Natural coastal vegetation of California changes from xeric, open oak woodland in the south to dense conifer rainforest in the north. The complex mosaic of southern California scrub oak (Quercus), oak woodland savanna, and oak-dominated foothill woodlands with isolated groups of closed-cone pine and cypress (Pinus radiata and Cupressus pygmaea) interfingers with chaparral and lowland sage scrub. Conifer forests develop upslope in which open pine woodlands with small, scattered stands of incense cedar (Libocedrus decurrens) are succeeded at higher elevations by mid-montane parkland with 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).
In Northern California, redwood (Sequoia sempervirens) distinquishes the southern extension of the Pacific Northwest evergreen coniferous forests, which are unique in size and longevity among temperate forest regions of the world (Waring and Franklin, 1979). Associated with redwood are western hemlock (Tsuga heterophylla), spruce (Picea sitchensis), Douglas fir (Pseudotsuga menziesii), and western red cedar (Thuja plicata). Inland, xerophytic lowland deciduous oak communities (Quercus garryana, Quercus lobata, and Quercus douglasii) and oak/pine/grassland mosaics develop. At higher elevations in the Coastal Range, montane forest formations develop with fir (Abies), hemlock, Douglas fir, pine, and evergreen oaks (Barbour and Major, 1977). Just north of ~4°N in Oregon, western hemlock and Sitka spruce dominate along the Pacific Ocean, with hemlock on the coast and spruce more prominent in the interior. Common forest associates include Douglas fir, western red cedar (T. plicata), and alder (Alnus) in moist habitats. 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 contrasting vegetation types of the California coast (southern California oak woodland and Pacific Northwest conifer forests) reflect significant 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 in Oregon, 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 roughly coincides with the frequency and intensity of frontal storms south and north of the atmospheric and oceanic polar fronts.
Close to the ocean, California and Oregon temperatures and effective precipitation are moderated by fog associated with upwelling (Barbour, 1988; Barbour et al., 1980) and by seasonal variations in sea-surface temperatures (SSTs) of the southward-flowing California Current and the poleward-flowing seasonal Davidson Current. 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 (Gardner et al., 1997).
During the past glacial cycle, changes in the California Current system and in California maritime vegetation and climate inferred from pollen data appear to be related. Near-synchronous high-frequency variations in the abundance of oak in Southern California and SST in the Santa Barbara Basin characterize oxygen isotope Stage (OIS) 5 (Heusser, 1995). Similar variations in redwoods in Northern California correspond to temporal and geographic changes in offshore upwelling and in movement of the Polar Front during OISs 3-1 (Doose et al., 1997; Gardner et al., 1997; Heusser, 1998; Lyle et al., 1992; Sabin and Pisias, 1996; Sancetta et al., 1992). Here we use three pollen records from cores taken on the northeast Pacific continental margin to document millennial-scale variability of coastal North American ecosystems from OISs 1 through 13.