Geological Survey Professional Paper 387-A Botanical Evidence of the Modern History of Nisqually Glacier Washington

AGE SAMPLING AND MAPPING PLAN

APPEARANCE OF THE FOREST

Several botanical characteristics are considered in the problem of seeking the oldest trees in several localities in order to date past ice positions. The gross appearance of a forest provides the primary means of making a broad estimate of the relative age of the surfaces upon which the forest grows (Lawrence, 1950). Just as a population of older people can be distinguished by sight from a population of younger people, so can an old forest be distinguished from a young forest. Within each area the ages of the oldest trees were estimated by their appearance before the trees were cored.

OLD FOREST

FIGURE 3.—Forest of living trees, 300 years old, fallen logs, and rotted stumps. Old surface is downvalley from 1840 terminal moraine of Nisqually Glacier. (See fig. 4.)

The largest trees in an area grow in the old forest, and at Mount. Rainier most of them have trunks that range from 3 to 5 feet in diameter (fig. 3). A few trees are as large as 6 feet in diameter. Ancient trees bearing broken tops, open foliage, or branches only part way around the crown are characteristic. The forest floor is covered with fallen logs in all stages of decay, of which the soundest are of about the same diameter as the largest living trees. Seedlings and small trees grow on the still identifiable rotted logs.

Scattered through the forest are barely perceptible mounds and pits that may be the erosional remnants of soil mounds formed by the fall of ancestral trees. Such mounds are common in the deciduous forest of the eastern United States and persist for a long time (Shaler, 1892, p. 273—274; Lutz, 1940; Goodlett, 1954, p. 66—81; Denny and Goodlett, 1956, p. 59—66). Thick humus layers almost completely cover the forest floor except on the youngest mounds formed by uprooted trees and along banks of small streams.

Humus, as used in this report and by foresters (Munns, 1950; Trimble and Lull, 1956, p. 2—8), is defined as the organic matter and intermixed mineral matter in soil profiles. It includes the partly decomposed surface litter in which plant parts can be recognized by sight. The humus consists of layers of rotten wood derived from old logs lying at all angles one on top of the other and differing in the degree of decay. The humus alone appears to represent a long interval of time. The largest living trees, whose ages can be estimated only from a count of annual rings found in a core of part of the radius, appear to represent only the most recent of several generations of trees.

Study of soil profiles reveals additional evidence of the antiquity of the forests on the old surfaces. Large fragments of charcoal and partly charred wood are found commonly beneath humus layers and close to the roots of the largest living trees. The charcoal is evidence that fire swept through an ancient forest that existed before the growth and death of the trees which rotted to form the humus and before the present forest began to grow. Volcanic ash layers, where present, are used to estimate the relative age of different surfaces.

YOUNG FOREST

FIGURE 4.—Forest of young trees, smaller logs, and nearly bare boulders near the downvalley limit of the 1840 terminal moraine of Nisqually Glacier; trees are 1 to 2 feet in diameter. (See fig. 3.)

The largest living trees in the young forests are smaller than the average size trees in the old forest and have well-formed crowns and dense foliage (fig. 4). Fallen trees are present, but logs are smaller in diameter than the trunks of standing trees indicating that they were members of a more dense stand in its early stage of development. Humus layers generally are thin, consisting primarily of decayed leaf litter. Bare or nearly bare boulders protrude through the humus, and mineral soil can be exposed by merely scratching the humus.

The only evidence of earlier forest growth in soil profiles on the younger surfaces consists of humus which is being formed from litter from the living trees. All evidence indicates that the present forest is the first to grow; thus the age of the oldest trees nearly equals the time the surfaces have been exposed to plant colonization.

SELECTION OF TREES

Within each forest population, that is, within the old forest and within different age groups in the young forest, trees at many localities were selected for detailed study. In localities where moraines exist, trees on both sides of the moraine or close to the presumed historic position of the ice were chosen. They included the oldest tree in the population, whose age was determined in order to provide an estimate of the minimum age of the surface at the trees' location. Individual trees that appeared to be the oldest were selected for sampling.

Trees in the Mount Rainier area apparently reach a maximum diameter of 4 to 6 feet. Because of the relatively rapid growth rate, only about 300 years is required for trees in the Nisqually River valley to grow to this diameter. Near Emmons Glacier, on the other hand, more than 400 years are required for trees to reach the maximum size before they die. However, some trees near Nisqually Glacier are growing more slowly than any trees sampled near Emmons Glacier. Thus the maximum age of trees cannot by itself be used to determine the age of an old surface, and it is necessary to search for additional evidences of age. Furthermore, if the growth rate of trees in a given area is determined to be relatively rapid, then the chances are low of finding an individual significantly older than the average of those sampled. If, on the other hand, the growth rate is relatively slow, diligent search might disclose an ancient specimen that is significantly older than its neighbors.

The general appearance of the bark of trees is the best single criterion for estimating the relative age of several trees of the same species within a stand. Because the bark consists primarily of cork produced continuously during the growing season by a layer of living cells within the inner bark, the outer cells remain unaffected by growth, and thus the bark of older trees looks more weathered and eroded than the bark of younger trees.

Twisted and gnarled trees that have stems bent in several directions generally are older than larger trees which are more symmetrical. Close study of the branches of these bent trees shows small closely packed needles and exceedingly short annual increments of twig growth. These anual increments can be identified as the length of the twig between successive minute scars remaining from the abscission of the terminal-bud scales. These scars are closely spaced lines in the bark running normal to the axis of a twig. Exceedingly short annual increments of twig growth during recent years combined with a twisted form suggests that such trees have always grown slowly and thus may be of considerable age.

Because the diameter of the trunk is a function of the growth rate as well as of age, it is the poorest single criterion to use in selecting trees for a core sample. For example, three Pacific silver fir trees growing in the same area on a slope above the Nisqually River all are about the same age, but differ markedly in diameter as shown in the table below.

The largest tree within a given species was cored, but other trees which appeared to be older on the basis of the other criteria previously outlined, were also cored. If all trees within a group appeared to have nearly the same vigor, the largest of each species was cored.

Many trees must be sampled at many different sites in order to determine the age of the oldest tree in the forest, primarily because the mortality of individual trees is extremely high. A so-called good stand of established seedlings ranges from 500 to 1,000 seedlings per acre. Thus, where the oldest trees in any area of several acres number only one or two it is apparent that the mortality rate from seedling to maturity is extremely high. On many newly formed morainic surfaces there are far fewer than 500 to 1,000 seedlings per acre, yet the ages of the oldest trees on different parts of what appears to be a single moraine are remarkably similar. This demonstrates the effectiveness of the sampling procedure used in this study.

Because of the many environmental influences that lead to the destruction of trees at Mount Rainier, many trees were sampled at each place selected for study. Initially, in the life of a stand on the newly formed moraines, seedlings are subject to uprooting and burial on the unstable surfaces. Some slow-growing trees die in dense young stands in which most other trees are growing rapidly, probably because of excessive amount of shade. Rock and snow slides, insects, disease, and fire destroy individuals and large areas of forest. One or more of these influences have been of such magnitude and frequency in certain areas that trees in them were not used as samples in the present study.

FIGURE 5.—Diagrams showing possible relations between successive advances of a glacier, solid lines show position of moraines that remain after readvance. A. Similar in downvalley extent but with axes of flow displaced laterally; B. closely similar in downvalley extent but differing in longitudinal gradient; C. Different downvalley extent but with same axis of flow. (click on image for an enlargement in a new window)

In summary, many characteristics must be taken into consideration in selecting forest stands and individual trees for sampling to determine the maximum age of the forest. The appearance of individual trees, as well as the species and trunk diameters, is used to determine which individuals should be sampled once the stand is selected for study.

GEOGRAPHIC DISTRIBUTION OF KEY TREES AND AGE GROUPS

Experience in tracing the position of moraines quickly demonstrates that a given advance of a glacier does not necessarily follow the same pattern as its predecessor. Therefore, a younger and an older moraine may not necessarily parallel each other in all parts of the valley. In fact, such symmetry seems to be the exception. Thus it is necessary to obtain the ages of trees that grow within geographic bounds sufficiently extensive to cover virtually all parts of the valley.

The dissimilarity in the pattern of movement of two successive glacial advances may, for convenience, be classed as: those that are dissimilar because of a lateral shift in the axis of flow (fig. 5A); those that result from a difference in slope of the surface of the ice streams (fig. 5B); and those that differ because the last advance did not extend as far downvalley as an earlier one but that both follow the same axis of flow (fig. 5C). In the class of two advances similar in downvalley extent and surface slope, but with lateral displacement of the axis of flow, the lateral moraine of the younger will show on one side of the valley, the lateral moraines of both the younger and the older will show on the other side of the valley, and the moraines will intersect somewhere near the terminal positions. In the class of two advances nearly similar in downvalley extent, but with differing longitudinal slopes, the lateral moraines of the younger or of both the younger and the older will be found on both valley sides, and the moraines will intersect at two places near the terminal positions.

In the third class of the last advance extending a shorter distance downvalley than an earlier one, the resulting moraines are roughly parallel and the younger ones occur within the older ones. Furthermore, a pre-existing moraine tends to modify the course of a later advance, especially where the glacier moves through a breach in the older moraine.

In Nisqually River valley the most recent major advance overrode and obliterated all evidence of the moraines left by possible earlier, less extensive advances. This advance fits the class in which the terminal moraine and lateral moraines in the area studied are of the same age.

Limited observation of Emmons and Tahoma Glacier moraines indicate, however, that the existence of one class alone may be uncommon. Combinations of various degrees of each class produce a complex pattern of relationships between the positions of two glacial advances. A previous position of the glacier may be represented by only small widely separated segments.

Thus careful tree-age sampling on an extensive geographic pattern is required to correlate the identifiable ice-front positions. Although the complex pattern of ice-front positions adds to the sampling problem, it is also a fortunate circumstance in that the evidence of a larger number of glacier positions is more likely to exist than if the advances followed a simple pattern. The sequence of advances and recessions of Emmons Glacier formed a complicated pattern of moraines, and studies to date are insufficient to unravel the complete history. At Tahoma Glacier, an older advance in the area studied is represented by a small segment of a lateral moraine that is bounded by a younger moraine.