A timberline is best defined as the

1.

the altitude above sea level at which timber ceases to grow. the arctic or antarctic limit of tree growth. Examples for timberline Expand

Meadows, little strips of alpine freshness, begin before the timberline is reached.

The Land of Little Rain Mary Austin

Already weak, they did not get down to timberline the first day.

Watched by Wild Animals Enos A. Mills

The territory dominated by old " timberline " had an area of about eighty square miles.

The Grizzly Enos A. Mills