tree line vs timber line

tree line and timber line both are nouns.

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

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

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

Other synonyms of tree line include timberline.

tree line (noun)	timber 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