The Bridegroom's Oak

Tucked into the northernmost part of Germany is the town of Eutin. On a nice day, you can drive out to Dodauer Forest, where you’ll find a 500-year-old oak tree with a ten-foot ladder resting against it. At the top of this ladder, you’ll find a knothole the size of a fist, and inside that hole, there is likely to be a stash of envelopes. Who writes letters to a tree? And why?