Re: Treeline / Timberline

The timberline, as you have observed, is not a constant phenomenon with respect to elevation. For example, it occurs at about 1070m (3500 ft) in central Alaska, 2290m (7500 ft) in Alberta, and 3660m (12,000 ft) in southern Colorado. Elevation is only one factor determining the height of the timberline, or "alpine tree line." It interacts with other factors such as latitude, climate (predominate weather patterns, timing and amount of precipitation), temperature (length of frost-free period, severity of daily fluctuations), and topography (slope direction, size of mountain range), just to name a few. The primary factor determining the location of the alpine tree line seems to be summer temperatures. Even the hardiest conifers require a frost-free growing season of approximately two months in order to complete their life cycle. The tree line is very closely correlated to the 10 C isotherm for the warmest month. That is, if you draw a line on a map connecting the places where the average temperature in July (usually the warmest month in the Northern Hemisphere) is 10 C, it's almost the same line as the tree line. Places that never have a month averaging above 10 C generally can not support trees.

Snowfall is another major factor. Trees actually need a certain amount of snowfall to protect them from extreme cold, ice, and drying winds. Too much snow, however, can cause breakage, and snow that lasts too late in the year can prevent new trees from becoming established.

For more information about the timberline, check out Timberline: Mountain and Arctic Forest Frontiers, by Steven F. Arno (Mountaineers, 1984). It is very easy reading and offers a good explanation of how these factors and others interact to determine the location of the timberline in any given area.