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illustrated with a selection of examples in a narrow arealand temporal section. In any case, such an approachbegins with the dating of human remains, such as housesor objects of art. Only then is it possible to attempt todiscover expressions of human behavior. Once asufficiently large dendrochronological data set for aspecific pool of human artifacts in a defined geographi-cal area and time window has been compiled, one canstart asking questions regarding general and abstractexpressions of past human life: e.g., what was the totalamount of building timber used? how large was theforest area needed for timber acquisition? which forestmanagement practices were applied? was there a serioustimber shortage? and the like. If this paper succeeds inencouraging and inspiring others to pursue this line ofresearch, it has achieved its purpose.

Acknowledgments

I am much obliged to Manuela Romagnoli, Viterbo,who organized EuroDendro2005 and invited me to

present this paper. I am also whole-heartedly grateful toSigrid Wrobel, Hamburg, and Tomasz Wazny, Torun,who both contributed substantially to this work. But Iam also aware of many colleagues around the worldwho deserve to be mentioned; however, it is not possibleto be exhaustive at this point.

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Degn, C., Muuß, U., 1965. Eine Landeskunde in 80 farbigen

Luftaufnahmen. Wachholtz, Neumunster, 77pp.

Degn, C., Muuß, U., 1968. Eine Landeskunde in 72

Luftaufnahmen. Wachholtz, Neumunster, 82pp.

Jakrzewska-Sniezko, Z., 1995. Gdansk w dawnych rycinach.

Ossolineum, Wrocław-Warszawa-Krakow-Gdansk-Łodz,

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Scheifele, M., 1996. Als die Walder auf Reisen gingen. Braun,

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