Nineteenth-century explorers of the Sierra Nevada found a finely tuned biological world, responding to the changing conditions of the post Ice Age period. As the ice melted, plant and animal populations shifted in response to the warming climate. During the Ice Age numerous cold-tolerant plants and animals colonized California. As the weather warmed, these organisms found their ranges increasingly restricted. At the same time, the shift towards warmer, drier weather increased habitat for many plants and animals of meso-American origin. In recent millennia, northern plants have been confined mostly to the upper altitude portions of the southern Sierra; at lower altitudes, more drought-tolerant species have established themselves.

Regardless of specific locations, nearly all the organisms which survive in the Sierra have had to accommodate some inescapable truths. Exact climate data for the southern Sierra is largely limited to the past fifty years, but even so, it is complete enough to give some idea of the patterns that have prevailed over the past 10,000 years. Temperature variation, for example, is a predictable function of altitude: every thousand feet increase in altitude usually results in a drop of three to five degrees Fahrenheit. The implications of this fact on a mountain range with 14,000 feet of altitude variation are easily appreciated. On an average day, the summits of the highest peaks will be 42 to 70 degrees colder than the western base of the mountains. Variation in precipitation is nearly as predictable. During the past fifty years, average annual values for the region have varied from about ten inches of precipitation in Visalia and Fresno, to a probable sixty inches of annual precipitation at the 9,000-foot level, an increase of 600 percent. These values undoubtedly have varied considerably over the past few thousand years, but the relationship between them probably has remained relatively stable. Another critical factor in any understanding of the Sierran climate in recent times is the Mediterranean cycle of precipitation. Under this pattern, which seems to have applied to the Sierra since the end of the Ice Age, precipitation comes mostly during the winter months, at a time when many plants are unable to use the moisture due to cold and lack of sunlight. A necessary corollary is summer drought, another inescapable fact of life (or death) for most Sierran life forms.

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In an endless game of experimentation and testing, plants and the animals that depend upon them have found ways to survive and even prosper in the face of these challenges. Up to about 5,000 feet in altitude, mild, sometimes rainy winters and intense summer drought have been the pattern for the past few thousand years. In this world, vegetation is oak-dominated broadleaf forest or dense brush—a plant community usually called chaparral. Above 5,000 feet, not only has precipitation increased to an average of about thirty-five to forty inches, but also a critical winter temperature threshold has been reached. Somewhere around 5,000 feet winter rains begin to freeze into snow. At 7,000 feet the southern Sierra snowfall average has been over 250 inches annually during the twentieth century. At 9,000 feet the total is probably 30 percent greater.

Plants must reflect these changes in temperature and precipitation, and they do. Between 5,000 and 9,000 feet the southern Sierra Nevada is clothed with an extensive conifer "snow forest." Nearly a dozen species of tall, narrow, evergreen trees grow together in this zone. The annual pressure of heavy snowfall makes the pines, firs and cedars similar in superficial appearance, but each is uniquely adapted to fill a slightly different ecological niche. Many of these species grow quite large, with six-feet-thick, 200-feet-tall trees common. Here grow the giant sequoias, with the largest specimens exceeding 300 feet in height and 30 feet in basal diameter! Scattered through the forest, where soils are too wet for trees, are lush wetlands or meadows.

Above 9,000 feet a third natural world appears. Here winters are long and cold, spring snowmelt is slow, and summers are cool and short. Life is difficult here for many reasons and the forest shows it. Tree growth tends to be patchy and sparse, and above 11,000 feet there usually is no forest, only rock and small, ground-hugging alpine perennials. This is tundra, a land of limited biological productivity and of limited ability to recover when damaged.

At the foot of the mountains, on the floor of the great valley to the west, was a fourth world, now largely superseded by intense human activity. Much of the valley floor was a dry, seasonal grassland which blended seamlessly into the spare oak savannas of the lowest foothills. But in some areas of the valley floor, mostly along rivers, were better watered freshwater marshland and deciduous forest. The richest of these were the lush deltas of the Kings and Kaweah rivers. Both streams supported extensive riparian mazes that began as soon as the rivers entered the valley floor. Ultimately both streams, together with the smaller Tule River and even part of the Kern, terminated in a giant freshwater marsh known as Tulare Lake. With a thousand square miles of wetlands, Tulare Lake was an enormously productive biological factory, supporting millions of birds and fish. [3]