One of the main factors that influences vegetation communities is soil type. To help understand patterns in vegetation communities, the watershed is divided into subsections that have certain characteristic soil types. Vegetation communities also vary due to variations in precipitation and evapotranspiration, especially the increase in evapotranspiration that occurs with increasing distance from the coast.

Soils on the coastal plain are formed from marine terrace deposits and are dominated by agriculture. Types of vegetation on the coastal plain include agricultural crops, pasture, grand fir-Sitka spruce forest, redwood forest, coastal grass-shrub, and coastal prairie-scrub mosaic. In the Smith River estuary, coastal salt marshes are found (Table 13, California 1977).

Sedimentary deposits of the Northern Franciscan subsection have developed into deep fertile soils (California Department of Fish and Game 1980). In these areas, marine influence is strong and fog is common in the summer. Because of favorable soils and low evapotranspiration stress, this subsection supports outstanding redwood and Douglas fir forests. Redwoods dominate low elevation forests. Tanoak and madrone are also common. Many forests have been logged and are now in early seral stages. Blocks of old growth forest remain, the largest being within Jedidiah Smith State Park.

The Gasquet Mountain Ultramafic subsection is characterized by serpentine soils developed from Mesozoic ultramafic intrusive rocks. Due to harsh soils, this subsection has more open forests such as the Jeffrey pine forest type. Typically in these areas, Jeffrey pine is the only conifer although incense cedar, western white pine, sugar pine, knobcone pine, huckleberry oak, and manzanita are sometimes present (Franklin and Dyrness 1988). The mixed evergreen forest type is also found in the Gasquet Mountain Ultramafic subsection. Dominant species of the mixed evergreen forest are madrone, chinquapin, tanoak, and Douglas fir. Because this subsection is farther inland, marine influence is reduced and more drought tolerant vegetation is found.

The greatest concentration of endemic plants in North America occurs on the ultramafic soils of the Smith River watershed and neighboring areas. Especially in the North Fork subbasin, serpentine soils support unique plant communities and 27 rare plants (Cooperrider and Garrett 1995, Table 14).

The parent materials for the Western Jurassic subsection are predominantly Mesozoic sediments, Jurassic-Triassic metavolcanics, Mesozoic basic intrusives, Upper Jurassic marine sediments, and Mesozoic ultra-basic intrusives. Although this subsection has a variety of soil types, the most common vegetation type is mixed evergreen forest dominated by Douglas fir, madrone, tanoak, canyon live oak, and chinquapin. Areas of Jeffrey pine, incense cedar, western white pine, huckleberry oak, manzanita, and ceanothus are also found.

The soils and vegetation of the Siskiyou Mountains are diverse. Parent materials include Mesozoic sediments, Mesozoic metavolcanics, and Mesozoic ultramafic intrusives. Soils developed from sedimentary parent materials tend to be steep and support Douglas fir forests. Granitic geology occurs mostly at high elevations where evapotranspiration stress on vegetation is less. Soils developed from granitic parent materials generally support mixed conifer forests including true firs, Douglas fir, and mountain hemlock. Soils developed from the metamorphosed volcanic parent materials are usually low to medium in timber productivity and may be brushy. This area also has ultramafic soils that support unusual vegetation communities.

In the drier fire-prone inland areas, there are scattered stands of knobcone pine (Pinus attenuata) and lodgepole pine (P. contorta) (Table 15). These "closed cone" pines depend on fire to produce conditions favorable for reproduction. Their cones generally remain sealed until the heat of a forest fire causes them to open and release their seeds (Tuffly 1995). In inland areas, forests at low elevations consist of a mixture of conifers and hardwoods, typically including Douglas fir, tanoak, canyon live oak, and madrone.

The Siskiyou Mountains have the highest diversity of conifers in North America (Table 16). The diversity of conifers and other vegetation is partly due to the diversity of soils and highly variable topography and microclimate. In addition, climatic and geologic history has contributed to the vegetative diversity of the area. Because the Smith River watershed has been continuously vegetated for 65 million years, it has developed a greater diversity of plants than areas subjected to extreme climatic conditions or submergence (Cooperrider and Garrett 1995).

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