Why a birch?
In Slavic tradition the birch is a friend you can confide in. People tied ribbons to its branches to leave a wish or a worry with the tree, and hugging its trunk was said to pass calm and strength back to you.
Hug a Tree takes that old, gentle gesture and makes it something you can do in a couple of minutes, wherever you are: give the tree what's weighing on you, breathe, and let the branch hold it for a while.