That’s my parents in the main photo.  They’re making a rough estimate of the chest-high circumference of a big yellow birch near their cabin in northern Vermont. They noticed it was way bigger than the trees around it and wondered how old it was. The state forester told them that the average width of a growth ring of a yellow birch is about 1/8″.  They determined the circumference was about 160″, and they did some math from there to figure out the tree’s age.  (The details are left to the reader – you didn’t really think I was going to tell you, did you?) The area where the tree is was clear-cut by a paper company around the turn of the last century, so this tree really stands out as one of the few trees that was left standing.