“Stone walls are the Mass Pike of the New England forest,” says our guide from the New Roxbury Land Trust, pointing to the long stone wall jutting into the woods. She explains how leaves collect along it and rot, creating habitat for all manner of bugs, beetles, and amphibians, which mammals eat. I am lost in thought… when I jerk my head up, startled by her next sentence. “The problem is earthworms, they’ve invaded the forest — fungi are dying off.” She begins to describe the mysterious symbiotic relationship between fungi and the roots of trees. Fungi in the soil invade tree roots for their own benefit and, in doing so, create miles of network that link the roots of forest trees; they are the fiber optics of the forest, miles of dense web that trees use to exchange nutrients and information.