Original: Types of tree lines There are several types of tree lines defined in ecology and geology: Alpine tree line The highest elevation that sustains trees; higher up, it is too cold or snow cover persists for too much of the year, to sustain trees.
Simplified: There are several types of tree lines defined in ecology and geology: Arctic tree-line The farthest north in the Northern Hemisphere that trees can grow; farther north, it is too cold. Alpine tree-line The highest height where trees grow: higher up, it is too cold, or snow covers the ground for too much of the year.