The annual growth rings of treeline pine have been used to create a growth ring calendar spanning more than 7600 years. It can be used in the detailed study of the annual changes in weather and climate over a long period. The annual growth rings reveal, to the year, the age of old wooden objects and buildings, ancient volcanic eruptions, flooding rivers and the composition of the atmosphere.

The annual growth rings treeline pine also contain traces of global climate changes, as evidenced by traces of the NAO (North Atlantic Oscillation) phenomenon that affects the macro climate in the northern hemisphere. The concept of NAO is used to describe the annual fluctuation of the weather in the Northern Atlantic environment.

Powerful volcanic activities are also visible in treeline pines. The pine has extremely narrow growth rings during times of large volcanic eruptions in the world; for example, the eruption of Mount Tambora in Indonesia in 1815 and Huaynaputina in Peru in 1600 AD, and Santorini in Greece around 1640 BC.

Dendrochronology studies the past with the help of a time series built from wood

The term dendrochronology derives from Greek and refers to the science of studying the past with the time series built from wood. The most commonly used method is to study the width, hardness, cellular structure and chemistry of the annual growth rings to learn about climate changes that took place thousands of years before the invention of the thermometer.

The annual growth ring calendar spanning more than 7600 years was built in Lapland using pine trees lifted from small, muddy lakes. It is the third-longest calendar span in the world. It was a conscious decision to use the annual growth rings of the treeline pine, because pine trees growing on the treeline are very sensitive to changes in temperature during June and July.

In addition to dendrochronology, forest researchers study past climates using the needle trace method. They can also use phenological observations, such as plants coming into leaf or flower, berries ripening, and leaves falling off, to study the effects of changes in climate. Phenological observations have been recorded since the early 1800s.

Research methods utilised globally

The series of annual growth rings spanning over 7600 years was a result of international research cooperation. The research was held back for a long time due to lack of evidence for the period of 350–170 BC. The issue was only resolved in 1999 through an EU project involving nine countries. The needle trace method was developed in the late 1980s in the Needle Trace Laboratory of the Finnish Forest Research Institute (the current Natural Resources Institute Finland).

Picture on top of the page: Essi Puranen, Luke