Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

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.