| A great band of evergreen conifer
forests extends around the northern hemisphere
in the subarctic latitudes.
Within this zone, the growing season is brief and the winter is long and
cold. These circumstances exclude most broadleaf deciduous
trees, which require more time to regrow their leaves than the subarctic climate
provides. Evergreen
trees, on the other hand, are ready to resume photosynthesis whenever the
air grows mild enough. Furthermore, the compact, cylindrical shape and
waxy skin of the needle-like leaves retard moisture loss during the
winter, when environmental water is unavailable because it is frozen. |
|
| Compared with the forests
of lower latitudes, the boreal
forests—often called by their Russian name of taiga—are
rather uniform in structure and species
composition. Trees, often in dense stands of even age, dominate. Many
assume a narrow, pointed habit that permits snow to be shed readily.
Ground vegetation
within the shady forest is sparse, comprising low shrubs, many of them in
the heath
family, and a layer of mosses. Another conspicuous feature of the taiga
region, from which the ice
sheets retreated only about 10,000 years ago, is the countless ponds
and bogs
that pock the flat and poorly drained terrain.
In Eurasia,
the taiga spans the continent
from the Russian Far East to Scandinavia.
The westernmost forests are very monotonous, but the number of species
increases as one moves to the east, and the forests of Hokkaido
in northern Japan
are comparatively diverse. An exception to the evergreen character of most
of the taiga is the deciduous larch forests of eastern Siberia,
where the bitterly cold winters of the continental interior make the
retention of any leaves, even needles, a liability.
The boreal forests of North
America are richer in species than their Eurasian counterparts
but are in other ways similar. The north–south alignment of North
American mountain
ranges means that taiga-like conifer forests extend far into the temperate
zone along the spines of the Cascade
Range and Sierra
Nevada, the Rocky
Mountains, and the Appalachians. |
