One Hundred Hikes in Yosemite

The Living Yosemite—Edaphic Influences

Yosemite's soils are derived primarily from granitic rocks. The decomposition of these rocks creates the soil that High Sierra hikers are so familiar with—gravel-size pieces of feldspar and quartz with a few flecks of mica and hornblende. On mid-elevation slopes these weathered minerals can accumulate to produce a deep, well-drained soil, which will support a moderately dense forest of pines and firs, such as on slopes along much of the Tioga and the Glacier Point roads. In soils that are extremely porous, a pure stand of Jeffrey pines can develop, as in volcanic soils of the Mono Basin, east of the Park. Where a gravelly Sierran soil is also shallow, drought-tolerant shrubs replace pines.

It is the dark minerals of granitic rock that are important in supplying certain elements to plants. These dark minerals often are the first part of the bedrock to decompose, and the products are often carried as ions in solution, only to be deposited lower down. Hence richer soils exist in flat areas, where these nutrients accumulate. Dense forests, particularly lodgepole forests, then develop, provided that the ground water is low enough. Where ground water saturates the soil for a few weeks or more, meadows develop.

A small fraction of Yosemite's soils are derived from metamorphic rocks, which are found mainly along the Park's highest elevations. At these heights an alpine plant definitely thrives better on the resulting metamorphic soils than on adjacent granitic soils. This is because metamorphic bedrock fractures into smaller pieces than granitic bedrock does, thus creating a greater water-storage capacity for plants. Furthermore, this rock is much richer in dark minerals, so it yields a more nutrient-rich soil. And, being darker in color than their granitic counterparts, metamorphic-derived soils absorb more heat, which is very important to plants at these alpine altitudes.

A final point to be made about soils is that in the range there were a lot of thick soils before glaciers removed them, and this altered the distribution of at least one notable species, the giant sequoia. In the fossil record it is associated with red firs, but today it grows mostly with white firs. Why did the sequoias switch allegiance from red firs to white firs? They didn't. There is an obvious explanation for this "switch," and it was unwittingly first put forth, in rudimentary form, by John Muir in 1876: Glaciers overran most of the trees, eliminating them from their preferred habitat, the red-fir belt. Glaciers removed the belt's deep, preglacial, groundwater-rich soils, and the modern, postglacial ones, while adequate for red firs, are too water-deficient for sequoias. They do grow in one almost pure, shady, red-fir grove in the southern Sierra, the Atwell grove, which, extending up to 8,800 feet in elevation, is the range's highest grove. They grow there because the slopes never were glaciated.

Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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