As one proceeds poleward or as elevation increases, the height of the trees gradually decreases while the spacing between them increases until a point is finally reached where the trees give way to tundra. This is called the tree line.

Arctic tree lines form a ring around the Arctic Ocean and extend southward to Labrador and westward around the Bering Sea from Alaska to Siberia. In oceanic regions Arctic tree lines are characterized by birches, while in the interior Arctic larches and spruce are more common. Firs are present in some Arctic tree lines. Antarctic tree lines are more abrupt, as very little tundra vegetation exists in these areas.

The shape of trees also changes with altitude. Broad-leaved trees are more common at lower altitudes, as at the base of a mountain. These tree forms gradually give way to pines and sometimes birches as the altitude increases. Spruce and fir tend to dominate forests at the highest elevations. Local conditions determine whether Alpine timberlines arise gradually or abruptly as the altitude increases. Abrupt timberlines give way to Alpine meadows and then boulder fields, followed by bare rock with life-forms limited to lichens.