Botkin Environmental Science Earth as Living Planet 8th txtbk

12.1 Forests and Forestry 241
FIGURE 12.5 Forest ownership in the lower 48 states of the United States in 2008.
(Source: U.S. Forest Service, Northern Research Station.)
As we mentioned, wood is a major energy source in
many parts of the world. Some 63% of all wood produced
in the world, or 2.1 million m 3 , is used for firewood. Firewood
provides 5% of the world’s total energy use, 10 2% of
total commercial energy in developed countries, but 15%
of the energy in developing countries, and is the major
source of energy for most countries of sub-Saharan Africa,
Central America, and continental Southeast Asia. 11
As the human population grows, the use of firewood
increases. In this situation, management is essential, including
management of woodland stands (an informal
term that foresters use to refer to groups of trees) to improve
growth. However, well-planned management of firewood
stands has been the exception rather than the rule.
How Forests Affect the Whole Earth
Trees affect the earth by evaporating water, slowing erosion,
and providing habitat for wildlife (see Figure 12.6). Trees
can also affect climate. Indeed, vegetation of any kind can
affect the atmosphere in four ways, and since forests cover
so much of the land, they can play an especially important
role in the biosphere (Figure 12.7):
1. By changing the color of the surface and thus the
amount of sunlight reflected and absorbed.
2. By increasing the amount of water transpired and
evaporated from the surface to the atmosphere.
3. By changing the rate at which greenhouse gases are
released from Earth’s surface into the atmosphere.
4. By changing “surface roughness,” which affects wind
speed at the surface.
In general, vegetation warms the Earth by making the
surface darker, so it absorbs more sunlight and reflects less.
The contrast is especially strong between the dark needles
of conifers and winter snow in northern forests and between
the dark green of shrublands and the yellowish soils
of many semiarid climates. Vegetation in general and forests
in particular tend to evaporate more water than bare
surfaces. This is because the total surface area of the many
leaves is many times larger than the area of the soil surface.