At the University of Innsbruck there is a marked expertise in dendrochronology, also known as tree-ring dating research. The aim of science is to date wood and to draw conclusions about architectural history, climate and glacier development.
Tree-rings are evidence of tree growth. They reveal individual growth phases from which scientists are able to extract various kinds of information. But above all, it is the age of trees that can be determined by this method. American Andrew Ellicott Douglass is the pioneer of this science. In the early twentieth century, he succeeded for the first time in putting the annual ring sequences from various wood sorts into an accurate chronological order.
Annual rings are never the same as those from previous years. We are able to study this variable, which in turn enables the compilation of characteristic sequences – the so-called tree-ring curves,” explains geographer Kurt Nicolussi, head of the Alpine Dendrochronology Group at the University of Innsbruck.
Compiling a tree-ring calendar
When it comes to living trees, annual rings are dated from their outer circumference to their inner circumference. The ring directly under the bark is related to the current year, the following ones to the preceding years. After that, older wood material is added. As a result, a yearly tree-ring calendar gradually emerges that extends ever further into the past. This is recorded in specialized computer programs and a reference database is then generated. These enable comparison with wooden artifacts of an unknown age plus the exact dating of ancient wood.
Characteristic regional patterns
The annual ring pattern that is created should typically have eighty or more rings. This does not apply to pieces of wood with fewer annual rings. Sequences of annual rings is then compared with existing reference databases and an attempt is made to precisely classify them by year, Nicolussi explains. Annual ring sequences have characteristic patterns. These patterns can be found not only within a single tree, but also in a similar form in various tree specimens and even in a larger regional area.
Precise dating of wood
Extremely precise time specifications can be made using dendrochronological methods. These have already been used in archaeology to accurately date lake settlements around the Alps over the last 6000 years. We can trace the origins of villages built thousands of years ago. Throughout the history of construction, wood from old castles and churches is able to be analyzed in order to draw conclusions about architectural history.
Tree trunks can also help to better understand the development of glacier regions. Nicolussi came to dendrochronology through this branch of research which he has been involved in since the 1980s. Today, he and his team have the most extensive tree-ring range of a mountain region in the world. The database spans the past 10,000 years.
Reconstruction of glacier formation
Analysis of a tree trunk that is found under an ice apron of a glacier is able to reveal:
- when the tree was alive;
- how long the tree lived;
- if the tree grew locally, which indicates when this place was free of ice.
As a result, this tree trunk can help reconstruct the glacier formation of the past.
A lot of wood has been exposed in the last three decades due to glaciers melting.
“These findings document lengthy retreat phases from the past, but glaciers today are lagging behind present climates and need to melt further back in order to regain equilibrium. That’s why we’re seeing some glacier faces collapse today because less and less ice is coming in from the accumulation zones,” says Nicolussi.
Conclusions on climate
As well as dating, dendrochronology also allows conclusions to be drawn on the climate over the past 10,000 years. Under certain conditions when comparing various annual ring sequences, similar fluctuations can be seen in the growth of singular annual ring sequences. “However, we also encounter limitations when making a comparison as the location of trees is crucial for the climatic signals contained in the tree rings,” explains Nicolussi. He cites the very hot and dry summer of 2003 as an example. At that time, trees in the lowlands lacked the moisture for growth and this is reflected in their very narrow annual rings. Trees at higher altitudes, on the other hand, had very good growth rates.
Volcanic eruption as a global phenomenon
Annual rings show the regional climatic signal. However, there are also events that can have a supra-regional and global impact on tree growth. One example of this are major volcanic eruptions. Ash and sulphur dioxide are hurled into the stratosphere and this leads to a global cooling of the climate. This can result in simultaneous, abrupt declines in annual ring growth on various continents,” Nicolussi explains.
Annual rings can still be seen in charred wood. This enables scientists to draw conclusions about historical mining. The benefits of wood and charcoal for science are manifold. The archive at the Institute for Alpine Dendrochronology contains more than 6000 samples.