Scott Johnston has his feet on the ground but his heart in the air. Day in and out, he trusts his life to a harness, coils of half-inch rope and 35 years of knowledge gained by working in trees. Do you want me to rig up and climb a tree? he asked. Thats all right, I said, clinging to my notebook. Im not sure my nerves could take it, even as a spectator. Johnston, who is 58 and looks like a maturing matinee idol, thinks nothing of getting high into an 80-foot oak or tulip tree. As an independent certified arborist, he has devoted his working life to trees, is passionate about them and, if you scratch below the cambium, is vexed by the way we treat them. His firm, Johnston Tree Care (motto: A cut above), is based in Marshall, Va. I caught up with him at the State Arboretum of Virginia, just east of Winchester, where he was giving a class on pruning. The students were gathered under the canopy of a yellowwood, a beautiful native tree that can be finicky: The same smooth, gray bark that