Biology Reference
In-Depth Information
cognitive foundation might be one factor explaining the resistance to sci-
entifi c discoveries that affect understanding of such an agent, giving rise to
the present dichotomy between science-based thinking and the spiritually
grounded thought of members of some religious groups—a dichotomy that
must have existed almost from the beginning.
The Boreal Region
Cooling molten slag began to form on the Earth's surface about 4.5 Ga (bil-
lion years ago) to constitute the fi rst masses of granitic rock, called cratons
or shields, around which the continents gradually formed. The central and
eastern boreal region of North America is underlain by the Canadian Shield.
The fi rst life reported from the Precambrian was cyanophycean bacteria
(blue-green algae), possibly fungi, and later bacillus bacteria in the Gun-
fl int Chert of southern Ontario (Tyler and Barghoorn 1954). The deposits
date from 1.9-1.83 Ga. Even earlier life is now known from structurally
preserved fossils in rocks dated at 3.2-2.5 Ga, and from biogenic stromato-
lites in Australia at 3.5 Ga (Knoll 2004, pers. comm., 2009).
The vast Laurentian ice sheet was centered over Hudson Bay, and in the
coldest phases of the Pleistocene, it fused with the Cordilleran ice sheet
of the northern Rocky Mountains. The boreal region is mostly fl at from
multiple glaciations, and from the glacial outwash that has fi lled in much
of the lowland. It is covered by numerous lakes,
Sphagnum
(peat) bogs, and
a boreal coniferous forest of
Abies
(fi r),
Larix
(larch),
Picea
,
Thuja
(cedar),
Tsuga
(hemlock), and a few species of
Pinus
(pine), along with
Alnus
(alder)
and
Salix
(willow). In the east along its southern border there is a Lake
States forest, transitional to the deciduous forest to the south, that adds
Betula alba
(white birch) and
Populus tremuloides
(quaking aspen). The
plants and animals that have adapted to this environment are compara-
tively few, and these ten genera constitute the prominent trees of the boreal
forest.
Growing conditions in the boreal region are harsh but less so than in
the Arctic. Rainfall is about 500 mm a year; instead of permafrost, soil
moisture is frozen for about eight months of the year; the minimum win-
ter temperature is
35°C; and the growing season is between seven and
twenty-fi ve weeks. The boreal coniferous forest contains one-third of the
world's trees, and it covers 28 percent of North America north of Mexico. It
extends south as a disjunct montane-subalpine coniferous forest just below
the alpine tundra in the mountains to the west, along the western coast, and
at the highest elevations in the Appalachian Mountains to the east.
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