The Sierra Nevada run NW to SW and are approximately 400 miles long and 50 mile wide. The range is highest towards the south with several peaks over 14,000 ft. Several large river vallies dissect the western slope with steep canyons. The eastern escarpment is much steeper than the western slope, in general. The range supports a diverse set of natural communities with many endemic species and extraordinary habitats.
The Sierra Nevada ecoregion harbors one of the most diverse temperate conifer forests on Earth displaying an extraordinary range of habitat types and supporting many unusual species. Fifty percent of California s estimated 7,000 species of vascular plants occur in the Sierra Nevada, with 400 Sierra endemics and 200 rare species (CWWR 1996). The southern region [section] has the highest concentration of species and rare and endemic species, but pockets of rare plants occur throughout the range. The eastern slope west of the Owens River valley is also noted for many unusual plant species. Geographic patterns of plant diversity are complex, with the species compositions of communities changing dramatically with altitude and between watersheds along a north-south gradient (i.e., high beta diversity).
Different communities are distributed in elevational belts on both sides of the range, with counterpart communities occurring at higher elevation on the eastern slope. Above the chaparral and foothill woodlands on the west slope of the Sierra Nevada, part of the Interior chaparral and woodland ecoregion, occurs a series of forests at montane and subalpine elevations. The montane elevation is itself sorted into several extensive forest types. First is the ponderosa pine forest which replaces chaparral and woodland, then the mixed conifer forest dominates. Here five conifers, ponderosa pine, sugar pine, Douglas-fir, and white fir mix on the most productive soils in the Sierra. Sugar pine is the worlds tallest and largest pine species of this forest type. Some seventy-five groves of giant sequoia (Sequoiadendron giganteum) exit within the mixed conifer forest. Giant sequoias are the most massive trees on Earth, with some trees 273 ft tall, over 36 ft in diameter, and over 3,200 years old. Sequoias and many other tree, shrub, and herbaceous plant species tolerate frequent fires and some require fire for their regeneration. Sequoias benefit from fires reduction of fungal pathogens and carpenter ants that weaken structural integrity in larger trees, clearing of understory trees and brush that compete with seedlings, and opening of cones through heat (Schoenner 1992). Fires in the past were often frequent and of low intensity because fuel loads were generally low. A diverse assemblage of herbaceous plants and shrubs is also found in this zone, including numerous local endemics and species specialized on particular soil types such as serpentine. Above the mixed conifer forest grows simple forests dominated first by white fir and then by red fir. They occur the zone of deepest snow.
From 7,000-9,000 ft , in the subalpine zone, lodgepole pine forms extensive forests mixed with meadows and montane chaparral. Jeffrey pine, western white pine, mountain juniper, pine, and aspen are locally common. Between 9,000-10,000 ft mountain hemlock, whitebark pine, foxtail pine, and limber pine are characteristic. Above timberline, alpine meadows, talus slopes, and rocky outcrops cover the land. On the eastern slope, the alpine zone extends down to around 11,800 ft, lodgepole pine or red fir-dominated forests occur down to 8,000 ft, and Jeffrey pine forests to 6,000 ft. Below this elevation the pinyon-juniper woodlands grade into sagebrush scrubs of the Great Basin. Extensive talus slopes, meadows, montane chaparral, lakes, and rock outcrops throughout the Sierra.
Approximately 400 terrestrial vertebrate species occur in the Sierra Nevada , although around 100 of these are largely distributed elsewhere. Around 60 percent of Californias vertebrate species are found here. Thirteen vertebrate species (e.g., salamanders, frogs, rodents, birds) are endemic to the range, this includes some of the highest levels of mammal endemism in the United States and Canada. Some endemics include the Yosemite toad, Mount Lyell salamander, Limestone salamander, Kern salamander, long-eared chipmunk, alpine chipmunk, western heather vole, Walker Pass pocket mouse, yellow-eared pocket-mouse, and the golden trout. A diverse vertebrate predator assemblage once occurred in the ecoregion including grizzly bear (Ursus arctos), black bear (Ursus americanus), coyote (Canis latrans), cougar (Puma concolor), ringtails (Bassariscus astutus), fishers (Martes pennanti), pine martens (Martes americana), wolverines (Gulo gulo, and several large owls, hawks, and eagles.
The Sierras support a diverse invertebrate fauna with a number of endemics, including Behrs colias butterfly (Colias behrii), restricted to a small area around Tioga Pass. Many other invertebrate species have very local distributions. New plant and invertebrate species are being added to lists with further investigations in the region. Between 1968 and 1986 sixty-five new plant species were described for the Sierras (CWWR 1986).
A century of intensive logging, mining, railroad building, development, fire suppression, and grazing by sheep and cattle have left only around 25 percent "intact" natural habitat in the Sierra Nevada. Much of this intact habitat occurs at higher elevations, often in non-forested alpine or in less productive forests and woodlands. More than 60 percent of the ponderosa pine or mixed conifer forests have been altered with many remaining forests degraded through logging and fire suppression. Overall, around 19 percent of the entire mid-elevation conifer belt is still relatively intact (CWWR 1996). Between 7 percent and 30 percent of late-successional old growth forests of middle elevations remain, the percentage depending on the forest type. One third of the original extent of Giant Sequoia groves have been harvested, with the U.S. Forest Service allowing these rare and unique trees to be cut in some of the northern groves as recently as the 1980's. The greatest percentage loss of habitat has occurred in late-successional forests, foothill woodlands, and riparian habitats.
Remaining Blocks of Intact Habitat
Four National Parks, Lassen,Yosemite, Sequoia, and Kings Canyon, and some National Forest Wilderness Areas, harbor the largest remaining blocks of relatively intact montane mixed conifer forests. Most of the rest of this zone has been cut over at least once. There is extensive high montane and subalpine forests and meadows, and alpine habitats in national park and forest service wilderness areas. Much of the higher elevations were once heavily grazed, and Forest Service wilderness areas are still subjected to grazing. Most remaining habitats, including late-successional forests suffer from significant alterations of historic fire regimes and consequent changes to processes (e.g., dense in growth of shade-tolerant species). A variety of changes resulting from intensive exploitation and current management of remaining forests contributes to the lower resiliency of forests to fire and epizootic disturbances. The SNEP analysis came to the conclusion that historic logging, creating simple forests, was more important in creating the fire problems in the Sierra, next comes fire suppression it self. These problems include forest simplification, removal of older trees with their structural and genetic resistance, loss of natural firebreaks such old growth patches with sparse understories and moist riparian vegetation, replanting schemes using genetically-similar seedlings of a single tree species, and intensive application of pesticides that also destroys natural predators on epizootics.
Degree of Fragmentation
Fragmentation varies inversely with altitude. The last remaining less distrubed habitats at lower elevation are severely fragmented.
Degree of Protection
The National Parks share a high degree of protection, including the few remnant lower elevation habitats they encompass. Recent changes in fire management policies within National Parks are helping to restore natural disturbance regimes and successional processes. Wilderness Areas, which usually are located at higher elevations, are protected from commercial logging, but they are still intensively grazed by domestic livestock causing significant damage to riparian habitats and other vegetation types.
Types and Severity of Threats
The vast majority of native forests have already been largely converted to tree plantations. Intensive forestry practices have simplified forest structure and composition in most remaining forests, causing reduced ecologic resilience, genetic variability, and impaired function. Forest simplification and fire suppression together contribute to greatly increased probabilities of catastrophic fires and increased frequencies and severity of widespread mortality from epizootics such as bark beetles and fungal pathogens (CWWR 1996, Dellasala et al. 1996). Most remaining fragments of unlogged forests outside National Parks are threatened by logging, fire suppression, and grazing. Meadows throughout the range outside the national parks continue to be significantly damaged by sheep and cattle grazing and pack animals. Soil compaction from logging and development activities is altering natural succession patterns.
Introduced pathogens from livestock harm native species, the entire herd of 65 bighorn sheep in the Warner Range, a small range northwest of the Sierra Nevada was extirpated by an introduced virus from a single sheep in 1988 (Jensen et al. 1993). Sugar pines in many areas are being killed by the introduced white pine blister rust (Cronartium ribicola) which is rapidly spreading. Continued cutting of larger and potentially resistant sugar pine trees may preclude any effective conservation programs. The creation of clear-cuts, a commonly used timber extraction practice, promotes the growth of gooseberry and currants, vectors of the rust (Johnston 1994). Tree mortality from Anosus root rot fungus (Heterobasidion annosum), a native pathogen, continues to increase from logging, which creates stumps that are a foci for the fungus, and from dense stands of young trees resulting from fire suppression (Johnston 1994).
In the middle-elevation conifer belt, oxidant-induced air pollution damage caused by ozone and other chemicals from air pollution from coastal and valley cities, is damaging many tree species, including Abies concolor, Pinus ponderosa, and P. jeffreyi. Lichens have also suffered significant damage and reduction due to air pollution.
Across the range, 218 endemic plant species are considered rare or threatened, and three plant species (Monardella leucocephala, Mimulus whipplei, Erigeron mariposanus) are believed to be extinct. Sixty-nine terrestrial vertebrate species (17% of the fauna) are considered at risk by government agencies. Many amphibian species at all elevations have severely declined and are disappearing in many areas throughout the range (Drost and Fellers 1996). Introduced fish appear to be a major cause of amphibian decline at higher elevations, although increased UV radiation, viruses, loss of habitat, acid rain been suggested as additional causes of this dramatic decline. Dams and impoundments, as well as grazing and logging in surrounding catchments, have degraded most aquatic habitats. Only 10 percent of the original habitat of anadromous fish in the Sierra are reachable for spawning.
Relationship to other classification schemes
The Sierra Nevada ecoregion used in this analysis is based on Omerniks (1995) Sierra Nevada ecoregion. This ecoregion generally covers the same habitats as Baileys (1994) Sierra Nevada Section (M261E) except that Bailey identifies the northern fifth of the range as the Southern Cascades and has an extension of the Sierras running southwest towards the Tehachapi Mountains. This ecoregion covers several of Küchlers (1977) vegetation classes including Big Trees, North Jeffrey Pine, Sierran Montane, Alpine, Upper Montane Subalpine Forests, Sierra Yellow Pine, Juniper-Pinyon Woodland, and Yellow Pine-Shrub Forests. Prepared by: D. Olson and J. Sawyer
This text was originally published in the book Terrestrial ecoregions of North America: a conservation assessment from Island Press. This assessment offers an in-depth analysis of the biodiversity and conservation status of North America's ecoregions. All text by World Wildlife Fund © 2001
For more general information on this ecoregion, go to the WildWorld version of this description.