At the tree line, tree growth is often sparse and stunted, with the last trees forming densely matted bushes, known as krummholz (German for "crooked wood").:58
The tree line, like many other natural lines (lake boundaries, for example), appears well-defined from a distance, but upon sufficiently close inspection, it is a gradual transition in most places. Trees grow shorter towards the inhospitable climate until they simply stop growing.:55
An alpine tree line is the highest elevation that sustains trees; higher up it is too cold, or the snow cover lasts for too much of the year to sustain trees.:151 The climate above the tree line of mountains is called an alpine climate,:21 and the terrain can be described as alpine tundra. In the northern hemisphere treelines on north-facing slopes are lower than on south-facing slopes because the increased shade on north-facing slopes means the snowpack takes longer to melt. This shortens the growing season for trees.:109 In the southern hemisphere, the south-facing slopes have the shorter growing season.
The alpine tree line boundary is seldom abrupt: it usually forms a transition zone between closed forest below and treeless alpine tundra above. This zone of transition occurs “near the top of the tallest peaks in the northeastern United States, high up on the giant volcanoes in central Mexico, and on mountains in each of the 11 western states and throughout much of Canada and Alaska”. Environmentally dwarfed shrubs (krummholz) commonly forms the upper limit.