Chaparral soils are typically coarse-textured, deep, and poorly developed.
Soil as used here includes all porous material (regolith) in which weathering
and roots are active. The distinction between soil depth and solum depth
(A and B horizons) is critical, since most of the soil supporting chaparral
is in the C horizon. Usually the A horizon is only a few inches thick,
and the B horizon is commonly absent. Soil texture varies from cobbly
and gravelly loamy sand to gravelly loam. Slopes of 60 to 70% are common.
All aspects are represented.
The C horizon, which can be as much as 30 to 40 ft deep, is hydrologically
important, even though total porosity may only be 20 to 25 %. Because
of deep weathering, this zone is able to store much of the winter rain,
which the deep-rooted shrubs use during dry periods.
The soils on the Whitespar watersheds (near Prescott, Arizona) are classified as Lithic Ustorthents,
loamy skeletal, mixed mesic, non-acid.
Chaparral shrubs grow on a variety of geologic rock types, all of which
weather to produce a deep, coarse regolith (Hibbert
et al 1974). In contrast, rock types such as basalt, limestone, and
quartzite, which weather to relatively fine-textured shallow regoliths,
normally do not support chaparral, even though rainfall and elevation
are similar. These soils in Arizona usually support ponderosa pine, pinyon-juniper,
or grass. On the Sierra Ancha Experimental Forest, shallow quartzite soils
support grass with scattered shrubs, while adjacent soils derived from
intrusive diabase support a medium-dense stand of brush.
Utah and alligator juniper stands dominate the central portion of the
Beaver Creek drainage south of Flagstaff at elevations ranging from 5,000
to 6,500 ft where annual precipitation averages 18 to 23 inches. On Mingus
watersheds, 30 miles west of Beaver Creek at similar elevation and precipitation,
the dominant vegetation is chaparral with scattered juniper and pinyon
trees. Soil is the only apparent difference in the sites that would account
for the difference in vegetation. The soils on Beaver Creek are derived
from basalt and are relatively fine textured and shallow, compared with
the soils on Mingus, which are derived from sedimentary rock that is shattered
and weathered to a greater depth.
To the Southwest on the Whitespar watersheds at 6,500 ft, the chaparral
is even better expressed on granite-derived soils, which appear to be
somewhat deeper than at Mingus. Here precipitation is 22 to 24 inches
and brush gives way to ponderosa pine on the higher, more moist sites.