Natural vegetation of coastal California ranges from xeric southern oak woodland to mesic northern rain forests. In Southern California, a diverse and complex mosaic of scrub oak (Quercus), oak woodland savanna, and oak-dominated foothill woodlands with isolated groups of closed-cone pine and cypress (e.g., Pinus radiata and Cupressus pygmaea) interfinger with chaparral and lowland sage scrub. At higher elevations, conifer forests develop: open pine woodland with small, isolated stands of incense cedar (Libocedrus decurrens) is succeeded upslope by parklike mid-montane conifer forests of scattered pine and incense cedar, upper montane juniper (Juniperus occidentalis) woodland, and subalpine coniferous forests with lodgepole pine (Pinus contorta; Barbour and Billings, 1988; Barbour and Major, 1977; Franklin and Dyrness, 1973; Kuchler, 1977).
The unique redwood (Sequoia sempervirens) forests of coastal Northern California, which include other conifers such as western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), and Douglas fir (Pseudotsuga menziesii), are replaced inland in drier situations by oak woodland and at higher elevations by montane forest formations with fir (Abies), hemlock, Douglas fir, pine, and evergreen oaks (Barbour and Major, 1977). Just north of ~42°N in Oregon, western hemlock and Sitka spruce are the major tree species along the Pacific Ocean, with hemlock dominating the coast and spruce more prominent inland. Common forest associates include Douglas fir, western red cedar (Thuja plicata), and, in disturbed and riparian areas, alder (Alnus). Above the narrow band of lowland forest are montane and subalpine forests of fir (Abies concolor and Abies amabilis), pine (Pinus ponderosa and Pinus lambertiana), and mountain hemlock (Tsuga mertensiana).
The two end-members of coastal California vegetation (southern California oak woodland and Pacific Northwest conifer forests) reflect regional differences in mean annual temperature and precipitation (first-order controls of vegetation distribution). South of ~40°N-42°N, mean annual temperatures and precipitation average 19°C and 30 cm, and upper montane temperature and precipitation are ~8°C and 57 cm. To the north on the Oregon Pacific coast, mean annual lowland temperature and precipitation are ~12°C and ~300 cm, and subalpine temperatures and precipitation average ~10°C and 140 cm (Elford, 1974; Sternes, 1974). The south-north transition from excess evaporation to excess precipitation reflects the frequency and intensity of frontal storms south and north of the atmospheric and oceanic polar fronts (Lawford, 1993).
Near the Pacific Ocean, temperature and effective precipitation are moderated by fog associated with upwelling and by seasonal variations in sea-surface temperatures (SSTs) of the southward-flowing California Current and the poleward-flowing, seasonal Davidson Current (Barbour et al., 1980; Barbour and Major, 1977). South of ~42°N, northerly winds drive near-coastal persistent seasonal upwelling; to the north, upwelling intensity is more variable. Off California and Oregon, waters north of 40°N-42°N are subarctic in type, with mean SSTs of ~12° to ~13°C; off Southern California, mean SSTs of ~14° to ~15°C reflect the presence of subtropical waters (Mortyn et al., 1996). Changes in the California Current system during the last glacial are reflected in changes in Pacific maritime vegetation. The virtual absence of coastal redwood in Northern California in glacial-age pollen records, for example, is probably related to decreased fogdrip associated with decreased nearshore upwelling and to lower SSTs (Adam et al., 1981b; Gardner et al., 1988; Sancetta et al., 1992). During oxygen isotope Stage 3, brief warming events in California and Oregon appeared correlative with interstadial events in waters offshore (Doose et al., 1997; Gardner, 1997; Gardner et al., 1997; Kennett and Ingram, 1995; Lyle et al., 1992; Sabin and Pisias, 1996; Thunell and Mortyn, 1995).