timber line vs tree line

timber line and tree line both are nouns.

Word	Noun	Adjective	Verb	Adverb
timber line	Yes	No	No	No
tree line	Yes	No	No	No

As nouns, timber line and tree line are synonyms defined as:

timber line and tree line: line marking the upper limit of tree growth in mountains or northern latitudes

Other synonyms of timber line include timberline.

timber line (noun)	tree line (noun)
line marking the upper limit of tree growth in mountains or northern latitudes	line marking the upper limit of tree growth in mountains or northern latitudes