|
These measures
deal with the incidence and area affected by both native and nonnative
plants, insects, and diseases.
The proportion
of forest composed of nonnative plants, because nonnative ("exotic")
plants that spread rapidly ("invasive") can displace native plants species,
damage wildlife habitat, and dramatically alter forest composition.
(Fig. 1)
The area of
forest damaged by both native and nonnative insects and diseases,
as a measure of the extent to which both exotic and native insects and
diseases affect forests. Native insects play vital roles in forests,
but major outbreaks can adversely affect recreation, wildlife habitat,
wood production, and ecological processes. (Fig. 2,
3)
Newly introduced
insects and diseases of concern, because newly introduced species,
if they gain a foothold, can dramatically change U.S. forests. Historical
examples include the chestnut blight and Dutch elm disease, both of
which are nonnative, and which transformed American forests by virtually
eliminating once-dominant trees. (Fig. 4)
Nonnative plants
cover one-fourth of the area of the forest "understory" layer (smaller
trees and shrubs) in California, and 20 percent of that in the Mid-Atlantic
region. Coverage in other regions where studies have been conducted
is lower.
Five species of
insects defoliate about 3 percent of U.S. forests (21 million acres)
each year, although individual species go through boom-and-bust cycles.
All except the gypsy moth are native, and all showed major variations
in prevalence around their 10-year average. Fusiform rust (a disease)
affects about 6 percent of southern forests (13 million acres), and
dwarf-mistletoe (a parasitic plant) affects roughly 13 percent of western
forests (29 million acres).
One example of
a newly introduced, damaging insect, the Asian longhorned beetle, has
been detected both in native trees and in imported packing materials
in warehouses. This beetle has the potential to severely damage maple
and other popular trees.
| Exotic
Plants In Selected Regions, 1994 (1) Technical
Note |
Average
Acreage Damaged Per Year,
1986-1995 (2) Technical
Note |
 |
 |
| Source:
USDA Forest Service, Forest Health Monitoring |
Source:
USDA Forest Service, Forest Health Monitoring |
Cycles
Of Insect Activity,
1986-1995 (3) Technical
Note |
A
New Invader—
Asian Longhorned Beetles (4) Technical
Note |
Source:
USDA Forest Service, Forest Health Monitoring |
 |
|
|
| Source:
USDA Animal and Plant Health Inspection Service |
|
STATUS
OF DATA & OTHER NOTES
|
No consistent
and comprehensive information is available on the existence of
exotic plants in forests. The data here are from the Forest Service's
Forest Health Monitoring Program, which will cover additional
areas in the future.
Data on insect
damage are from continuing monitoring programs of the Forest Service.
Data on forest diseases are less complete than are those for insects.
Other invasive
insects and diseases certainly exist, and efforts are under way
to develop authoritative "watch lists" for such pests.
Please see
the Technical
Notes for additional information.
|
|
