California's north coastal forests are divided into separate communities that intergrade with one another. The largest and most important of these communities are the coastal redwood, Douglas-fir, and mixed-evergreen forests. From the coast inland, species composition is dictated by moisture gradients. These gradients are determined by rainfall and the ability of soils to retain water. In the wettest coastal areas, where fog alone can account for up to ten inches of added yearly precipitation, coast redwood, Sequoia sempervirens; Sitka spruce, Picea stichensis; western hemlock, Tsuga heterophylla, and canoe cedar, Thuja plicata, are dominant. In moderately moist areas, farther inland, Douglas-fir, Pseudotsuga menziesii, takes over. Mixed-evergreen forests occur in warmer areas and are highly variable in their species composition.
Closed-cone coniferous forests are another unique California community occurring in patches along the coast from Humboldt to Santa Barbara County. The name of this community derives from the fact that the seed-bearing cones remain closed for several years, a reproductive adaptation that ensures survival. Only age, excessive hot weather, or fire opens them.
Riparian woodlands occur in ribbon-like bands along stream beds where rich soils and high humidity produce a natural greenhouse effect. Although this unique community accounts for less than one per cent of California's total forest acreage, it supports one of the most diverse ecological communities of plants and animals.
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