10,000,000Acreage burned (number of acres)9,000,000
8,000,000
7,000,000
6,000,000
5,000,000
4,000,000
3,000,000
2,000,000
1,000,000
0
Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec
Month2011
2001–2010
(average)U.S. acres burned in 2011, compared to 2001–2010 average.NASA Images© Albuquerque Journal/ZUMAPress/CorbisCourtesy of National Oceanic and Atmospheric Administration (NOAA 2011)A time exposure image highlights the scale of
New Mexico’s Las Conchas fire, June 27, 2011.Wildfires
A wildfire is any unexpected—and unwanted—fire that
burns in grass, shrub, and forest areas. Whether started
by lightning or by humans, wildfires are an important
environmental force in many geographic areas, especially
places with wet seasons followed by dry seasons, such as
chaparral. Vegetation that grows during the wet season dries
to tinder during the dry season. After fire ignites the dry
organic material, wind spreads the fire through the area.
At the peak of the wildfire season in the American West
and Southwest, an area prone to wildfires, hundreds of new
wildfires can break out each day. In 2011, dry conditions
triggered U.S wildfires that consumed 3.5 million hectares
(8.7 million acres), an annual total of destruction that
ranked third in the past 12 years, whereas the number of
fires reported in 2011 was about average for that period
(see graph). Arizona, New Mexico, and Texas in particular
experienced record wildfires (see photos).
Fires have several effects on the environment. First,
combustion frees minerals locked in dry organic matter.
The ashes left by fire are rich in potassium, phosphorus,
calcium, and other nutrient minerals essential for plant
growth. Thus, vegetation flourishes after a fire. Second, fire
removes plant cover and exposes the soil, which stimulates
the germination of seeds that require bare soil and the
growth of shade-intolerant plants. Third, fire increases soil
erosion because it removes plant cover, leaving soil more
vulnerable to wind and water.
Fires were a part of the natural environment long
before humans appeared, and many terrestrial ecosystems
have adapted to fire. Grasses adapted to wildfire have
underground stems and buds. After fire kills the aboveground
parts, the untouched underground parts send up new
sprouts. Fire-adapted trees such as bur oak and ponderosa
pine have thick, fire-resistant bark; others, such as jack pine,
depend on fire for successful reproduction because the fire’s
heat opens the cones and releases the seeds.
Human interference also affects the frequency and
intensity of wildfires, even when the goal is fire prevention.
When fire is excluded from a fire-adapted ecosystem, organic
litter accumulates. As a result, when a fire does occur, it
burns hotter and is much more destructive than ecologically
helpful. Decades of fire suppression in the West are partly
responsible for the massively destructive fires that have
occurred there in recent years. Prescribed burning is an
ecological management tool that allows for controlled
burning to reduce organic litter and suppress fire-sensitive
trees in fire-adapted areas.
CASE STUDY
✓✓THE PLANNERNASA satellite image shows two major wildfires burning in
Arizona, June 8, 2011.