Life and climate in an ice age - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 10

1.3 ICE AGE FORESTS

Bonnicksen (2000) described some of the climatic effects of the great ice sheets in
the last ice age. Desert basins in the American Southwest filled with water from
heavy rains and meltwater from mountain glaciers, and cold air reduced evapora-
tion. There were more than 100 such freshwater lakes, with the largest being Lake
Bonneville in Utah that occupied an area of 19,000 square miles and reached
depths up to 300 meters. Even Death Valley, California filled with water. Cold air
blew off the sides of the ice sheets at high speed generating gale force winds. In the
winter when conditions were dry, these winds produced huge sand and dust storms
that blew silt across the central part of America. According to Bonnicksen, Ice
Age silt ''covers 30% of the U. S. but it lies beneath forests and grasslands.''
Bonnicksen provided an elegant description of Ice Age forests. He described
the Ice Age as an ''alien world of modern and extinct animals living among well-
known plants mixed in unusual ways.'' The territories of cold-weather trees such
as spruce, fir, and bristlecone pine spread out during cold periods and contracted
when it became warmer. Conversely, warm-weather trees such as oak, hickory,
and ponderosa pine spread poleward when the glaciers retreated and moved
toward the south when the glaciers expanded. In this process, ''the trees shifted
and sorted themselves into unique forests while moving around the landscape.''
The most recent sorting process began when the climate warmed shortly after the
Last Glacial Maximum about 18,000 years ago. Prior to that, North America's
Ice Age forests endured an uneasy stability for thousands of years. According to
Bonnicksen, summers were cooler but differences between seasons were less
extreme than today. Exotic mixtures of plants and animals were able to form
complex patchworks of communities that had ''a greater diversity of species,
higher numbers of animals, more large animals, and larger animals than any that
existed from then until now. Many of these primeval communities fell apart after
the ice sheets melted .''

Bonnicksen described immense tracts of open white spruce forests:

...

''The white spruce is a short tree with a thin trunk and low-hanging
branches that form a narrow cone of pointed blue-green needles. It grows on
relatively dry soils

. Spruce forests grew in a band hundreds of miles wide from
the Rocky Mountains to the East Coast. They hugged the southern edge of the
tundra and cold steppes at the foot of the glaciers. Occasionally fingers and
patches of spruce also protruded into the tundra and worked their way up to
the edge of the ice.''

...

As the glaciers expanded, the tree line in the western United States descended
by up to 700-800 meters. The continental ice sheets redirected the jet stream
southward, causing changes in precipitation patterns. The Puget Sound area
became drier while the Southwest benefited from increased rainfall. California's
weather became cooler, drier, and more continental. Santa Ana winds also
increased, blowing great clouds of sand from the Mojave Desert westward over