The Southern Rockies and Southwest Ranges have two timberlines. The lower treeline is the border below which annual precipitation is insufficient to support forest growth (20-25 inches per year). At this border, the forest gives way to shrublands, often dotted with juniper, pinon pine and other drought tolerant trees. In Colorado, this lower timberline occurs near 6500 feet; it is higher in southern Colorado and on south-facing slopes where increased temperatures and solar evaporation reduce the soil moisture content.
The alpine timberline is the high altitude border above which forests cannot develop. While low temperatures, intense solar radiation, high winds and heavy snows all play a role, the location of this border coincides most closely with the 50 degree (F) isotherm for the warmest month of the year (usually July in the Northern Hemi-sphere); in other words, along the timberline zone, the average July temperature is 50 degrees F (this average includes all day and night temperatures through the month). Above this border, which has an average elevation of 11,500 feet in Colorado, the July isotherm is below 50 degrees F and the annual growing season is generally less than 2 months. As with the lower timberline, the alpine treeline is higher in southern regions and on south-facing slopes; in this case, greater solar radiation raises the isotherm for any given elevation.
Despite harsh conditions above the alpine timberline, more than 300 plant species (grasses, sedges, low shrubs and wildflowers) colonize the tundra; almost all are perennials, capable of storing nutrients through winter for the short summer of growth and reproduction. Like desert plants, most are also equipped with succulent leaves and extensive root systems, adaptations to the dessicating effects of high wind and intense solar radiation. Readers interested in a thorough discussion of alpine timberline ecosystems are referred to the books by Zwinger, Arno and Hammerly, listed in the right column of this blog.
