[pvaldes]

That depends entirely on the species of tree. Absolutely crucial in this case. Do they mention this data in the article?

[roywiggins]

Yes, at least two species of tree. You can read which ones in the paper!

[flyingfences]

The species is actually not absolutely crucial in this case. The trees were dated by looking for solar flare activity in the rings; the trees are known to have been cut by Vikings because they were cut with steel tools.