Tree rings keep track of time

I WAS in Denmark last week and one morning I got on a train in Copenhagen to make a personal pilgrimage that I had been promising myself for some time.

I’ve always been fascinated by Vikings and anyone who shares that interest needs to visit the pleasant little town of Roskilde in central Denmark.

It’s an ancient town at the eastern end of a very long and winding fiord and it’s home to the Viking Ship Museum, the focal point of my pilgrimage. The museum is there because about 900 years ago several Viking ships were scuttled at a narrow point in the fiord at a place called Skuldelev, a short distance to the west. This was to partially block access to Roskilde by sea-borne raiding parties of unwelcome Norwegian Vikings.

In 1962 the remains of the ships were discovered and the painstaking work of excavating and preserving them was started. The results are now on display in the museum.

The largest of them, a 30 metre-long warship called Skuldelev 2, was built in Dublin out of unseasoned oak that was felled in June and July in 1042. An exact replica of it was built in Roskilde in 2004 and named the Sea Stallion of Glendalough. The replica was sailed to Ireland in 2007, taken out of the water and put on display for a year. It then sailed back to Roskilde and last week was in winter storage on dry land.

Perhaps the most remarkable aspect of this story is the fact that we know precisely when and where Skuldelev 2 was built. It’s all thanks to the amazing science of dendrochronology.

Dendrochronology was invented in 1937 by an astronomer at the University of Arizona called AE Douglass. He was studying sun-spots and, with a rather brilliant piece of lateral thinking, he decided that solar activity might be reflected in tree rings.

When he started to examine the annual growth rings under a microscope he discovered that they were each as individual as a fingerprint, reflecting the particular growth conditions and weather of that growing season. There are also regional differences which means that, once you build up an adequate data base, you can analyse a piece of timber and tell the precise years in which it lived and died and the approximate place where it grew.

Remarkably there are now databases for some species in some areas that extend back for over 10,000 years and tree ring studies are so accurate that they’re used to calibrate radio-carbon dating.

GREY HERON - Ardea cinerea

Herons, sometimes misleadingly called cranes, are very large birds. They can be a metre in height with a 2 metre wingspan. This means they really can’t be mistaken for any other common Irish bird. They are found everywhere that there‘s wade-able water with fish in it. They will also stab and eat frogs, other birds and small mammals if they get a chance. The stab is very powerful — an injured one once killed a man with a single blow to the head. They nest in trees in colonies, called heronries, that typically hold 70 to 100 pairs. During the breeding season the male’s beak briefly changes from yellow to bright pink. In Holland they’ve become common city birds, visiting markets and snack bars and begging from anglers. There is evidence that this is starting to happen in Irish cities too. Several herons visit Dublin Zoo to rob fish from the penguins at feeding time and they are quite common on the canals in the city.

Dick Warner

More in this section