I have written a lot about historic temperature proxies based on tree rings, but it all boils down to “trees make poor thermometers.” There are just too many things, other than temperature, that can affect annual tree growth. Anthony Watts has a brief article from one of his commenter that discusses some of these issues in a real-life way. This in particular struck me as a strong dose of common sense:
The bristlecone records seemed a lousy proxy, because at the altitude where they grow it is below freezing nearly every night, and daytime temperatures are only above freezing for something like 10% of the year. They live on the borderline of existence, for trees, because trees go dormant when water freezes. (As soon as it drops below freezing the sap stops dripping into the sugar maple buckets.) Therefore the bristlecone pines were dormant 90% of all days and 99% of all nights, in a sense failing to collect temperature data all that time, yet they were supposedly a very important proxy for the entire planet. To that I just muttered “bunkum.”
He has more on Briffa’s increasingly famous single hockey stick tree.