Geoscience Reference
In-Depth Information
soils of the North, which have formed only in the last six to nine thousand years since the glaciers retreated.
They are able to extract the nutrients in this organic-poor and acidic soil through a mutually beneficial relation-
ship with mycorrhizae, fungi that are intimately interwoven with their root tissue. (Most plants of the taiga, in-
cluding mosses, ferns, and angiosperms, take advantage of this ancient relationship.) The fungi supply the
roots with nutrients and water, while the higher plant feeds the fungus with carbohydrates obtained through
photosynthesis.
The most critical adaptation is to cold in the north, where temperatures can plunge below -40°C (-40°F).
Hardy trees of the eastern deciduous forest, such as maples and oaks, can supercool the liquid within their cells
even when temperatures drop below the freezing point, without the formation of ice crystals, which would
damage the cell's proteins and organelles, killing the trees. Once the temperature drops below -40°, however,
the liquid inside the cells of the hardwoods freezes. In the hardier conifers, these liquids are squeezed out
through the cell membranes into the empty spaces between the cells, where the ice crystals that inevitably form
at these temperatures can do no damage. A few hardwood trees typical of the boreal forest—paper birch, trem-
bling aspen, and balsam poplar—also have this extracellular freezing capacity.
Much of the boreal forest in eastern North America arises on the granitic rocks of the Precambrian Shield,
the core of the continent. Four types of vegetational communities grade into each other along a south-to-north
latitudinal cline. In sequence, they are closed coniferous forest, open coniferous forest, forest-tundra, and tree-
less tundra. The trees gradually thin out until they disappear at the continental Arctic tree line and are replaced
by tundra. This tree line is anything but straight, however, swinging north and south, with fingers of forest oc-
curring along sheltered river valleys and giving way to treeless tundra on exposed uplands. In between are the
ubiquitous lichen woodlands, a forest of widely spaced spruces, which appear as dark, steeplelike silhouettes
against a pale ground carpet of lichens, interspersed with scattered northern shrubs and herbaceous plants.
The lichens of greatest importance in boreal forests, in both extent and sheer biomass, are the reindeer
lichens. They grow on soils, both organic and inorganic, rather than on bare rock, bark, or decayed wood. One
of their preferred habitats is rocky outcrops covered with the thinnest veneer of soil, where they have a com-
petitive advantage over mosses and vascular plants. In general, lichens thrive in drier places, whereas mosses
like wetter environments. But the greatest limiting factor for the growth of ground lichens is light. Few lichens
can grow in the shaded habitat occupied by common boreal mosses, such as sphagnum moss. Lichen prefer
thinly wooded habitat, where light penetrates to the forest floor. For this reason, lichens are pioneer plants and
may be the first plants to predominate on a disturbed site—after a fire, for instance. In general, mosses pre-
dominate in moist oceanic climates, and lichens in drier continental ones.
Animals of the North Woods
As in all ecosystems, the flow of energy through the boreal forest begins with plants and photosynthesis. But
in Arctic and subarctic ecosystems, such as prevail in the more northerly parts of the boreal forest, short peri-
ods of intense primary production are followed by long, cold, dark periods when the accumulated energy can-
not be passed through the food web.
Even so, northern forests support surprisingly large numbers of boreal animals. The mammals found here
tend to be habitat generalists, such as wolf, red fox, beaver, muskrat, and deer mouse, which can adapt to chan-
ging conditions and thus have some of the widest distributions among North American mammals. Other typic-
al boreal mammals include the red squirrel, nocturnal flying squirrel, woodchuck, porcupine, snowshoe hare,
moose, black bear, and caribou, as well as smaller rodents such as mice and voles—which are largely herbi-
vores. Among the major carnivores are coyote, lynx, marten, fisher, weasel, mink, and otter. Each of these spe-
cies has its own food needs and preferences, which allows all of them to survive the whole year, even the
winter, with its extreme cold and snow.
