With only the highest summits reaching above the timberline, a large part of the mountains lies within the White Mountain National Forest.
Birch can be found up to the timberline at 14,000 feet (4,200 metres).
The two zones are separated by the tree line, or timberline, defined in this case (the term also applies to the upper limit of arboreal growth at high elevations) as the absolute northern limit of treelike species, although even beyond it the same species may be found in low shrubs and dwarfed forms.
Timberline, upper limit of tree growth in mountainous regions or in high latitudes, as in the Arctic.
In the interior forests the timberline in many places is less than 600 feet (180 metres) above sea level, although it is generally between 1,500 and 2,000 feet (450 to 600 metres).
The reason low temperatures affect timberline is that they slow biological processes, which decreases the production of dry matter, a condition that is exacerbated by the shortened growing season.
Elsewhere in the Sierra lie hundreds of smaller lakes, some above the timberline in regions of tumbled granite and smooth-walled canyons.
One of the state's principal tourist destinations is Mount Hood National Forest, which covers an area of some 1,700 square miles (4,300 square km) and is notable for its scenic views and Timberline Lodge (built on the mountain in 1937).
The timberline is between 11,400 and 12,000 feet (3,475 and 3,660 metres); above it rise nearly 2,500 feet (760 metres) of bare granite.
The timberline in the central Rockies and Sierra Nevadas is around 3,500 metres (11,500 feet), whereas in the Peruvian and Ecuadorian Andes it is between 3,000 and 3,300 metres (10,000 and 11,000 feet).
In much of the central and southern Rockies there is a double timberline: the usual high timberline below which there is a belt of normal tree growth; and then a low timberline below which no trees grow because of low precipitation and high temperatures.
