A line above or below which trees of any dimension do not grow, governed by temperature, solar energy, water supply, soil conditions, and exposure. The upper elevation of tree growth is called the cold tree line, to distinguish it from a lower tree line or dry tree line, below which the moisture supply in certain regions doesn’t facilitate growth. In northern New Mexico and southern Colorado, the cold tree line is around 12,000 feet in elevation, while in northern Colorado it is around 11,000 feet. (The lower line here happens to be at 5,600 feet.) The upper limit drops progressively to the north, as growing seasons become shorter, the line descending to 8,000 feet in Montana’s Glacier National Park, and to sea level at the edge of the Arctic lowlands north of the Mackenzie Mountains in the Canadian Yukon.