tim·ber·line / ˈtimbərˌlīn/ • n. (on a mountain) the line or altitude above which no trees grow. Also called tree line. ∎  (in high northern (or southern) latitudes) the line north (or south) of which no trees grow.

Timberline

The elevational or latitudinal extent of forests. Above upper timberline, either at high elevations or at the Arctic or Antarctic limits, atmospheric and soil temperatures are too cold for forest development. Below lower timberline, conditions are too dry to support forests. Timberlines are more common and pronounced in the western than in the eastern United States because of more extreme topographic and climatic conditions in the West.

timber-line (waldgrenze) A line that marks the altitudinal limit of trees that are in a close canopy and that grow erect and tall. It occurs below the baumgrenze or tree-line proper, and below the kampfzone (in which the trees often show the krummholz condition).