4 
PLANTS OF YELLOWSTONE NATIONAL PARK 
for the third type of lichen. This type has upright, fruiting 
branches and is called a fruticose form. Along with fruticose lichens 
there usually come the mosses and with the upright lichens and 
mosses growing on the rock the development of a soil will proceed 
somewhat more rapidly, although it will still be pretty slow. These 
plants continue the disintegration of the rock surface, they catch 
wind-blown materials, some of the plants die and their dead bodies 
are added to the soil as humus, and so in time there is soil enough 
for certain grasses and flowering plants. The building of a soil will 
now proceed still more rapidly until, finally, there will be soil and 
water enough in the soil to support woody plants, first the smaller 
shrubs, but eventually trees. 
Thus it comes to pass that we have a forest where at first there 
was nothing but bare rock. To be sure the development is usually 
not so orderly as we have briefly outlined it, for there are practically 
always destructive agencies to interfere. There is abundant evidence 
that this has been true in Yellowstone Park. In the northeastern 
part of the park entire forests were completely buried by volcanic 
ash and later became petrified. There were glaciers that completely 
destroyed the existing vegetation over considerable areas. There 
have been fires and other agencies that have practically destroyed the 
plant life in more limited areas. In general, however, the vegetation 
of Yellowstone Park must have developed much as we have described 
it, and it is still developing. Under present climatic conditions the 
climax of vegetation development in the park is a spruce-fir forest. 
Theoretically, therefore, as the leveling forces of nature continue 
their work, thus bringing about more nearly medium conditions with 
respect to the water supply throughout the park, there should come 
a time, in the far distant future, when most of the park would be 
occupied by a spruce-fir forest. 
PRESENT TYPES OF VEGETATION IN THE PARK 
As a result of the developmental processes that have been de¬ 
scribed, we find several more or less distinct types of vegetation in 
the park at the present time. All of the developmental stages men¬ 
tioned are represented in the park. In some places all ,uiay be found 
within an area of a few square feet. Also there is a comparable 
series of stages in the development from a free water surface to a 
forest, and all of these may be found to a limited extent in the park. 
However, disregarding the minor units and subdivisions, a general 
birdseye view of the park shows us seven outstanding types of plant 
communities. These may be briefly characterized as follows: 
1. Aquatic communities .—These are found in the streams and lakes 
and are made up of plants that grow in flowing or standing water. 
