Ecological Sections: Sierra Nevada - (MAP)
This subsection is near the southern end of the block-faulted northeastern
part of the Sierra Nevada. It is between Sierra Valley and Lake Tahoe.
It has a cold and semi-arid to humid climate. MLRA 22e.
Tahoe - Truckee
Lithology and Stratigraphy. Cenozoic volcanic rocks
predominate in this subsection. There are some Mesozoic granitic rocks,
Jurassic marine sedimentary rocks, and Jurassic and older metavolcanic
rocks. The Cenozoic volcanic rocks are mostly Pliocene andesite,
basalt and pyroclastic rocks and Pleistocene basalt. Pleistocene
glacial till and outwash are common and there are small areas of Quaternary
alluvial and lacustrine deposits.
Geomorphology. This subsection is on a gently sloping
to moderately steep plateau with some steep mountains that is dissected
by streams in canyons that have steep to very steep sideslopes. Most
of the canyons on the west side of the subsection have been modified by
glacial erosion and are U-shaped there. The elevation ranges from
about 5000 along the Truckee River and adjacent to Sierra Valley up to
9143 feet on Mt. Lola. Faulting, mass wasting, and fluvial erosion
and deposition are the main geomorphic processes.
Soils. The soils are mostly Ultic and Mollic Haploxeralfs;
Lithic Ultic Haploxerolls; Ultic and Lithic Ultic Argixerolls; Andic Xerochrepts;
and Andic Xerumbrepts. There are some Cryumbrepts at the highest
elevations. Soils on granitic rocks are mostly Entic Haploxerolls
and shallow Typic Xeropsamments, but they are not extensive. Soils
on glacial till are mostly Pachic, Entic, and Typic Xerumbrepts.
The soils are mostly well drained, but some on basin floors are somewhat
poorly drained. Soil temperature regimes are mostly frigid, but some
are cryic. Soil moisture regimes are mostly xeric. Soils with
aquic moisture regimes are present in glaciated terrain and small valleys,
but they are not extensive.
Vegetation. The predominant natural plant communities
are Ponderosa pine series, Mixed conifer series, White fir series, Red
fir series, and Big sagebrush series. Jeffrey pine series is common
in drier areas on shallow and rocky soils. There are a few small
patches of Mountain hemlock series at the highest elevations. Lodgepole
pine series prevails on many wet soils and on drier soils where cold air
drainage and frost limit the regeneration of other trees. Sedge meadow
communities are common, but they are not extensive. Black cottonwood
series is common in riparian areas and there is some Aspen series.
Characteristic series by lifeform include:
Climate. The mean annual precipitation is about 20 to 40 inches;
most of it falls as snow. Mean annual temperature is about 35°
to 45° F. The mean freeze-free period is about 25 to 75 days.
Grasslands: Alpine habitat, Ashy ryegrass series, Breaked sedge
series, Fen habitat, Idaho fescue series, Montane meadow habitat, Needle-and-thread
series, Rocky Mountain sedge series, Shorthair reedgrass series, Shorthair
Shrublands: Big sagebrush series, Bitterbrush series, Low sagebrush
series, Parry rabbitbrush series, Rubber rabbitbrush series.
Forests and woodlands: Aspen series, Jeffrey pine series, Lodgepole
pine series, Mixed conifer series, Mountain hemlock series, Ponderosa pine
series, Red fir series, Washoe pine series, Western white pine series,
White fir series, Whitebark pine series.
Surface Water. Runoff is rapid from most of the area.
Most of it flows to the Truckee River, but runoff from the northern part
of the subsection flows to Sierra Valley, which is at the head of the Middle
Fork of the Feather River. Maximum flow in these rivers is during
spring when snow is melting rapidly. There are many small and a few
large natural lakes or ponds in glaciated terrain, and some reservoirs.