The Natural History of the North Fork of the American River

The plants and animals of the North Fork American and its surrounding upland regions are much like those of the Sierra Nevada at large. The Mediterranean climate of California, with mild wet winters and warm dry summers, is modified by the Sierra in many ways. The prevailing westerly winds bring moist martime air masses across California from the Pacific, and these are forced to rise while crossing the Sierra. On an average, the lapse rate of an air mass (the rate at which temperature diminishes with increased elevation) is 3.5 degrees Fahenheit per 1000 feet of elevation. Thus, as the air masses cross the Sierra, they are chilled, perhaps, chilled below the dew point, at which ratio of temperatur and humidity, water contained in the air becomes saturated and condenses into droplets. A cloud results, or a vast army of clouds, stretching up and down the range. Let the air contain yet more water, or be chilled even more, and actual precipitation results.

On an average, precipitation increases about one inch per year per 100 feet of elevation. Suppose that Sacramento, say, at elevation 52 feet, receives 18 inches of precipitation per year. Then a location of elevation of 1052 feet in the foothills will likely receive 28 inches per year; at 2052 feet, 38 inches, and so on, until at about 6052 feet, and 78 inches of precipitation per year, a maximum is reached, and annual precipitation decreases slightly at higher elevations, as one approaches the crest. Above 5000 feet, most of the precipitation falls as snow.

East of the Sierra crest, the climate changes abruptly; the air masses have had much of their moisture wrung from them already, and as they descend, adiabtic warming tends to suppress further precipitation. The climate is much more continental, and less maritime, than on the west slope of the Sierra. There are greater extremes of temperature, notably, extremely cold low temperatures are common in the desert basins east of the Sierra. Bridgeport once recorded 56 degrees below zero.

In a crude way, we can say, big water, big trees. And in a similar vein, vegetation follows climate, and wildlife follows vegetation. As we proceed from the Central Valley up the west slope of the Sierra, it is much as if we were traveling north from San Diego to Oregon, Washington, and on up to the treeless tundras of Arctic regions. This is an extremely fruitful analogy, one which was codified quite a while ago, in what is called the Merriam system of ìlife zonesî of the Sierra. It has fallen out of favor in recent years, as more complicated systems have come to the fore. The Merriam system remains my own favorite.

The life zones are named in part after geographic regions: Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine. Each life zone exhibits a characteristic mix of trees, shrubs, and wildlife. The life zones should not be taken as hard and fast and absolute indicators of plant and animal life. however; in the Sierra, microclimate often overtakes climate, in its effects upon vegetation.

More when time permits!

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