Formation of the Soils
The forests in the park make up the dominant ecosystem that affects soil development. Because the parent material is stable in areas covered by forests, organic matter accumulates and begins to decompose, which releases nutrients for use by micro-organisms. The major types of forest communities in the park are dominantly mountain hemlock, Shasta red fir, lodgepole pine, ponderosa pine-fir, whitebark pine, and Douglas fir (See General Vegetation Map).
The mountain hemlock communities cover most of the park. These communities typically occur in areas where precipitation is about 60 to 70 inches and at elevations of more than 5,000 feet on the western side and 6,000 feet on the drier eastern side. These communities are in the cryic soil temperature regime and the udic soil moisture regime. Most of the udic soils in the park, including Llaorock, Castlecrest, Timbercrater, Sunnotch, Unionpeak, Grousehill, and Umak soils, are associated with these communities.
The Shasta red fir communities are at lower elevations and receive slightly less precipitation that do the mountain hemlock communities. The Shasta red fir communities are mainly on south-facing exposures in the eastern part of the park. The soils associated with these communities are similar to those associated with the western hemlock communities; however, unique map units are included in the survey to identify these ecologically different communities. If a map unit or map unit component is identified as “dry”, it is associated with the Shasta red fir communities.
Lodgepole pine forests are throughout the park, and in many areas these forests occur as the early stage of forest succession after a fire. Over time these areas will return to the historic climax forest type. Where cold air frost pockets inhibit the survival of other tree species, lodgepole pine is the historic climax forest type. Examples of forests that are dominantly lodgepole pine are on the bench surrounding the depression at “Pumice Desert”, the bottom of Desert Creek, and the southeastern portion of Pinnacle Valley. The soils in these areas commonly are low of Castlecrest and Collier soils.
Ponderosa pine-fir communities are in the drier areas of the park, where the average annual precipitation generally is less than 50 inches. These areas are in the cryic soil temperature regime and the xeric soil moisture regime and are along the eastern and southeastern borders of the park. Maklak, Collier, Lapine, and Oatman soils support these communities.
Whitebark pine communities are dominant in higher lying, windswept positions in the park, near rock outcroppings and on the edges of alpine meadows. They occur at elevations of more than 6,000 feet and commonly are associated with mountain hemlock communities.
Douglas fir communities occur only on the steep, south-facing slopes along Red Blanket Creek, in the southwestern corner of the park. The soil temperature regime is frigid, and the soil moisture regime is xeric. Soils of the Donegan series support these communities.
Because most of the park is covered by forests, areas where forests are absent stand out in stark contrast. These areas consist of meadows and deserts, areas of Rock outcrop, and areas of Rubble land. The only vegetation in the areas of Rock outcrop and Rubble land is in the associated areas of minor soil components. The vegetation in the meadows is mainly sedges, grasses, and forbs with a few trees, and the vegetation in the deserts is mainly a sparse cover of sedges and forbs. Cleetwood soils are in the meadows and deserts. Because the root systems of the sparse vegetation are insufficient to stabilize the ash and sand, these soils are subject to wind and water erosion. The lack of plant cover allows winds to dry the soil surface rapidly after the snow cover has melted; thus, encroachment by forests is limited by high seedling mortality. Moisture remains in the subsoil well into the dry season because of a unique “summer fallow” effect, which allows a few seedlings to become established. Cleetwood soils exhibit only a few weakly expressed soil characteristics because the lack of stabilizing vegetation and disturbance by erosion has greatly limited soil development.