Traveling Through the Life Zones of the Central Sierra Nevada
by Mary Ann Resendes
California has the greatest natural diversity of any area of equal size in North America. The diversity of California’s flora and fauna is due to the variation in topography and its influence on the climate. The Sierra Nevada is an excellent example of this diversity because the drastic increase in altitude of the Sierra Nevada creates a wide range of life zones within a short distance. As you drive into the Sierra Nevada, you will notice that the landscape changes as the elevation increases. This is due to adiabatic cooling, the decrease in temperature as elevation increases. In the Sierra Nevada, temperature decreases about 3 to 5 degrees F per 1000 feet. As an air mass moves east across the Central Valley and rises over the western slope of the Sierra Nevada, the air becomes chilled and releases moisture in the form of rain or snow. By the time the air mass reaches the crest much of the moisture has been lost, leaving little or no precipitation for the eastern slope of the Sierra Nevada. This phenomenon, called the rain shadow effect, is responsible for the many life zones from the valley grasslands that rise up into the oak woodland and chaparral, to the yellow pine forest, the snowy lodgepole-red fir forests, and finally, the windblown environment of the subalpine.
Foothill Woodland /Upper Sonoran Beyond the patchwork of agricultural fields in the Central Valley lie the rolling hills of the foothill woodland. This life zone ranges from 300 to 5000 feet. Before the arrival of European settlers, the hillsides were covered with both annual and perennial native grasses. However, non-native seeds were unintentionally transported via fur or droppings of imported cattle, in hay, or mixed in with crop seeds. The seeds slowly made their way into soils surrounding the settlements, then spread into the woodlands where they currently make up 50 to 90 percent of the ground cover. These grassy hills are dotted with blue oak, Quercus douglasii, interior live oak, Quercus wislizenii, and gray pine, Pinus sabiniana. Blue oak stands out from other oaks because of its blue-green foliage. This oak is able to survive on dry, rocky slopes due to several mechanisms that increase the oak’s tolerance to its hot and dry environment. For example, under severe conditions, Blue Oaks will drop their leaves until the next spring. Like the Blue Oak, the evergreen Interior Live Oak has mechanisms that allow it to exist in a hot dry environment. The Grey Pine is only conifer that is commonly found throughout the foothill woodland. It is easy to recognize because of its gray-green needles, and, unlike most other conifers, the Gray Pine divides into several trunks. Chaparral In some areas of the foothill woodland, the vegetation becomes dense and shrubby, a characteristic of chaparral. Here vegetation has developed adaptations to drought and fire. Many plants have scelorpytic, or thick waxy leaves that resist water loss. The California buckeye’s, Aesculus californica, strategy to survive drought is to put out big, lush leaves and clusters of white flowers early in the season. It then drops its leaves midsummer, leaving seedpods dangling from bare branches. Other plants include manzanita shrubs, Archtostaphylos spp., which has light green leaves with wavy, wine red branches, which can also be found in the Yellow Pine and Lodgepole-Red Fir Forest. Fire is an important component of this environment, and intense fires are common. Some plants adapt by crown-sprouting or sprouting roots from a belowground burl when the rest of the shrub has been consumed by fire. Other plants need fire in order for their seeds to germinate. The Acorn Woodpecker, Melanerpes formicivorus, is one of the more interesting animals found in both the foothill woodland and the yellow pine forest. The woodpeckers store acorns by drilling holes in decayed parts of living oaks, dead trees, and fence posts. Other animals found in the Oak woodland include the Western Rattlesmake, Crotalis viridis, found up to the lodgepole-red fir zone, Common King Snake, Lampropeltis getulus, and the California Ground Squirrel Citellus beecheyi. Mule Deer, Odocoileus hemionus, migrate along the western slope of the Sierra Nevada, from the foothills to the higher forests. The boundaries between the life zones are not distinct, and plants and animals can be found in more than one life zone. Yellow Pine Forest / Transition Forest As the elevation increases to 2500 feet, road begins to wind through the shade of tall pines trees. Two of the most common pine trees, Ponderosa, Pinus ponderosa, and Jeffrey pines, Pinus jeffreyi, are almost indistinguishable—both trees grow very tall, 60 to 130 feet in height, and both have evergreen needles. Ponderosa pine needles grow from 4 to 8 inches in bundles of two to five, while Jeffrey pine needles grow three in a bundle and 5 to 10 inches long. One way to tell them apart is to remember prickly ponderosa and gentle Jeffrey. Ponderosa cone have prickly spines, while Jeffrey pines do not. Another way is to smell the bark—Jeffrey pine bark smells like vanilla. Jeffrey Pine often grow on rocky outcrops. Giant sequoia, Sequoiadendron giganteum, is the most magnificent of the Yellow Pine Forest and is found in seven groves in the Sierra Nevada. Most of these trees are found in Sequoia and Kings Canyon National Parks. Other trees in this life zone include the aromatic Incense Cedar, Calcedrus decurrens and Sugar Pine, Pinus lambertianna. Douglas Fir, Pseudotsuga menzieii, grow in moist pockets of the forest, and Black Oaks, Quercus kellogii, are scattered among the conifers on open slopes and dry ridges. The Western Gray Squirrel, Sciurus griseus, can be seen scurrying along the branches of the Black Oaks. These squirrels remain active through the winter, surviving on food that they have buried in the forest floor. Black Bears, Ursus americanus, also inhabit this forest. The Black Bear is the only bear that lives in the Sierra Nevada; Grizzlies are no longer found there. Birds include the Black-headed Grossbeak, Pheucticus melanocephalus, the Dark-eyed Junco, Junco hyemalis, and the Stellar Jay, Cyanositta stelleri. Lodgepole-Redfir Forest / Upper Montane Forest / Canadian Life Zone The lodgepole-red fir forest receives the most snowfall of all of the life zones in the Sierra Nevada. At this altitude, 5500 to 7500 feet, much of the moisture in the air drops out, producing heavy snow falls that range from 35 to 65 inches per year. Red Fir, Abies magnifica, is also known as the “Silver Tip” Christmas tree because of the silver appearance of the new needles. This species is named for its red bark, which is deeply furrowed into narrow ridges. While lodgpole pine, Pinus contorta, can be found from mixed conifer forest to the lower subalpine zone, it is found in greater abundance in the Red Fir-Lodgepole pine forest. At slightly higher elevation and into the subalpine zone, you will see a rather striking tree, the Sierra Juniper, Juniperus occidentalis. In response to the harsh climate of this higher elevation, the sturdy trunk grows gnarled and twisted over time. The cinnamon colored, deeply grooved bark adds to the unique features of the tree. The Sierra Juniper grows on rocky outcrops, snaking its roots through crevices in the granitic rock. If you walk among the Red Firs and Lodgepole Pines, you will likely hear the loud chattering of the Chickarees, Tamiasciurus douglasii. The Godlen-mantled Ground Squirrel, Spermophilus lateralis, and species of chipmunk, Tamius spp, make their home here as well. Birds found in the forest include the Hermit Thrush, Catharus gutttatus, and the Blue Grouse, Dendragapus obscurus. The Great Gray Owl, Strix nebulosa, the largest Sierran owl, is found mainly in this zone. Subalpine Forest / Hudsonian Life Zone Subalpine forests occur at elevations of 8,000 to 10,000 feet, and are the highest point at which trees are found. Harsh environmental conditions limit the types of vegetation that can survive in this zone. While the subapline may offer spectacular views to the passing observer, the inhabitants must struggle with a rugged landscape that is shaped by fierce winds and cold temperatures. These conditions result in interesting adaptations. Some trees like Whitebark Pine, Pinus albicaulis, whose forms have been bent and twisted by the brutal wind appear to be struggling up the mountainside. This tree will develop a number of growth forms depending on environmental conditions. Its ability to grow horizontally allows it to survive in windy conditions where most other trees cannot. It can also grow as a straight single-trunk tree, a stunted multi-stem tree, or as a shrub. Trees may also exhibit flagging, a condition in which branches grow only in one direction. Branches on the windward side of the trees are not able to survive the force of the wind due to abrasion by wind-borne particles that can tear at the cuticle on conifer needles. Some animals from the Lodgepole Pine-Red Fir Forest, like the Gold-mantled Ground Squirrel venture up into the subalpine zone, and species of chipmunks such as the Alpine Chipmunck, Tamias alpinus are also found in this zone. Clark’s Nutcrackers spend the milder months in this zone, and spend the winter months in lower elevations. Other birds such as the Hermit Thrusch, the American Robin, Turdus migratoris, and Mountain Bluebirds, Silia currucoides, also spent the milder seasons here. Bibliography Arno SF. 1973. Discovering Sierra Trees. Yosemite Association and Sequoia Natural History Association in Cooperation with the National Park Service. Johnston VR. 1994. California Forests and Woodlands. Berkeley: University of California Press. Johnston VR. 1998. Sierra Nevada. The Naturalist’s Companion. Berkeley: University of California Press. Little EL. 1998. National Audubon Society. Field Guide to Trees Western Region. New York: Chanticleer Press Ornduff R. 1974. Introduction to Plant Life. Berkeley: University of California Press. Schoenherr AA. 1995. A Natural History of California. Berkeley: University of California Press. Storer TI & RL Usinger. 1963. Sierra Nevada Natural History. Berkeley: University of California Press.
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