The white-bark pine (fig. 21) grows close to timber line in the
mountains of the Pacific coast from British Columbia to southern
California. In the Canadian Rockies it extends north to the fifty-third
parallel. It is the most alpine of all the pines. Its lower limit on
Mount Rainier is about 5,000 feet above sea level. In sheltered places
where the soil is deep the trees are sometimes 30 to 40 feet high and 20
inches in diameter. The trunks are free from limbs for 8 or 10 feet. The
outer bark, from which the tree derives its name, consists of thin,
light-gray scales.
As the white-bark pine advances up the mountain its habit changes
rapidly. The stem shortens and becomes gnarled and twisted. The tough,
flexible branches reach the ground and spread over it to a great
distance from the tree. On rocky summits and the bleak crests of
wind-swept ridges the twisted trunk and branches are quite prostrate and
the crown is a dense flattened mass of foliage.
The roots of the tree are deep, long, and tenacious. They spread wide
and deep and cling so firmly to the rocks that the tree is rarely
overthrown by the violent winds that sweep over the mountain.
[Illustration: FIG. 21.--A white-bark pine (_Pinus albicaulis_) in its
characteristic mountain habitat, Mount Rainier National Park.