COMPARISON WITH LIVING FORESTS.
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.