At my visit with Richard Primack to Nuclear Metals Superfund site in West Concord, I gathered quite a bit of bark from a fallen Hemlock tree.  Hemlock’s have one of the highest concentrations of tannin in their bark, leading to over 70 million being felled in a few decades during the industrial era.  The remaining Hemlock’s have been blighted by the Woolly Adelgid insect.