From Longman Dictionary of Contemporary Englishtimberlinetim‧ber‧line /ˈtɪmbəlaɪn $ -ər-/ noun [singular] technical 1 TMHBPthe height above the level of the sea beyond which trees will not grow2 TMHBPthe northern or southern limit in the world beyond which trees will not grow
Examples from the Corpus
timberline• These valleys well below timberline are dusty already, even though it is early spring. 