I can see how you can use tree rings to correlate major climatic events, such as the slowdown in growth caused by a major eruption, like a Krakatoa magnitude eruption.
However, to try and define temperatures from tree rings on an annual basis is just plain voodoo, because of:
1. Rainfall variations.
2. Shade variations from other trees.
3. Fertilisation from nearby forest fires.
4. Fertilisation from the occasional animal dump.
5. Nitrate boost from lightning storms.
6. Variations in growth depending on the side of trunk.
7. Occasional insect infestations on leaves.
8. Variations in competition from roots of nearby trees for water and nutrients.
9. Variation in ring widths depending on the age of the trees.
10. Variation in soil temperatures depending on slope direction.
I have seen much more comprehensive lists, but the bottom line is this: the only place you can magically transform BS readings into ‘hard facts’ is in climate science, where the use of voodoo statistical techniques is considered both acceptable and normal.