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Dendrochronology by Peggy Edwards

Updated: May 15, 2020

Gillies Hill Scots Pine

Did you know that the first person to mention that trees form rings annually and that their thickness is determined by the conditions under which they grew was Leonardo da Vinci way back in about 1490!

If you like him, have ever stopped to look at a cross-section of a fallen tree you would have noticed the thin concentric lines known as tree rings, growth rings or annual rings. The study of these ring patterns is called dendrochronology from the Ancient Greek dendron, meaning "tree;” khronos, meaning "time;” and logia, meaning "the study of".

Tree rings are formed each year in a layer of cells near the bark. Throughout the year the growth rate of a tree changes in a predictable pattern that responds to seasonal changes.

Spring growth is rapid and forms less dense and wider rings which narrow and darken as growth slows down in the summer and autumn. The rings are more visible in temperate zones where trees produce one growth-ring each year with the newest adjacent to the bark.

A year-by-year record for a tree with annual rings reflects the age of the tree AND the climatic conditions in which the tree grew. A wide ring reflects adequate moisture and/or warmth whereas a narrow ring may reflect a dry and/or cold year. Ring growth may also be affected by fire, insect or fungus damage.

In 1937 the first formal lab established to study dendrochronology was the Laboratory of Tree-Ring Research (LTRR) at the University of Arizona in Tucson.

In Scotland in 1984 LTRR’s researcher, Malcolm Hughes, reconstructed Edinburgh summer temperatures going back to the early 18th century through the use of tree ring data. Since then St. Andrew’s Tree-Ring Laboratory has established the Scots Pine Project one of whose aims is “to extend living chronologies using extant, historical or sub-fossil tree-ring material.” In Scotland wood found submerged in lochs, on crannogs, bogs, and historical buildings can be used to establish ancient chronologies. This page from the Scottish Pine Project shows a terrific chart of tree-ring records for the last millennium Samples for the study included wood from Stirling Palace floorboards, St Andrews Queen Mary’s House, Jedburgh Abbey, Perth High Street, and the oldest, Aberdeen Gallowgate.

Although tree-rings from Sierra, coast and dawn redwoods in Scotland have not been studied (to my knowledge) it would certainly be possible to begin researching those of the past 164 years. The trees are small enough to use an increment borer (appropriately) to extract pencil thin core samples from selected healthy trees. Additionally, the beginnings of a redwood data bank could be created by preserving cross sections of any redwoods felled in the future.

Two additional redwood tree-ring studies worth mentioning are Allyson Carroll’s (Humboldt State University sponsored by Save the Redwoods League’s Redwoods and Climate Change Initiative (RCCI) program) study that demonstrated how redwoods in California have responded to past and current climate conditions by collecting core samples from trees in 16 plots. (Coast redwoods have been back dated to 328 AD; and giant sequoia to 474 AD.)

Another study by Todd Dawson and his team of the University of California, Berkeley, of lighter and heavier CO2 and H2O isotope imprints derived from the cellulose of tree rings has paved the way for scientists to extrapolate climatic conditions of the past, and to show how changes in water availability impact current redwood growth.

Many more studies exist, too many to mention in this blog. Suffice it to say that it’s an exciting time for the science of dendrochronology – a vital branch of research for analysing past and present climate change and attempting to predict that of the future.

Blog written by Peggy Edwards, January 2019

Written by Peggy Edwards. December 2018.

Peggy Edwards is a botanical illustrator and redwood aficionado. She has a BSc from the University of California and a Graduate Certificate Natural Science Illustration from the University of California, Santa Cruz. She has been a field Botanist, Naturalist and K-12 Teacher. She is one of the co-founders of the Scottish Redwood Trust.

Next Time: The Story of the rarest of all the sequoias - the Dawn Redwood

St Andrews Tree-Ring Laboratory – The Scottish Pine Project

University of Arizona – Laboratory of Tree-Ring Research

UK Tree-ring Services - Neolithic Pine Project

Allyson Carroll’s publication Millennium-Scale Crossdating and Inter-Annual Climate Sensitivities of Standing California Redwoods

Todd Dawson’s (U.C. Berkeley) presentation Chemical Signals of Climate and Physiology in Redwoods and the use of stable isotope analysis to unlock the climate history recorded in redwood tree rings

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Coming from California myself it was nice to find out more about what I had always considered "our" trees. Thanks

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