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central California. h ese awe-inspiring trees are the world's tallest living
organisms, some towering higher than 360 feet. h e coast redwoods require
summer fog to survive, and, because of this, they are intimately linked with
the oceanographic conditions just of shore in the eastern Pacii c Ocean. h e
fog reduces temperatures during the warm, dry summers and provides mois-
ture for the trees by a process known as “fog drip”—that is, when moisture in
the fog condenses on the redwood's i ne needles, it drips to the ground where
it can soak into the soil and travel to the roots.
Coastal fog is thickest along the Pacii c coast during the warm, dry sum-
mers. h e fog forms when moist air l ows inland of the ocean and passes
over the cold surface waters of the coast, which causes condensation of the
water vapor, resulting in fog. Simultaneously, the warm land surface heats
the overlying air, causing it to expand and rise, and, as it rises, that warm air
is replaced by the cool marine air and fog l owing onshore. In regions of the
coast with small mountain ranges, this onshore movement of fog can appear
like a cloud waterfall pouring over the coastal mountains, bringing precious
moisture to coastal ecosystems during the dry summer months.
h e Pacii c coastal redwoods form a geographic boundary, or ecotone,
between the more arid oak woodland and open scrub of Southern California
and the wetter western hemlock, spruce, and alder lowland coastal forests
of Oregon and Washington. An analysis of pollen from cores taken along
the California and Oregon coasts suggests that coastal upwelling and the
development of summer fog were coincident with a great expansion of these
majestic redwood forests that began about 5,150 years ago.
human migrations
Against this backdrop of a warming and drying climate in the West during
the mid-Holocene, it is clear that the early humans living in the region faced
tough times. Archaeologists have found evidence that humans were likely suf-
fering from the severe droughts, especially in southeastern California and the
Great Basin. Prolonged and severe droughts appear to have forced large-scale
migrations out of the driest inland regions as people went in search of food
and water. Douglas Kennett, an archaeologist at Penn State University who
has studied these migrations, reasons that the moderate climate and abundant
marine resources along the coast would have attracted native human popula-
tions from the drier interior deserts. Kennett has collaborated with his father,
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