Bristlecone pines rank among the slowest growing organisms on Earth- their girth increases by roughly a meter per millennium, and some have sat atop the California tree line since before the invention of writing. As the world's oldest trees, they naturally contain the world's oldest living climate record, reflected in the rings of their growth, and play an important role in efforts to understand palaeoclimate.
But they record a lot more than changes in the weather over time- each set of rings is a palimsest overwritten by changes in ecology that change micronutrient availability, a capsule history of all the natural fertilizers left behind by generations of animals, dead and alive. In the last half century isotopic systemics and mass specroscopy have done much to improve and deconvolute the proxy climate record tree rings provide, much as they have figured in advances in atmosphic chemistry and transport. Most observers embrace this as normative scientific progress, but some deny the most advanced instrumental results in favor of 19th century natural history- guess who?