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Trees of Yosemite (1932, 1948) by Mary Curry Tresidder

Introduction

Life Zones

Plant and animal life varies with varying conditions of temperature, humidity, and rainfall. Through a mountain area such as this, we have a range of altitude which gives us several well-defined belts of conditions for growth, and groups of associated forms of life in those belts. These are known as life zones. To say that a plant or a bird belongs in a certain life zone is to describe in a general way the factors it requires, the company it keeps. Conversely, seeing a certain plant or animal known to keep strictly to its own preserves serves to identify the zone through which one is passing. The chart on page 9 shows the distribution of trees through the life zones of our region. The altitudes given are those of the Yosemite latitude; farther north, the boundaries are lower; farther south, they are pushed upward.

These boundaries of altitude are only an approximation, of course, not a hard and fast line. Exposure makes a great difference in the climatic conditions; a wide variety of plant life is encountered in the Yosemite Valley, where along the south wall conditions are those of the true Transition Zone, while the north will, absorbing and reflecting the heat, accommodates that side of the Valley to the warmth-loving trees and plants of drier regions.

Thus in the Yosemite Valley proper—between Mirror Lake and the Cascades—we find indigenous specimens of all the trees commonly encountered in our region except the Digger Pine from the Sonoran Zone, the Sequoia gigantea from its isolated stands at the upper edge of the Transition Zone, the Western White Pine from the Canadian Zone, and the Mountain Hemlock and White-Bark Pine from the Hudsonian Zone. The Mountain Hemlock and the Sequoia gigantea have been planted here and are thriving.

A motor trip from El Portal through Yosemite and over the Tioga Road gives an idea of the sequence and significance of these life zones, and an acquaintance with all the trees generally encountered in our region. At El Portal the traveler is just leaving the foothills— the Upper Sonoran Zone, with its Digger Pine and Blue Oak and its chaparral belt—and entering the Transition Zone, in which the Yosemite Valley lies. Here the Ponderosa Pine predominates, with the White Fir, the Incense Cedar, and the California Black Oak as its most constant companions. Douglas Fir and Sugar Pine also enter the picture, the latter at its best in the upper portion of the zone.

As we turn up the Big Oak Flat Road, on the warm northern side of the Valley, the talus slope is covered with Quercus chrysolepis, or Canyon Live Oak, almost to the exclusion of other trees except the California Laurel, with its aromatic leaf. Near Gentry’s the road turns west, then north, away from the full glare of the sun, and in little ravines along the watercourses are Douglas Fir and Ponderosa Pine again, with Incense Cedar and White Fir appearing here and there. Across Lily Creek the road climbs abruptly; noble Sugar Pines make the road a Hall of Columns, and Tamarack (or Lodgepole) Pines enter into the picture more and more, as Tamarack Flat witnesses.

We reach the Canadian Zone, with its Red Fir and Jeffrey Pine, then drop down to Crane Flat and pass through the Tuolumne Grove of Big Trees, few in number but glorious. At Carl Inn the Tioga Road turns north and east through a similar forest, on the border between the Transition Zone and the Canadian Zone. Aspens at Aspen Valley, Sierra Junipers along the rocky bluffs above us as we climb over the ridge and drop into Yosemite Creek Basin, a magnificent forest of Red Fir and Western White Pine as we reach Porcupine Flat—these are what cause the Canadian Zone to thrill the heart of the lover of trees, with its great clear shafts glowing with life, its play of sun and shadow through the foliage upon the thick carpet of needles, its occasional meadow of delicate flowers for contrast with these dark giants.

Near Snow Flat, the Mountain Hemlock puts in an appearance; along Tenaya Lake, Lodgepole Pine and Juniper are still present, but Hemlock occupies the moist and shadowed places. Lodgepole Pine leads through Tuolumne Meadows, with Western White Pine as runner-up, particularly on the slopes rising from the meadows, while Junipers cling to the ledges and granite domes of the region. As the road winds up toward Tioga Pass the Hemlock recurs, and at last comes our sturdy, weather-beaten pioneer of timber line, the White-Bark Pine, above whose territory only dwarf shrubs and the hardiest Alpine plants venture.

CHART TWO
Yosemite Trees Classificed by Life Zones

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