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,