From some information found on the Denali National Park website:
Tundra is simply the land above treeline. Treeline varies with latitude, existing at about 18,000' at the equator, about 10,000' in Colorado, about 2700' in Denali and drops to sea level in the vicinity of the Arctic Circle. Tundra comes in two basic forms: dry tundra, which is usually found on well-drained mountain slopes and ridges, and moist tundra which is damp to downright boggy. The word tundra comes from a Lapp word meaning "marshy plain."