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The elevation where trees fail to grow is called the tree line. The biotemperature of the subalpine zone is between 3 and 6 °C (37 and 43 °F). Above the tree line the ecosystem is called the alpine zone or alpine tundra, dominated by grasses and low-growing shrubs

Explanation:

Timberline is the word that describes the forest vegetation boundary.

Explanation:

The forests are composed mostly of trees and shrubs. Both the trees and the shrubs need particular conditions in which they can survive, so they thrive where they have them but are absent in places that don't offer the same. The only boundary, elevation-wise, is up, where there is roughly a line above which the forest vegetation cannot grow anymore.

This line is called timberline or tree line. Basically, this is the point where the trees and shrubs don't have conditions that support their existence, thus they can not grow. Because of this, the forests sharply end at a certain elevation, being replaced by short grasses and mosses.