Forests are widespread over the tropical and middle latitude regions of the
globe. Humans have actively utilized and cleared this resource causing marked
lessening of areal coverage in the last few hundred years and a striking decrease
in the last few decades (seeDeforestation). The net effect is that global forest
cover has decreased by about one-fifth since prehistoric times. As is true with
other physical elements of Earth, forest cover is unequally distributed. According
to the Food and Agricultural Organization of the United Nation, about two-thirds
of Earth’s forest cover is split among ten countries, with the largest amount of
forest in the Russian Federation. The forested area of Earth is roughly 38 million
sq km (14.7 million sq mi). This constitutes about 30 percent of Earth’s land area.
About half the forests are in the tropics, a third in upper middle latitudes and lower
polar latitudes of the Northern Hemisphere, a tenth in temperate climates, and the
rest in the subtropics.
Forests require a growing season long enough to add significant biomass via
photosynthesis and conditions in which precipitation is greater than evapotranspi-
ration to ensure a reasonable supply of soil moisture. Forests were able to evolve
in various climates so there are various types and subtypes of forests. At the great-
est level of abstraction, three forest types are usually delineated: tropical, temper-
ature, and boreal.
Tropical forests are associated with the wetter tropical places. The tropical for-
est has some subtypes. Therainforesthas the most superlatives attached to it.
Found associated with the rainforest climate, this subtype is associated with the
greatest number of species (both trees and other life), the tallest species (some
50 m and more), and the greatest biomass per hectare on Earth. Some rainforests
receive more than 7,500 mm (250 in) of rain per year; there is no dry season.
The trees are evergreen, losing and gaining leaves gradually rather than in antici-
pation of seasonal dryness or freezing temperatures. Themonsoon forest(moist/
dry deciduous forest) has a considerable dry season in the low-sun part of the year,
huge precipitation totals in the high-sun period, and no freezing temperatures.
These tropical trees are broadleaf species and seasonally drop their leaves in
anticipation of winter dryness. Thetropical deciduous forestis found in climates
in which the precipitation totals are not as great as in the first two subtypes and
the drier season longer. These forests are sometimes known as semi-evergreen for-
ests because the taller trees are deciduous while the understory trees are evergreen.
Temperate forests are found in central and western Europe, the eastern half of
North America, northeastern Asia, and in middle latitudes along the west coasts
of continents. Growing seasons are up to six months long with annual precipitation
greater than evapotranspiration. Winter and a concomitant lowering/stoppage of
photosynthesis are limitations to growth. Temperate types are delineated primarily
on the basis of seasonality and amount of precipitation. Mountains and high132 Forests