Chaparral Shrublands

Soils

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.

• More information on erosion and sedimentation

Plant-Soil Relations

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.