EVERY year, regular as clockwork, I get a letter from John McLoughlin near the end of February. It tells me about National Tree Week, organised annually by the Tree Council of Ireland. This year's NTW began yesterday, and its high light, as always, will be the Augustine Henry memorial lecture thin Wednesday.
The Augultine Henry lecturer this year is Prof Mike Baillie of Queen's University, Belfast. His personal specialised subject dendrochronology, is the technique of using tree rings to fix the age of wood. And at 8 p.m. on Wednesday, in the Merrion Room of the RDS in Dublin, he will explore "What Tree Rings Can Tell Us About the Past 7,000 Years of Irish History". Admission is free and all are welcome.
The science of dendrochronology is based on the fact that if you slice through the trunk of a tree, the resulting disc is embossed with concentric circles, or tree rings, one for each year since the tree began to grow.
The width of each tree ring varies with the weather of its formative year low temperature and low rainfall both producing narrower rings than usual. Each tree, therefore, displays a pattern which tells the story of the climate during its lifetime.
Now a living oak, say, 250 years old, shares a common pattern of 150 rings with a similar tree that was felled a century ago. By using tree ring "patterns that overlap like this, dendrochronologists can build a "master chronology" for any region the tree-ring pattern running backwards through time for thousands of years.
Once this master chronology has been established, anyone who wishes to know the age of a particular piece of timber in an ancient building, for example, has only to take a sample and compare the growth ring pattern with the standard sequence.
Here, Prof Baillie and his colleagues have assembled a precisely dated record of oak growth for every year back to 5400 BC, against which any fragment of that timber can be dated.
In addition, the record allows us to see, as Prof Baillie puts it, "what the trees thought of conditions in the past history according to the trees".
The national record highlights, for example, periods of severe or benign weather, times of possible plague, and intervals when volcanic eruptions may have had adversely affected our climate.
Moreover, by comparing anomalous Irish timber with records in other countries, it has been possible to see evidence for trade in oak timbers between Ireland and other parts of Europe long before it was suspected to exist.