Dendrochronology is a method used for studying tree-rings. The growth of trees often reflects annual variations in climate. Trees from the same species will thereby often have a matching pattern of annual tree-rings over large areas. In Norway the growth of conifers often correlates with the annual variation in summer temperature as demonstrated in Fig.2.

Fig. 2 Comparison between the temperature (May-August tetratherm) from 1870 to 1988 in Trøndelag (lower curve) and the corresponding sequence of tree-ring widths in pine (upper curve). Temperature data provided from the Norwegian Meteorological Office.

 A warm summer normally provides a wider tree-ring than average, a cold summer a narrower tree-ring while an average summer has average growth. This will over a long period produce a tree-ring pattern unique for those years. A matching of the growth pattern from a sample with long tree-ring chronologies will thereby enable us to identify the actual period the tree was growing and thereby to perform a dendrochronological dating. If the number of tree-rings is sufficient and the outermost tree-ring is present in the sample, which can be seen if some bark is intact, dating of the felling year can be performed using dendrochronology.

Today more than 500 houses and approximately 1000 samples from archaeological excavations have been dated with dendrochronology in Norway.

The basis for dendrochronological activity is the construction of absolute tree-ring chronologies as far back in time as possible. Matching the tree-ring pattern from two samples from trees grown during the same period is called cross-matching. Matching the tree-ring pattern from a sample of unknown age to a tree-ring pattern related to calendar years performs a dating. By starting with living trees a long chronology back in time can be constructed by overlapping of successively older tree-ring series, demonstrated in Fig. 3.

Fig. 3 The principle of the construction of a tree-ring chronology. The inner tree-rings from a sample overlap in time with the outer tree-rings of an older sample.

A cross-matching of a tree-ring curve with the constructed chronology is demonstrated in Fig. 4 where the thick line is a part of the chronology. The thin line is from building timber cross-dated against the chronology.

Fig. 4 Cross-correlation of the tree-ring series from a timber building to the period 1650-1747. As the timber contains bark, the felling year of the trees must have occurred during the winter 1747 – 1748.

Before a house can be dated it is normal procedure to take samples from at least 10 different logs, and to construct a mean curve from the logs that contain a matching tree-ring pattern. This is necessary, as tree-ring growth is also sensitive to other environmental factors then climate.

As the tree-ring pattern matches the summer temperature, long tree-ring chronologies are known to contain valuable information on climate, thus giving rise to the sub-field of dendroclimatology, which studies climatic variations back in time.

Since tree-ring growth is also sensitive to many other environmental factors like pollution, geomorphological changes, river floods, etc., other sub-fields also exist, such as dendroecology, dendromorphology and dendrohydrology, which, respectively, study the ecology of biotic communities, geomorphic processes and the history of flooding.

Examples of dendrochronologically dated objects from Norway:

Urnes Stave Church. The material of the present church has been felled during the period from 1129 to 1131 according to the dendrochronological datings. However, re-use of material from the predecessor(s) of the present church is still going on, cf. article written by architect Ola Storsletten, NINA NIKU, in Fortidsminneforeningens årbok (annual book from the Norwegian Heritage Foundation) in 2002 (pages 91-100).

Sculpture (mask) originally situated in Ål Stave Church, demolished in 1880. The tree-ring serie measured along one radius on the cross section of the sculpture was dendrochronologically dated from 1011 on the innermost tree-ring to 1134 on the outermost tree-ring.

Storehouse from the farm Stave normally referred to as “Staveloftet”. The present location is at Hallingdal Folk Museum, Nesbyen. The felling year of the timber is dendrochronologically dated to the winter 1323 - 1324.

Storehouse originally from the farm Heierstad, today situated at Vestfold County Museum. Among the very first dating after the Black Death. The timber is felled during the period 1405 to 1407, and the building year is probably in 1407, only 57 years after the plague.

This cottage is situated at the farm Søre Gjellerud in Flesberg. An inscription “1300” misled many to believe the house was from that year. However, some building historians doubted this because of the building technique and meant the house had to be much younger. The dendrochronological dating proved that the timber was felled during the winter 1562-1563.