Essentials of Ecology

CONCEPTS 10-1A, 10-1B, AND 10-1C 221

to treetop, burning whole trees. Crown fires usually
occur in forests that have not experienced surface fires

for several decades, a situation that allows dead wood,
leaves, and other flammable ground litter to accumu-

late. These rapidly burning fires can destroy most veg-
etation, kill wildlife, increase soil erosion, and burn or

damage human structures in their paths.

As part of a natural cycle, forest fires are not a ma-
jor threat to forest ecosystems. But they are serious

threats in parts of the world where people intention-
ally burn forests to clear the land, mostly to make way

for crop plantations (Concept 10-1B). This can result in

dramatic habitat losses, air pollution, and increases in
atmospheric CO 2.

Sudden oak death

White pine blister rust Pine shoot beetle Beech bark disease

Hemlock woolly adelgid

Figure 10-10 Natural capital degradation: some of the nonnative insect species and disease organisms that
have invaded U.S. forests and are causing billions of dollars in damages and tree loss. The light green and orange
colors in the map show areas where green or red overlap with yellow. (Data from U.S. Forest Service)

Accidental or deliberate introductions of foreign

diseases and insects are a major threat to forests in the

United States and elsewhere. Figure 10-10 shows some

nonnative species of pests and disease organisms that

are causing serious damage to certain tree species in

parts of the United States.

There are several ways to reduce the harmful im-

pacts of tree diseases and insect pests on forests. One is

to ban imported timber that might introduce harmful

new diseases or insects; another is to remove or clear-

cut infected and infested trees. We can also develop tree

species that are genetically resistant to common tree

diseases. Another approach is to control insect pests

by applying conventional pesticides. Scientists also use