tree line vs timberline

tree line and timberline both are nouns.

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

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

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

Other synonyms of tree line include timber line.

tree line (noun)	timberline (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