Dating the Past through Tree Rings

The science behind the tree rings

Among the currently available techniques to date ancient wooden objects and structures, dendrochronology (i.e. tree-ring science) is the only one that can provide exact calendar years for each tree ring present in the wood. This is possible because trees growing in temperate climates produce every year during the growing season (approximately April to September, depending on the latitude and altitude) a new tree ring under the bark. As a result, if we count the tree rings present on a tree stump that we find in the forest, we will know how old the tree was when it was cut. And if we assign the calendar year of the past growing season to the tree ring right under the bark, we can move towards the pith assigning an exact calendar year to each tree ring. Although there are a couple of nuances and exceptions, this is what makes dendrochronology the most exact and accurate method to date wood.

But dendrochronology is much more than counting tree rings. Dendrochronologists measure, for example, the ring widths. The width of each new tree ring will depend primarily on the climatic conditions prevailing at the site where the tree grows (e.g. temperature and precipitation), but it can also be influenced by natural and anthropic disturbances, such as severe defoliations caused by plagues of insects, or by pruning practices carried out by humans. Disturbances aside, one of the pillars of dendrochronology is that trees of the same species growing on the same site will produce similar patterns of thicker and thinner tree rings. Therefore, if we collect samples from numerous trees in the same area and average their tree-ring series into a reference tree-ring chronology, this chronology will represent a barcode unique for that specific species growing at that specific site. Such reference chronologies can then be used to establish the date and provenance of historic timbers made from trees of the same species and geographical area.

Oak sample series

Tree-ring series of an oak sample from a tree that was cut in the autumn of the year 2014. Each dot in the series represents the ring width (tree growth) in millimetres of the tree in that specific year (image: M. Domínguez Delmás).


Developing the tools to date and provenance ship timbers

As dendrochronologist, my role within the ForSEAdiscovery project has been to develop a much needed set of reference tree-ring chronologies of oak and pine species in key areas of the Iberian Peninsula that supplied timber for shipbuilding form the 16th to the 18th century. These chronologies would represent a set of tools to establish the date and provenance of timbers from Iberian shipwrecks (understanding as Iberian those ships built in Spain and Portugal with wood from trees growing in the Iberian Peninsula, as well as with imported wood from elsewhere). For this, we selected oak and pine forests in the north and south of Spain respectively that could yield samples with a couple of hundreds of tree rings. At the same time, we selected historic buildings close to those forests, hoping that the timber structures supporting their roofs would provide another source of tree-ring data that would expand the chronologies form the living trees back in time. To our surprise, in the historic buildings we also found wooden stairs made in some cases from very old trees.

Through a joined team effort, we managed to collect hundreds of samples from living trees and historic buildings in several sampling campaigns, a fantastic bulk of raw data that could hold the key to many historical and environmental questions.

Sakana oaks

One oak chronology was obtained from trees cut from the Sakana forest (Navarra, North East of Spain) to build the replica of the San Juan whaler (photo: M. Domínguez Delmás).

black pine

Centuries-old black pine from the Cazorla and Segura Mountains (photo: M. Domínguez Delmás).

San vincente de arama

Wooden stairs made of oak at San Vicente de Arana church (photo: Marta Domínguez Delmás).


A very promising result

The hard work paid back, and after analysing more than 1500 samples from living trees and buildings I created almost 50 chronologies reaching back to the 14th century in the North of Spain (oak chronologies), and the 12th century in the South (black pine chronologies). The base towards dating and provenancing Iberian ship timbers has been set, and hopefully one day the bulk of reference data will be such that dating the past through the tree rings in Spain and Portugal becomes a normality instead of an exception.

Dr. Marta Domínguez Delmás carried out her research at the University of Santiago de Compostela as Marie Curie ITN fellow.