Almost every day, Dr. Simard takes a two-hour hike up the mountain behind her house in Nelson, British Columbia. High up on the trail, she stops at a Douglas fir that is more than 100 feet tall, with branches that hang to the ground—patting its bark and asking: “Hi, how are you?” But the tree closest to her heart is lower down the trail: a ponderosa almost as tall. Dr. Simard greets this tree, too, and sometimes leans against a flat spot on its bark, inhaling its scent. (It smells like vanilla, because it produces vanillin, she says.) “These trees have been there so long, living together peacefully as the world has gone nuts. They are solid. Predictable. I am just in awe of them,” says Dr. Simard, who is a professor of forest ecology at the University of British Columbia and author of the new book “Finding the Mother Tree: Discovering the Wisdom of the Forest.” “It’s a significant moment, and I instantly feel better.”