Answer: timberlines

AnswerCrossword Clue
TIMBERLINESThey mark the upper limit of tree growth
timberlines(in high northern (or southern) Latitudes) the line north (or south) of which no trees grow
timberlines(on a mountain) The line or altitude above which no trees grow
timberlinesTIMBERLINE, the upper limit of timber trees on a mountain