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As trees grow, the thin layer of living tissue just beneath their bark (the cambium) lays down new cells on top of older ones.
One of my undergraduate research projects demanded a rope and safety harness just to sample the trees. Once researchers find a stressed tree, they need to estimate where the pith sits within the tree, bore into it with what amounts to an overpriced, hollowed-out drill bit, and remove the core in one piece.
Once removed, they take the core back to the lab, mount it, surface it (with a sander or a razor blade), and measure its rings.
Scientists used to do that last part by hand, but software coupled with a properly calibrated scanner can now both identify rings and measure their widths. As trees age, their growth slows, and declining trends must be eliminated from the data.
For the next dozen years, Douglass scoured Arizona for Ponderosa pine—dead or alive—to construct his first chronology.
Completed in 1914, Douglass's chronology stretched back nearly 500 years, a feat accomplished by crossdating.
Ask any second grader what you can do with the rings on a tree, and they'll respond, "Learn the age of the tree!