I'm currently reading Oliver Rackham's "Woodlands". 
While the dust jacket says that he's one of Britain's best known naturalists, Rackham's is hardly a name that is often cited around most British breakfast tables. This is a pity, because he has written some masterful books, including the seminal History of the Countryside. His books are full of wonderful, arcane knowledge about the British landscape and the way land use has changed over the years. Woodland and trees turn out to be wonderfully counterintuitive. For example the presence of an ancient tree in a wood is a strong indicator that the wood is not ancient. This is because in woods, trees are felled on a regular basis. So if you see an ancient tree in a wood, it probably means that a wood has grown up around an a single ancient tree. I find that rather wonderful.
Rackham is a botanist: his specialisms are trees and woodlands - as a fellow of All Souls Corpus Christi Cambridge he is an acknowledged authority on his subject - which is why I was amused to read his thoughts on dendrochronology and paleoclimatology. It's possible that I may be inferring something into his words which is in fact not there. But I can't help but get a feeling of a gentle sarcasm, a wise old man raising an eyebrow at the antics of the young.
Tree rings have other uses. Because weather varies from region to region, the provenance of a timber can sometimes be determined: if the sequence matches a master curve from Poland rather than England, this is evidence that the sample is of Baltic oak. By removing year-to-year variation, leaving the long-term trends, it has been possible in America to use growth rates as a measure of climate change.
To get a result one normally measures at least 100 rings, preferably from each of several contemporary trees or timbers. Tree rings are affected by other factors besides weather, such as defoliating caterpillars. In view of the statistical 'noise' introduced by unknown factors, it is surprising that the method has been so successful and so seldom at odds with dating by other means.
[Emphasis is mine]