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Ars has a great article on the basics of dendrochronolgy, page 2 is particularly interesting in its exposition of the mechanics of dendroclimatology.

Everyone knows that you count a tree’s rings to tell its age.  Most of us know that the thickness of each ring relates to the quality of the growing season.  What I find fascinating is what the Ars article calls “cross dating”, lining up the rings in different trees and pushing backwards our historical knowledge of local climates.

I first became aware of this upon upon an early visit to the ancient bristlecone pine forest of California’s White Mountains.  These trees live in a marginal zone, at tree line in an arid region, making them ideal for historical climate studies.  Some of these trees are 3 to 4 thousand years old, giving us a long view into history.  Since dead trees are preserved in this zone, long dead trees can have their rings matched to trees of known age, allowing us to have a look back at historical climate data all the way to the last ice age.