A brief comparison of the fossil forests with the forests now living in
the Yellowstone National Park may be of some interest. The present
forests are prevailingly coniferous, the most abundant and widely
distributed tree being the lodgepole pine (_Pinus murrayana_), which
forms dense forests over much of the plateau region. It is distinguished
by having the leaves in clusters of two. It is a tree with a slender
trunk, usually 70 or 80 feet high, though in exceptionally favorable
localities it may reach a height of 150 feet. Its diameter rarely
exceeds 2 or 3 feet. The areas ravaged by forest fires are usually
reforested by this pine alone, and the young trees come up so close
together as to form thickets that can scarcely be penetrated.
There are two other pines in the park, both white pines, allied to the
common white pine of the Eastern States, and like it both have the
leaves in clusters of 5. One, known as the Rocky Mountain white pine
(_Pinus flexilis_) is a small tree, only 40 or 50 feet in height, and
usually grows singly or in small groves. The other, called the Western
white pine (_Pinus albicaulis_), is still smaller, being usually 20 to
30 feet high, and has a short trunk some 2 to 4 feet in diameter. It
grows on high slopes and exposed ridges.
Perhaps next in abundance to the lodgepole pine is the white or
Engelmann spruce (_Picea engelmanni_), a tall, handsome tree with
disagreeable smelling foliage. Another rather abundant tree is the
Douglas spruce, or red fir (_Pseudotsuga mucronata_), which, where best
developed on the Pacific coast, attains a height of 200 feet, though in
the drier interior it is rarely over 80 or 100 feet high. There are also
two species of fir, the white fir (_Abies grandis_) and the Balsam fir
(_Abies lasiocarpa_), and a single juniper (_Juniperus communis
siberica_), which is often scarcely more than a shrub.
The deciduous-leaved trees are almost a negligible element in the
present park flora, being confined to an occasional cottonwood (_Populus
angustifolia_) at the lower elevations, along the Yellowstone River, and
small groves of the quaking aspen (_Populus tremuloides_). Along the
streams and in wet places there are many species of willow (Salix) and
several alders (Alnus), and in mountain bogs and valleys there is a
small birch (_Betula glandulosa_). There are, of course, many small
shrubs, such as gooseberries, currants, and roses.