Timberline--where the trees end--is a biological boundary visible throughout North America to even the casual traveler. Where highways or hiking trails ascend to upper timberlines (ranging from below 2000-foot elevations in Alaska to over 11,000 feet in California), visitors see patchy forest and meadows giving way to stunted trees and finally to mere shrub-like trees and tundra. This book describes what timberlines are and why they exist, and what...