Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

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      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 СКАЧАТЬ