Instead, ecologists divide the Rocky Mountain into a number of biotic zones. Each zone is defined by whether it can support trees and the presence of one or more indicator species. Two zones that do not support trees are the Plains and the Alpine tundra. The Great Plains lie to the east of the Rockies and is characterized by prairie grasses (below roughly ). Alpine tundra occurs in regions above the treeline for the Rocky Mountains, which varies from in New Mexico to at the northern end of the Rocky Mountains (near the Yukon).