I am dangling 100 feet in the air. The only thing that keeps me from plummeting to the forest floor is a braided nylon rope as wide around as my finger. It is anchored to the top of a 500-year-old, 250-foot Douglas fir, a tree that travelers can scale and spend the night in swinging from a hammock.

The tree—one of the tallest in the country—is nicknamed Fuzzy for the moss and lichen that beard its branches. Each battered plate of bark is the size of a dinosaur's scale, some of them scorched from a...