71. Zetterberg, P. (2003). Dendrokronologisesti ajoitetut puulöydöt keskiajan tietoarkistona (Dendrochronologically dated timber as a data archive of the Middle Ages). Teoksessa Seppänen, L. (toim.): Kaupunkia pintaa syvemmältä - Arkeologisia näkökulmia Turun historiaan. Archaeologia Medii Aevi Finlandiae IX:383-392.

Dendrochronology is a discipline which studies the annual growth rings of trees and uses them both to provide absolute dates for the wooden material itself, and to date and study natural phenomena in the past. The absolute chronology of tree-rings makes it possible to determine e.g. the construction date of a building with a precision of one year, which is considerably more accurate than with other methods. Excavations in Turku have revealed a great deal of remains of wooden structures. Under favourable conditions, the timber of ancient buildings has preserved in the ground for as long as 800 years. During 1987-2003, some 350 samples from Turku have been analysed at the University of Joensuu Laboratory of Dendrochronology. The largest individual materials have come from excavations at the Åbo Akademi site and the Aboa Vetus site. The oldest dated timber dates to the turn of the 13th century. The dated tree-ring chronologies contain 28 sequences ending in the 13th century, 139 in the 14th century, and 77 in the 15th century. The oldest annual ring is assigned to 1012. The article presents the results for the samples from the Aboa Vetus excavations and makes interpretations of climatic changes in medieval Turku. The following are requirements for the dendrochronological dating of timber samples: 1) The sample must contain at least 50-70 annual rings. However, this might be insufficient for a tree that has grown fast, while 40 years may suffice for a slow-growing tree. 2) The growth of the timber must be undisturbed. The radial increment of living trees may be disturbed by many factors, such as damage to the trunk or boughs, tilting, needle damage or tree diseases. 3) The sampling point in the trunk must be chosen carefully. The base and crown (tension wood in the roots and the branches), as well as knots should be avoided.

4) The decay of wood limits the study of annual rings. They can be measured from a tree in a fairly advanced state of decay, but once the cellular structure has begun to break down, the original ring widths can no longer be measured. 5) The closer the preserved wooden material is to the original cambium, the greater the accuracy of the felling date. The bark of timbers found in the soil is rarely preserved. 6) Master sequences must be available as reference chronologies to which the sample’s tree-ring sequence is compared. The radial increment of the wood from the Aboa Vetus site was slightly less than 1 mm per year in the Middle Ages. Growth was fastest in the late 12th and the 13th century, but it decreased to 0.6 mm by the mid-14th century and then increased a little again in the early 15th century. In this material, the period of fastest growth, which reflects a favourable phase in climate, dates to 1183-1190, but the drop in growth that occurred in 1191 was the most drastic during the time period covered by this material. The drop is further emphasized by the fact that the period 1193-1214 was slightly more favourable for the growth of pine than on average. The earliest period of exceptionally slow growth, indicating unfavourable climatic conditions, dates to as early as the beginning of the 1150s. The subsequent unfavourable periods were experienced at the turn of the 1220s and in 1233-1234. After this followed a long period during which there were no dramatic drops in growth until 1326-1328. The most favourable conditions for the growth of pine in the 13th century date to the late 1220s, the second half of the 1240s, and to 1264-1270.