The tree’s calendar helps climate researchers
Climate models use variations from a thousand-year perspective to provide likely future forecasts. An important tool for verifying these models hides beneath the bark of trees.
THE OLDEST WEATHER OBERSVATIONS in Sweden date back 300 years,” says Kristina Seftigen, a researcher in dendrochronology. Thanks to trees’ growth rings, we can follow climate changes right back to the last ice age, says Kristina Seftigen, researcher in dendrochronology.
The method of carrying out analyses based on precisely dated growth rings is almost a century old. In 1929, American astronomer A. E. Douglass discovered that he could read the sunspot cycle in a tree’s growth rings. Since then, scientists have used growth rings to date trees, wooden structures and forest fires, and even to gain an insight into the weather and climate during the tree’s life. Most children learn to count the growth rings on a stump to work out the age of the tree when it was cut down.
– Some might turn their nose up at the idea of counting growth rings as an outdated science, continues Kristina. But dendrochronology has developed into an important research field within climatology.
KRISTINA SEFTIGEN SETTLED ON HER research field when she had the chance to join an expedition to Xi’an province in China. Together with researcher Hans Linderholm, they climbed high alpine terrain to collect tree samples.
–Several of my interests came together on that trip, she recalls. I like adventure and pushing myself, and I’m interested in history. Plus, this is fascinating research.
Sweden’s first weather station was estab lished in 1722, meaning that we have recorded weather data for the last 300 years – one of the longest measurement series in the world. However, every tree carries a weather calendar in its growth rings, and by looking at old trees that overlap in age we now have an unbroken time series that stretches back around 7,000 years in Sweden.
– Trees can of course be dated using the carbon-14 method, but this gives an uncertainty of around 50 years. However, once you have documented overlapping growth rings using the dendrochronological method, we can be sure right down to the very year.
GROWTH RINGS CAN TELL US a lot. A thicker ring shows that the tree’s growth period was favourable that year, while a thinner ring re- veals that the tree was exposed to some kind of stress. This could be a lack of water due to drought, or abnormally cold weather that reduced the tree’s photosynthesis. In order to obtain clear indications of the climate conditions, the researchers seek out areas where the growing conditions for a particular tree species are on the borderline. In the mountains, a small deviation in temperature can stop growth almost entirely for a year. In ar- eas that are normally fairly dry, a year with little rainfall can have a significant effect on the growth ring.
– Trees are individuals, so to carry out a truly reliable analysis you need to take samples from 20 to 30 trees in an area. Individual trees may have missed a growth ring for one reason or another. If the tree has been stressed, the effects can last for several growth rings until the tree has fully recovered.
THE THICKNESS OF THE GROWTH rings can be observed with the naked eye, or using a standard microscope. Kristina’s research team is also using new methods to measure the dimensions of the individual cells (tracheids) that make up a growth ring.
– We look at the anatomy of the wood. The thickness and size of the cell walls provides more reliable information about what the climate was like in a particular year than the thickness of the growth ring alone. This has allowed us to say that it has not been as warm as it is now for 1,200 years.
Previous interpretations of growth ring thickness led researchers to assume that there was a period in the Middle Ages that was as warm or perhaps even warmer than today’s climate, but this is not the case. Analyses of 50 million cells from 188 pine trees in Sweden and Finland were needed to obtain these results, using new methods that allow for a greater number of more thorough analyses.
– Thanks to AI, we can process much larger quantities of data across larger geographical areas. Trees are almost everywhere, and we can verify the models used to predict future climate change through growth ring analyses.
DISTINCT SEASONS in which the tree undergoes dormant periods are needed to produce clear growth rings for analysis. The tropics are therefore something of a black hole within this field of research. However, there are often periods of rain and drought that may be interpreted in a similar way in the tree’s wood. Research is currently being carried out.
– Then there’s the southern hemisphere, which has been very sparsely researched. There are excellent opportunities with trees that grow and have grown in the Andes in South America and in Tasmania where there are high mountains with distinct seasons. There’s still a lot of research to be done there.
Text: Olof Lönnehed
Photo: Dick Gillberg
Kristina Seftigen
Work: Dendroclimatologist and Associate Senior Lecturer at the Department of Earth Sciences.
Age: 38.
Family: Partner Jesper Björklund, who is also a researcher in dendrochronology, and two dogs, Monty and Stuart.
Spare time: Travelling, working on the house and garden.
THEN: Researchers counted the number of growth rings, and created overlapping time series using samples from different old trees. The width of the ring told them a lot about how the tree had grown. A good summer produced a thick growth ring.
NOW: By analysing the anatomy of the cells in growth rings and measuring the density using an X-ray camera, more accurate answers about temperature and precipitation can be obtained. Larger quantities of data can also be processed thanks to AI.
IN THE FUTURE: The methodology for dealing with tree samples will be developed. There is still a lot of work involved in preparing and scanning the wood anatomy samples. Methods for analysing tree growth – and thus climatic variations in the tropics – are currently being developed.
Dendrochronology comes from the Greek: ‘dendro’ meaning ‘tree’, and ‘chronology’ meaning ‘time counting’.