GRAND TETON NATURE NOTES

PLANT SUCCESSIONS IN THE GRAND TETON NATIONAL PARK

by Willis Smith
Ranger -Naturalist

A view of Jackson Hole from one of the Teton peaks or from an airplane discloses a striking distribution of the forested areas. The forests are limited to a narrow fringe around the shores of the glacial lakes and a few isolated areas such as Timbered Island and protrusions into the valley floor such as Burnt Ridge. Without exception these forests are growing on glacial moraines. To the east and extending across the floor of the valley a complete absence of forest is the conspicuous feature of the landscape. This is a desert plain populated with a typical sagebrush association of plants.

To the casual observer, particularly from a moving automobile, the moraine along the east side of Jenny Lake and Leigh Lake is populated with a dense and pure stand of Lodgepole Pine. One would expect the floor of the forest to be covered with the young of that same tree, but with the exception of the open places, where the sun reaches the ground, and along road clearings, this is not the case. Rather than lodgepole seedlings, a beautiful stand of Alpine Fir and Engelmann Spruce is found. Some explanation of this limited forest distribution and the young fir and spruce replacing the young lodgepole is needed.

Available water is a prime factor in plant distribution, and the size of the particles that make up a given soil is a prime factor in the availability of water. Clay soil holds more water than sandy soil because clay particles are very small. Although glacial moraines that have given rise to the forests are characterized by large boulders, great quantities of finely pulverized rock - fine sand, silt and clay are also present. These fine soil particles retained the water from melting snow and summer rains in the past as well as at the present time thus permitting the lodgepole forest to populate the moraines.

On examination of the soil in the floor of the valley where grows the sagebrush and its associates, a very different condition is revealed. The soil here consists of coarse sand and quartzite gravel. This extends to hundreds of feet in depth. The water from the melting snow and spring rains disappears from it as though it were a sieve, leaving the surface a dry and inhospitable place even for the hardy, drouth resisting Lodgepole Pine. In fact, it is so arid here that no Lodgepoles grow.

A few desert plants that have come down to us through countless generations of adjustment to extreme arid conditions thrive there. The cactus and sedums that store water in their stems and leaves, the buckwheat whose matted leaves shade the soil and accumulate humus, the lupine with its deep tap-root, and the sagebrush with a pubescence on the leaf that prevents excessive loss of moisture find in the floor of Jackson Hole a suitable habitat. Thus in the soil condition of the region is found an explanation for the limited distribution of forests.

Why are Alpine Fir seedlings found on the floor of a Lodgepole Pine forest? The answer is clear to the student of plant life. It is easy to assume that the plants of Grand Teton National Park were always as they are now, but this is a mistake. As the glaciers retreated back from the areas that are now the shores of the lakes and into the heads of the canyons, the granite boulders of the moraines and the canyon walls were naked and devoid of plant life. In time these bare surfaces were populated with an interesting plant which is a combination of a algae and fungi. This plant is called a lichen. The lichens are commonly seen making brilliant as well as subdued patches of orange, red, yellow, brown and gray color on the rocks. These plants grow very slowly but in time bring about a slight disintegration of the rocks. This film of dust along with the dead body of the lichen sifts into crevices where enough of this soil collects to permit other more complicated forms of plants such as mosses, certain grasses, and eventually the larger flowering plants to grow. This soil building process is very slow at first, but becomes more rapid as plants come. Thus the time comes when what was a bare, hard granite surface is a beautiful flower garden populated with the characteristic plants of the region. The Lodgepole Pine is a hardy shallow-rooted tree and is among the first of the higher plants to take its place in the succession. It will be noted, that in such a process as described that a given plant group will prepare the habitat for another type and thus be crowded out of the picture. Certainly when the rock surface was changed to loose soil the lichen no longer lived there.

FOREST ZONES IN GRAND TETON NATIONAL PARK
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Now in a similar manner the many generations of lodgepoles that have grown on the moraines along the lake shores have prepared the way for this new growth of fir and spruce. These two trees are usually found in moist shaded canyons or well up on the slope where more moisture is available. This extra demand for moisture is successfully met on the moraines today due to the presence of great quantities of humus or rotted plant bodies, mostly Lodgepole Pine. Even a greater factor in water holding capacity of soil than the size of the soil particles, is the presence of this humus. It acts much as a sponge, regulating and retarding the run-off of melting snow and summer rains. In this way the Lodgepoles on the moraines have prepared the habitat for a new type of tree.

Why do not Lodgepole seedlings also help to repopulate the forest floor? Largely because this tree, unlike the Spruces and Firs, is not tolerant of shade, and the young will not survive in the deep shade cast by the larger parents. Thus, while preparing the soil for other tree types, the Lodgepole does not permit the seedlings of its own species to develop. The Pine, then, must perpetuate itself in the open places and on the borders of the deep forest.

In turn, then, it is encroaching on the domain of the sagebrush association and in a similar manner will replace it. It does not take too great a stretch of the imagination to see in the future the floor of Jackson Hole densely populated with a beautiful Alpine Fir - Engelmann Spruce forest. Thus, down through the ages there has been a series of shifting of types of plants that have populated the Grand Teton National Park.