timberline

The elevation above which trees cannot grow — the boundary between forest and alpine tundra, drawn by cold, wind, desiccation, and the shortness of the growing season. Timberline is not a line but a zone: the forest thins, the trees shrink and twist into krummholz, and then, above a certain altitude, they stop. The line varies by latitude, aspect, and local conditions — roughly 11,500 feet in Colorado, 6,000 in the northern Cascades, at sea level in the Arctic.