The theory behind using tree rings to infer temperature changes is based on the fact that trees grow better when sunlight is brightest which likely occurs on warmer days; trees also grow faster when they have an abundance of carbon dioxide which suggests a certain synergism once the two become coexistent.
That's brings up another question I have. From what I gather, they use things like tree rings and coral growth as proxy data for ancient temperature measurement. But for more modern times, (1600 AD +) they use actual temperature measurements. Why not continue using tree rings for modern data and see how well it coincides with actual measurements?
I'm no forester, but I know there are a lot of factors involved in tree growth beside temperature.