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Chapter 3

PCT Natural History

Geology

It is very likely that the California section of the Pacific Crest Trail is unequaled in its diversity of geology. Many mountain trails cross glacial and subglacial landscapes, but which ones also cross arid and semi-arid landscapes? Some parts of your trail will have perennial snow; others are usually dry. Precipitation may be more than 80 inches per year in places, less than 5 inches in others. In each of the three major rock classes—igneous, sedimentary, and metamorphic—you’ll encounter dozens of rock types. Because the PCT provides such a good introduction to a wide spectrum of geology, we have added a liberal dose of geologic description to the basic text. By the end of your journey you’ll have developed a keen eye for rocks and understand the relations between the different rock types. Since we assume that many hikers will have only a minimal background in geology and its terminology, we’ll try to cover this broad subject for them in the next few pages. Those wishing to pursue the subject further should consult the list of references at the end of this book.

Rocks

First, you should get acquainted with the three major rock classes: igneous, sedimentary, and metamorphic.

Igneous rocks

Igneous rocks came into being when the liquid (molten) rock material (magma) solidified. If the material solidified beneath the earth’s surface, the rock is called intrusive, or plutonic, and a body of it is a pluton. If the material reached the surface and erupted as lava or ash, the rock is called extrusive, or volcanic.

Intrusive rocks: The classification of an igneous rock is based on its texture, what minerals are in it, and the relative amounts of each mineral present. Since intrusive rocks cool more slowly than extrusive rocks, their crystals have a longer time to grow. If, in a rock, you can see an abundance of individual crystals, odds are that it is an intrusive rock. These rocks may be classified by crystal size: fine, medium, or coarse-grained, to correspond to average diameters of less than 1 millimeter, 1–5, and greater than 5.

Some igneous rocks are composed of large crystals (phenocrysts) in a matrix of small crystals (groundmass). Such a rock is said to have a porphyritic texture. The Cathedral Peak pluton, which is well exposed on Lembert Dome at the east end of Tuolumne Meadows in Yosemite National Park, has some feldspar phenocrysts over four inches long. High up on the dome these phenocrysts protrude from the less resistant groundmass and provide rock climbers with the holds necessary to ascend the dome.

The common minerals in igneous rocks are quartz, feldspar, biotite, hornblende, pyroxene, and olivine. The first two are light-colored minerals; the rest are dark. Not all are likely to be present in a piece of rock; indeed, quartz and olivine are never found together. Intrusive rocks are grouped according to the percentages of minerals in them. The three common igneous groups are granite, diorite, and gabbro. Granite is rich in quartz and potassium feldspar and usually has only small amounts of biotite. Diorite is poor in quartz and rich in sodium feldspar, and may have three dark minerals. Gabbro, a mafic rock (rich in magnesium and iron), lacks quartz, but is rich in calcium feldspar and pyroxene, and may have pyroxene and olivine. You can subdivide the granite–diorite continuum into granite, quartz monzonite, granodiorite, quartz diorite and diorite. These rocks, which are usually called “granitic rocks” or just plain “granite,” are common in the Sierra Nevada and in most of the other ranges to the south.

Since it is unlikely that you’ll be carrying a polarizing microscope in your backpack, let alone a great deal of mineralogical expertise in your head, your best chance of identifying granitic rocks lies in making educated guesses based upon the following table.

At first you’ll probably estimate too high a percentage of dark minerals, partly because they are more eye-catching and partly because they show through the glassy light minerals. If the intrusive rock is composed entirely of dark minerals (no quartz or feldspar), then it is an ultramafic rock. This rock type, which can be subdivided further, is common along the trail from Interstate 5 at Castle Crags State Park northwest to the Oregon border.

Extrusive rocks: Extrusive, or volcanic, rocks are composed of about the same minerals as intrusive rocks. Rhyolite, andesite, and basalt have approximately the same chemical compositions as granite, diorite, and gabbro, respectively. As with the intrusive rocks, the three volcanics can be subdivided into many groups, so it is possible to find ordinary rocks with intimidating names like “quartz latite porphyry”—which is just a volcanic rock with quartz phenocrysts and a composition in between rhyolite and andesite.

Castle Crags, Section P

Texture is the key feature distinguishing volcanic from plutonic rocks. Whereas you can see the individual crystals in a plutonic rock, you’ll have a hard time finding them in a volcanic one. They may be entirely lacking, or so small, weathered and scarce that they’ll just frustrate your attempts to identify them. If you can’t recognize the crystals, then how can you identify the type of volcanic rock? Color is a poor indicator at best, for although rhyolites tend to be light gray, andesites dark gray, and basalts black, there is so much variation that each can be found in any shade of red, brown or gray.

One aid to identifying volcanic rock types is the landforms composed of them. For example the high silica (SiO2) content of rhyolite makes it very viscous, and hence the hot gases in rhyolite magma cause violent explosions when the magma nears the surface, forming explosion pits and associated rings of erupted material (ejecta). For the same reason, a rhyolite lava flow (degassed magma) is thick, short, and steep-sided and may not even flow down a moderately steep slope. The Mono and Inyo craters, north of Devils Postpile National Monument, are perhaps the best examples of this volcanic rock in California. You will find very little of it along the trail.

The landform characteristically associated with andesite is the composite cone, or stratovolcano. Mount Shasta and some of the peaks in the Lassen area, including Brokeoff Mountain, are examples. These mountains are built up by alternating flows and ejecta. In time parasitic vents may develop, such as the cone called Shastina on Mount Shasta; and the composition of the volcano may shift to more silica-rich dacite rock, an intermediate between rhyolite and andesite, which, like rhyolite, gives rise to tremendous eruptions, but also can produce lava domes such as Lassen Peak.

The least siliceous and also the least explosive of volcanic rocks is basalt. A basaltic eruption typically produces a very fluid, thin flow and a cinder cone, usually less than 1000 feet high. When in Lassen Volcanic National Park, take the alternate route up to the rim of the Cinder Cone. From this vantage point you can see what an extensive, relatively flat area its thin flows covered. Contrast this with Lassen Peak, to the west, California’s largest dacite dome.

Sedimentary rocks

We often think of rocks as being eternal—indeed, they do last a long time. But even the most resistant polished granite eventually succumbs to the effects of weathering, although on broad, unglaciated ridges and gentle slopes the rate of removal (denudation) is about a foot or less per million years. Granite rocks solidified under high pressures and rather high temperatures within the earth. At the surface, pressure and temperature are lower and the rock’s chemical environment is different, and in this environment it is unstable. The rock weathers, and the pieces are gradually transported to a place of deposition. This place may be a lake in the High Sierra, a closed basin with no outlet such as the Mono Lake basin, an open structure such as the great Central Valley, or even the continental shelf of the Pacific Ocean. The rocks formed of the sediment that collects in these basins are called sedimentary rocks.

Most sedimentary rocks are classified by the size of their particles: clay that has been compacted and cemented forms shale; silt forms siltstone, and sand forms sandstone. Sandstone derived from granitic rock superficially resembles its parent rock, but if you look closely you’ll notice that the grains are somewhat rounded and that the spaces between the grains are usually filled with a cement, usually calcite. Pebbles, cobbles and boulders may be cemented in a sand or gravel matrix to form a conglomerate. If these particles are deposited on an alluvial fan and then gradually cemented together to form a hard rock, collectively they become fanglomerate. Alluvial fans are usually formed where a stream debouches from the mouth of a canyon and drops its sedimentary load, or alluvium, over a fan-shaped area. Alluvial fans are seen along the south edge of the Mojave Desert, where it abuts the north base of the San Bernardino and the San Gabriel mountains. If the larger particles in a conglomerate or fanglomerate are angular rather than rounded, the sedimentary rock is called a breccia.

Granitic rock walls above Smedberg Lake, Yosemite National Park, Section I

Limestone, another type of sedimentary rock, is formed in some marine environments as a chemical precipitate of dissolved calcium carbonate or as cemented fragments of shells, corals and foraminifers. The individual grains are usually microscopic. If the calcium in limestone is partly replaced by magnesium, the result is dolomite.

Since the PCT attempts to follow a crest, you’ll usually find yourself in an area being eroded, rather than in a basin of deposition, so you’ll find very ephemeral sediments or very old ones. The young ones may be in the form of alluvium, talus slopes, glacial moraines, or lake sediments. The old ones are usually resistant sediments that the intruding granitic plutons bent (folded), broke (faulted) and changed (metamorphosed).

Metamorphic rocks

A volcanic or a sedimentary rock can undergo enough alteration (metamorphism) due to heat, pressure, and superhot, corrosive fluids that it loses its original characteristics and becomes a metavolcanic or a metasedimentary rock. Metamorphism may be slight or it may be complete. A shale undergoing progressive metamorphism becomes first a slate, second a phyllite, then a schist, and finally a gneiss. The slate resembles the shale but is noticeably harder. The schist bears little resemblance and is well-foliated, with flaky minerals such as biotite or other micas clearly visible. The gneiss resembles granite, but has alternating layers of light and dark minerals.

Hornfels is a hard, massive rock, common in parts of the High Sierra, formed by contact of an ascending pluton with the overlying sediments. It can take on a variety of forms. You might find one that looks and feels like a slate, but differs in that it breaks across the sediment layers rather than between them.

Quartzite is a metamorphosed sandstone and resembles the parent rock. The spaces between the grains have become filled with silica, so that now if the rock is broken, the fracture passes through the quartz grains rather than between them as in sandstone. Metamorphism of limestone or dolomite yields marble, which is just a crystalline form of the parent rock. Check out Marble Mountain, in northern California, when you reach it.

Geologic Time

You cannot develop a feeling for geology unless you appreciate the great span of time that geologic processes have had to operate over. A few million years’ duration is little more than an instant on the vast geologic time scale (see following Geologic Time Scale). Within this duration a volcano may be born, die and erode away. Dozens of major “ice ages” may come and go.

A mountain range takes longer to form. Granitic plutons of the Sierra Nevada first came into being about 240 million years ago, and intrusion of them continued until about 80 million years ago, a span of 160 million years. Usually there is a considerable gap in the geologic record between the granitic rocks and the older sediments and volcanics that they intrude and metamorphose—often more than 100 million years.

Geologic History

With the aid of a geologic section, like the one above, we can reconstruct in part the geologic history of an area. Our geologic section represents an idealized slice across the Sierra Nevada to reveal the rocks and their relations.

Through dating methods that use radioactive materials, geologists can obtain the absolute ages of the two granitic plutons, the andesite flow, and the basalt flow, which respectively would likely be Cretaceous, Pliocene, and Holocene. The overlying, folded sediments intruded by the plutons would have to be pre-Cretaceous. The metabasalt could be dated, but the age arrived at may be for the time of its metamorphism rather than for its formation. A paleontologist examining fossils from the marble and slate might conclude that these rocks are from the Paleozoic era.

Before metamorphism the Paleozoic slate, quartzite, metabasalt, and marble would have been shale, sandstone, basalt, and limestone respectively. The shale–sandstone sequence might indicate marine sediments being deposited on a continental shelf, then on a coastal plain. Lack of transitional rocks between the shale and the sandstone leads us to conclude that they were eroded away, creating a gap in the geologic record. We then have an unconformity between the two strata (layers), the upper resting on the erosional surface of the lower. The basalt, shale, and limestone sequence indicates first a localized volcanism, followed by a marine and then a shallow-water environment.

These Paleozoic rocks remained buried and protected from erosion for millions of years until the intrusion of granitic plutons and associated regional volcanism. Radiometric dating would show that the quartz-monzonite pluton was emplaced before the granodiorite pluton. Field observations would verify this sequence because the latter intrudes the former as well as the overlying sediments. During the Mesozoic period, plutonism and volcanism were at times accompanied by mountain building. This occurred when large pieces of continental crust, which were riding atop a plate that generally was diving eastward beneath the edge of the continent, were transported toward the range. Being relatively low in density, this continental crust did not descend with the rest of the plate, and so was forced against the range. The resulting compression caused uplift, and the Paleozoic rocks became folded, metamorphosed, and often faulted. Until plutonism ceased about 80 million years ago, the Mesozoic Sierra Nevada was just a small part of a much longer range that extended continuously along the western coasts of North America and South America. The climate was mostly tropical, and both weathering and erosion were intense; so as uplift occurred, these processes removed much of the Paleozoic rocks.

Geologic Time Scale

Light Marble Mountain and dark Black Mountain, Section Q

After plutonism ceased in California, late Cretaceous through early Tertiary faulting broke up the longer range and the Sierra Nevada became separated from the Klamath Mountains on the north, and the Coast, Transverse, and Peninsular ranges on the south. (This, and much that follows, cannot be deduced from the geologic section.) Before the breakup, the longer range was high, similar to today’s Andes, but with the faulting into smaller blocks there also was detachment faulting—the separation of upper crust from lower crust. This occurred when the lower continental crust, under tremendous pressure from the thick, overlying upper crust and from high heat flow below, started to flow laterally. The upper continental crust lacked sufficient heat and pressure to flow. Rather, this brittle layer detached at its base and was transported laterally, atop the flowing lower continental crust. Where the upper several miles of Sierran proper crust went is not yet known. In the southern Sierra, most of the upper crust was transported westward. Then, when the San Andreas fault system developed, it was transported northwest, slivering into linear blocks in the process.

With the upper crust removed—more than 65 million years ago for most of the Sierra—the unburdened lower crust rose to heights that probably were a bit higher than today’s. In the ensuing millions of years, broad summits such as Mt. Whitney’s have been reduced through weathering and erosion by only a few hundred feet, if that. Back in those early days following detachment, the range already had achieved a largely granitic landscape, since most of the exposed lower crust was granitic. Because stepped topography develops in granitic rocks, it would have begun generating cliffs and benches as well as streams, with almost level reaches alternating with rapids, cascades, and even falls. Like the Sierra Nevada, the Peninsular Ranges and the Klamath Mountains in the northernmost sections of the PCT had also experienced a similar postplutonic history of uplift and erosion to expose their lower continental crusts. This also may have been true for the eastern and central parts of the Transverse Ranges, but they have been so disrupted by faulting, especially over the last 30 million years, that some additional uplift probably has occurred.

Thirty million years ago was an important time. Roughly about then the climate began changing from one that was somewhat tropical to one that was drier and more seasonal. In the northern half of the Sierra Nevada the range was in part buried by extensive rhyolite-ash deposits. Furthermore, the San Andreas fault system was born, west of the modern coast of southern California. By 15 million years ago, California had acquired an essentially modern summer-dry climate; the northern half of the Sierra Nevada was buried under even larger amounts of andesitic deposits (burying the old, granitic river canyons); and the fault system was beginning to migrate eastward onto existing lands, thereby disrupting them. As today, lands west of any fault segment moved northward with respect to those on the east (right-lateral faulting). Also by 15 million years ago, the composite Sierra Nevada–Central Valley block had begun migrating from its location near the southwestern Nevada border, first west, then northwest, some 150–180 miles to its present location. Today on a very clear day, from Mt. Whitney’s summit you can see granitic Junipero Serra Peak, highest summit of central California’s outer coast ranges, about 170 miles west. Likewise, back then from the same summit, on a very clear day you could have seen the Grand Canyon plateau (no canyon yet), a similar distance east.

Most of the volcanic deposits in the northern Sierra Nevada were readily eroded, but the new canyons cut in such deposits were inundated by additional sediments. About 10–9 million years ago several massive outpourings of lava flowed westward from faults near the present Sierran crest. These faults were created by extension of the Great Basin lands, which before widespread down-faulting had been a rugged, mountainous highland. The floor of the Owens Valley sank, but the already high Sierra Nevada did not rise; the opposite-direction arrows along the fault in the idealized geologic section indicate only relative movement, not absolute up or down. Note that the fault cuts the bedrock but not the lateral moraine (an accumulation of debris dropped off the side of a glacier), and this indicates that no faulting has occurred since the moraine was deposited (or else it too would have been disrupted).

Significant parts of these lava flows still remain, and the remnants best preserved are those that lie directly atop old bedrock, as does the remnant of an andesite flow in the idealized geologic section. Such remnants stand high above the floor of today’s granite-walled canyons, which had been mostly exhumed of volcanic deposits before glaciation commenced. From this relation, geologists have concluded—incorrectly in my opinion—that major postflow uplift raised the flows to their present high positions, and that the steepened rivers then cut through thousands of feet of granite to their present low positions. According to this view, glaciers aided in the excavation, but misinterpretation of the field evidence has led geologists to infer major glacial erosion in some canyons, such as Yosemite Valley, and very little in others, such as the Grand Canyon of the Tuolumne River—two adjacent drainages both in Yosemite National Park.

The Sierra Nevada first experienced major glaciation about two million years ago, although it could have had episodes of minor glaciation long before that. These first large glaciers eroded the layer of rough, fractured, weathered bedrock, then retreated to leave behind much smoother surfaces. Where the bedrock floor was highly fractured and/or deeply weathered (in Yosemite Valley, the most extreme example, tropical weathering had penetrated some 2000 feet down), glaciers could excavate quite effectively, leaving behind bedrock basins that quickly filled with water each time the glaciers retreated, creating a bedrock lake, or tarn. (In some canyons a lake formed behind a terminal moraine, although such a lake exists not so much because of a moraine dam, but rather because of impervious bedrock that is buried by the moraine.) On the resistant, smoothed and polished bedrock, succeeding glaciers could do very little, despite a century of claims by glaciologists.

Some evidence for a lack of major glacial erosion lies along or close to the PCT, much of it in the southern half of the PCT. This is described in greater detail in Pacific Crest Trail: Southern California.

In the Sonora Pass to Echo Lake Resort section of the northern PCT, a descent from the Wolf Creek Lake saddle north takes you into the deep, glaciated East Fork Carson River canyon. After about two trail miles from there, and before you cross the river’s second tributary, there are remnants of volcanic deposits on the west slopes that descend to within 200 feet of the canyon floor. These remnants are dated at about 20 million years old, indicating that back then—before any supposed uplift and before any glaciation—the canyon was about as wide as it is today and almost as deep.

Mount Shasta, a volcano, rises beyond Bull Lake and Mt. Eddy, Section P

Cinder Cone lying 3.5 miles northeast of Lower Twin Lake, Section N

Returning to the idealized geologic section, we see both a lateral and a terminal moraine on the east side of the crest, these usually being massive deposits left by a former glacier. (However, some lateral moraines are thin, merely a veneer atop an underlying bedrock ridge.) If glaciers do not erode, then why are moraines so large? Rockfall is the answer. It can occur at any time, but it is especially prevalent in late winter and early spring (due to cycles of freeze and thaw that pry off slabs and blocks). During and after a major earthquake, a tremendous amount of rockfall occurs, as noted in the 1980 Mammoth Lakes earthquake swarm, which was centered near the town along the east base of the range. Rockfall was greatest along and east of the crest, and so perhaps it is good that the PCT lies a few miles west of it. The greatest amount of local rockfall along the PCT route was from the ragged southeast face of Peak 11787, north of Purple Lake in the southern California section (Map H16). What glaciers do best is haul out a lot of rockfall, from which moraines are constructed and with which rivers are choked. Over the last two million years there were 2–4 dozen cycles of major glacier growth and retreat, and the glaciers transported a lot of rockfall. At the head of each canyon, where physical weathering was extremely pronounced, there usually developed a steep-walled half-bowl called a cirque. Before glaciation these already existed in a less dramatic form, as can be seen in the unglaciated lands west of Rockhouse Basin (Section G).

In the idealized geologic section, the last significant change was the eruption of lava to produce a cinder cone, which partly overlapped the terminal moraine, thereby indicating that it is younger. A basalt flow emanated from the cinder cone during or immediately after its formation. A carbon-14 date on wood buried by the flow would verify the youthfulness of the flow. Weathering and erosion are oh-so-slowly attacking the range today, at a rate much slower than in its tropical past, but nevertheless they are seeking to reduce the landscape to sea level. This will not occur. Future PCT hikers in the distant geologic future can expect a higher range, for eventually the Coast Ranges of central California should be thrust across the Great Central Valley and onto the Sierra Nevada, the crust-crust compression generating a new round of mountain building.

For now, PCT hikers can study the existing landscape. When you encounter a contact between two rocks along the trail, you might ask yourself: Which rock is younger? Which older? Has faulting, folding, or metamorphism occurred? Is there a gap in the geologic record?

Biology

One’s first guess about hiking the Pacific Crest Trail—a high adventure rich in magnificent alpine scenery and sweeping panoramas—turns out to be incorrect along some parts of the trail. The real-life trail hike will sometimes seem to consist of enduring many repetitious miles of hot, dusty tread, battling hordes of mosquitoes, or slogging up seemingly endless switchbacks. If you find yourself bogged down in such unpleasant impressions, it may be because you haven’t developed an appreciation of the natural history of this remarkable route. As there is a great variety of minerals, rocks, landscapes and climates along the PCT, so also is there a great variety of plants and animals.

Even if you don’t know much about basic ecology, you can’t help noticing that the natural scene along the Pacific Crest Trail changes with elevation. The most obvious changes are in the trees, just because trees are the most obvious—the largest—organisms. Furthermore, they don’t move around, hike, or migrate in their lifetimes, as do animals. When you pay close attention, you notice that not only the trees but the shrubs and wildflowers also change with elevation. Then you begin to find latitudinal differences in the animal populations. In other words, there are different life zones.

Life zones

In 1894 C. Hart Merriam divided North America into seven broad ecosystems, which he called “life zones.” These zones were originally based primarily on temperature, though today they are based on the distribution of plants and animals. The zones correspond roughly with latitude, from the Tropical Zone, which stretches from Florida across Mexico, to the Arctic Zone, which includes the polar regions. Between these two are found, south to north, the Lower Sonoran, Upper Sonoran, Transition, Canadian and Hudsonian zones. All but the Tropical Zone are encountered along the California sections of the PCT.

Just as temperature decreases as you move toward the earth’s poles, so too does it decrease as you climb upward—between 3° and 5.5°F for every 1000-foot elevation gain. Thus, if you were to climb from broad San Gorgonio Pass for 10,000 feet up to the summit of San Gorgonio Mountain, you would pass through all the same zones that you would if you walked from southern California north all the way to Alaska. It turns out that 1000 feet of elevation are about equivalent to 170 miles of latitude. Although the California PCT is about 1600 miles long, the net northward gain in latitude is only about 650 miles—you have to hike 2.5 route-miles to get one mile north. This 650-mile change in latitude should bring about the same temperature change as climbing 3800 feet up a mountain. On the PCT you enter Oregon at a 6000-foot elevation, finding yourself in a dense, Canadian Zone pine-and-fir forest. Doing your arithmetic, you would expect to find an equally dense fir forest at the Mexican border 3800 feet higher—at a 9800-foot elevation. Unfortunately, no such elevation exists along the border to test this prediction. However, if we head 85 miles north from the border to the Mt. San Jacinto environs, and subtract 500 feet in elevation to compensate for this new latitude, what do we find at the 9300-foot elevation? You guessed it, a Canadian Zone pine-and-fir forest. Ah, but nature is not quite that simple, for the two forests are unmistakably different.

Lodgepole pines at Boulder Lake, Section J

Plant geography

Every plant (and every animal) has its own range, habitat and niche. Some species have a very restricted range; others, a very widespread one. The sequoia, for example, occurs only in about 75 groves at mid-elevations in the western Sierra Nevada. It flourishes in a habitat of tall conifers growing on shaded, gentle, well-drained slopes. Its niche—its role in the community—consists of its complex interaction with its environment and every other species in its environment. Dozens of insects utilize the sequoia’s needles and cones, and additional organisms thrive in its surrounding soil. The woolly sunflower, on the other hand, has a tremendous range: from California north to British Columbia and east to the Rocky Mountains. It can be found in brushy habitats from near sea level up to 10,000 feet.

Some species, evidently, can adapt to environments and competitors better than others. Nevertheless, each is restricted by a complex interplay of climatic, physiographic (topography), edaphic (soil) and biotic influences.

Climatic influences

Of all climatic influences, temperature and precipitation are probably the most important. Although the mean temperature tends to increase toward the equator, this pattern is camouflaged in California by the dominating effect of the state’s highly varied topography. As was mentioned earlier, the temperature decreases between 3° and 5.5°F for every 1000-foot gain in elevation. The vegetational changes reflect this cooling trend. For example, the vegetation along San Gorgonio Pass in southern California is adapted to its desert environment. Annuals are very ephemeral; after heavy rains, they quickly grow, blossom and die. Perennials are succulent or woody, have deep roots, and have small, hard or waxy leaves—or no leaves at all. Only the lush cottonwoods and other associated species along the dry streambeds hint at a source of water.

As you climb north up the slopes of San Gorgonio Mountain, not only does the temperature drop, but the annual precipitation increases. On the gravelly desert floor below, only a sparse, drought-adapted vegetation survives the searing summer temperatures and the miserly 10 inches of precipitation. A doubled precipitation on the mountainside allows growth of chaparral, here a thick stand of ocean spray, birchleaf mountain mahogany, Gregg’s ceanothus and great-berried manzanita. By 7000 feet the precipitation has increased to 40 inches, and the moisture-loving conifers—first Jeffrey pine, then lodgepole pine and white fir—predominate. As the temperature steadily decreases with elevation, evaporation of soil water and transpiration of moisture from plant needles and leaves are both reduced. Furthermore, up here the precipitation may be in the form of snow, which is preserved for months by the shade of the forest, and even when it melts is retained by the highly absorbent humus (decayed organic matter) of the forest soil. Consequently, an inch of precipitation on the higher slopes is far more effective than an inch on the exposed, gravelly desert floor. Similar vegetation changes can be found wherever you make dramatic ascents or descents. In northern California significant elevation and vegetation changes occur as you descend to and then ascend from Highway 70 at Belden, Interstate 5 at Castle Crags State Park, and Highway 96 at Seiad Valley.

Physiographic influences

As we have seen, the elevation largely governs the regime of temperature and precipitation. For a given elevation, the mean maximum temperature in northern California is about 10°F less than that of the San Bernardino area. Annual precipitation, however, is considerably more; it ranges from about 20 inches in the Sacramento Valley to 80 inches along the higher slopes, where the snowpack may last well into summer. When you climb out of a canyon in the Feather River country, you start among live oak, poison oak and California laurel, and ascend through successive stands of Douglas-fir and black oak, incense cedar and ponderosa pine, white fir and sugar pine, then finally red fir, lodgepole, and western white pine.

The country near the Oregon border is one of lower elevations and greater precipitation, which produces a wetter-but-milder climate that is reflected in the distribution of plant species. Seiad Valley is hemmed in by forests of Douglas-fir, tanbark-oak, madrone, and canyon live oak. When you reach Cook and Green Pass (4750’) you reach a forest of white fir and noble fir. To the east, at higher elevations, you encounter weeping spruce.

A low minimum temperature, like a high maximum one, can determine where a plant species lives, since freezing temperatures can kill poorly adapted plants by causing ice crystals to form in their cells. At high elevations, the gnarled, grotesque trunks of the whitebark, limber, and foxtail pines give stark testimony to their battle against the elements. The wind-cropped, short-needled foliage is sparse at best, for the growing season lasts but two months, and a killing frost is possible in every month. Samples of this subalpine forest are found on the upper slopes of the higher peaks in the San Jacinto, San Bernardino, and San Gabriel mountains and along much of the John Muir Trail. Along or near the High Sierra crest and on the highest southern California summits, all vestiges of forest surrender to rocky, barren slopes pioneered only by the most stalwart perennials, such as alpine willow and alpine buttercup.

Other physiographic influences are the location, steepness, orientation, and shape of slopes. North-facing slopes are cooler and tend to be wetter than south-facing slopes. Hence on north-facing slopes, you’ll encounter red-fir forests which at the ridgeline abruptly give way to a dense cover of manzanita and ceanothus on south-facing slopes. Extremely steep slopes may never develop a deep soil or support a coniferous forest, and of course cliffs will be devoid of vegetation other than crustose lichens, secluded mosses, scattered annuals, and a few drought-resistant shrubs and trees.

Edaphic influences

Along the northern part of your trek, at the headwaters of the Trinity River and just below Seiad Valley, you’ll encounter outcrops of serpentine, California’s official state rock. (Technically, the rock is serpentinite, and it is composed almost entirely of the mineral serpentine, but even geologists use “serpentine” for the rock.) This rock weathers to form a soil poor in some vital plant nutrients but rich in certain undesirable heavy metals. Nevertheless, there are numerous species, such as leather oak, that are specifically or generally associated with serpentine-derived oils. There is a species of streptanthus (mustard family) found only on this soil, even though it could grow better on other soils. Experiments demonstrate that it cannot withstand the competition of other plants growing on these soils. It therefore struggles, yet propagates, within its protected environment. Another example is at Marble Mountain, also in northern California, which has a local assemblage of plants that have adapted to the mountain’s limey soil.

A soil can change over time and with it, the vegetation. An illuminating example is found in formerly glaciated Sierran lands, where young soils today are thin and poor in both nutrients and humus. However, with passing millennia they will evolve into more-mature soils, and eventually could, given enough time, support sequoias up in the red-fir zone. These trees likely grew mostly in that zone, but glaciers removed the soils, so the trees that manage to survive today do so in the lower, unglaciated lands, that is, mostly down with the white firs and sugar pines. Once glaciation ceases in the Sierra Nevada, which could be a few million years away, the sequoias could recolonize the lands they lost some two or more million years ago.

Biotic influences

In an arid environment, plants competing for water may evolve special mechanisms besides their water-retaining mechanisms. The creosote bush, for example, in an effort to preserve its limited supply of water, secretes toxins which prevent nearby seeds from germinating. The result is an economical spacing of bushes along the desert floor.

Competition is manifold everywhere. On a descending trek past a string of alpine lakes, you might see several stages of plant succession. The highest lake may be pristine, bordered only by tufts of sedges between the lichen-crusted rocks. A lower lake may exhibit an invasion of grasses, sedges and pondweeds thriving on the sediments deposited at its inlet. Corn lilies and Lemmon’s willows border its edge. Farther down, a wet meadow may be the remnant of a former shallow lake. Water birch and lodgepole pine then make their debut. Finally, you reach the last lake bed, recognized only by the flatness of the forest floor and a few boulders of a recessional moraine (glacial deposit) that dammed the lake. In this location, a thick stand of white fir has overshadowed and eliminated much of the underlying lodgepole. Be aware, however, that lake-meadow-forest succession is very slow, the lakes being filled with sediments at an average rate of about one foot per thousand years. At this rate, about 20– 30,000 years will be required to fill in most of the lakes, and Tenaya Lake, between Tuolumne Meadows and Yosemite Valley, will take over 100,000 years. However, barring significant man-induced atmospheric warming, California’s climate should cool in a few thousand years, and another round of glaciation should commence.

When a species becomes too extensive, it invites attack. The large, pure stand of lodgepole pine near Tuolumne Meadows has for years been under an unrelenting attack by a moth known as the lodgepole needle-miner. One of the hazards of a pure stand of one species is the inherent instability of the system. Within well-mixed forest, lodgepoles are scattered and the needle-miner is not much of a problem. But species need not always compete. Sometime two species cooperate for the mutual benefit, if not the actual existence, of both.

Nearly all the plants you’ll encounter have roots that form a symbiotic relationship with fungi. These mycorrhizal fungi greatly increase the roots’ efficiency of water and nutrient uptake, and the roots provide the fungi with some of the plants’ photosynthesized simple sugars.