Peter Essick / Aurora Photosand are dominated by coniferous evergreen trees such
as spruce, fir, cedar, and hemlock. The boreal forest
biome is the world’s largest, covering about 11 percent of
Earth’s land. Harvested primarily through clear-cut log-
ging, boreal forests are the primary source of the world’s
industrial wood and wood fiber. The annual loss of boreal
forests is estimated to encompass an area twice as large as
the Amazonian rain forests of Brazil.
About 1 million hectares (2.5 million acres) of
forest in Canada—currently the world’s biggest timber
exporter—are logged annually (Figure 13.5). Most of
Canada’s forests are subject to logging contracts, known
as tenures, between provinces and companies. On the
basis of current harvest quotas, logging was widely con-
sidered unsustainable in Canada until recently. In 2010
logging companies and environmental groups brokered
an agreement in which logging was suspended in more
than 300,000 km^2 (about 116,000 mi^2 ) of boreal forest,
an area the size of Great Britain. An additional 385,000
km^2 (about 149,000 mi^2 ) were designated for restricted
logging using sustainable guidelines.
Extensive tracts of Siberian forests in Russia are
harvested, although estimates are unavailable. Alaska’s
boreal forests are at risk because the U.S. government
may increase logging on public lands in the future.America and Central America did not change between
2000 and 2010, and Europe and Asia actually gained for-
ested areas, through either natural regrowth or increas-
ing forest plantations.
Results of Deforestation Deforestation results in
decreased soil fertility, as the essential mineral nutrients
found in most forest soils leach away rapidly without trees
to absorb them. Uncontrolled soil erosion, particularly
on steep deforested slopes, affects the production of
hydroelectric power as silt builds up behind dams.
Increased sedimentation of waterways caused by soil ero-
sion harms downstream fisheries. In drier areas, defores-
tation contributes to the formation of deserts (discussed
shortly). Regulation of water flow is disrupted when a for-
est is removed, so that the affected region experiences
alternating periods of flood and drought.
Deforestation contributes to the extinction of many
species. (See Chapter 15 for a discussion of the importance
of tropical forests as repositories of biological diversity.)
Many tropical species, in particular, have limited ranges
within a forest, so they are especially vulnerable to habitat
modification and destruction. Migratory species, including
birds and butterflies, also suffer because of deforestation.
Deforestation contributes to regional and global cli-
mate changes. Trees release substantial amounts of mois-
ture into the air; in the hydrologic cycle, about 97 percent
of the water that roots absorb from the soil is evaporated
directly into the atmosphere and then falls back to Earth.
When a large forested area is removed, local rainfall may
decline, droughts may become more common in that
region, and temperatures may rise slightly. Studies sug-
gest that the local climate has become warmer and drier
in parts of Brazil where huge tracts of the rain forest were
burned. As the forest continues to shrink, it is possible
that the changing regional climate will no longer be able
to support the forest. Thus, large parts of what had once
been tropical rain forest could become savanna.
Deforestation also contributes to an increase in
global temperature by releasing carbon originally stored
in the trees into the atmosphere as carbon dioxide, which
enables the air to retain heat. When an old-growth for-
est is harvested, researchers estimate that it takes about
200 years for the replacement forest to accumulate the
equivalent amount of carbon stored in the original trees.
Boreal Forests and Deforestation Extensive
deforestation in boreal forests due to logging began in
the late 1980s and continues today. Boreal forests occur
in Alaska, Canada, Scandinavia, and northern Russia
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About 80 percent of Canada’s forest products are exported to
the United States.