Wildfire and Invasive Species
In the past, the stunning Mojave and Sonoran deserts
of the southwestern United States were generally considered
fireproof. There simply was not enough fuel to
carry a fire through the sparse shrubs and cacti in these
unique ecosystems.
Invasive grasses are
making desert landscapes
more prone
to fire.
Image courtesy of T. Esque,
USGS.Recently, however, some nonnative grasses have
become established in these deserts, transforming
fireproof desert into highly flammable grassland. For
example, buffelgrass, which hails originally from
Africa, is spreading rapidly over large parts of the
Sonoran Desert. Another grass, known as red brome, is
spreading through the Mojave.
These grasses are adapted to fire, sprouting again
quickly and densely after a fire sweeps through.
Unfortunately, their rapid return pushes out the native
species, including the iconic saguaro, or giant, cactus, which is not adapted to frequent fire.
While climate change is not directly implicated in the spread of these grasses, scientists are
concerned that warming temperatures will allow invasive grasses to spread further in the desert
Southwest and to extend into higher elevations.
The Piñon Pine
Large swaths of the American Southwest are covered with piñon-juniper woodlands—a vegetation
type too scrubby to be called a forest but with too many trees to be called a shrubland.
As its name implies, it is characterized by two types of evergreens: piñon pines and junipers.
Although these plants can typically tolerate drought, extreme conditions can push even these
tough species past their limits.
Photos taken from
the same vantage
point near
Los
Alamos, New
Mexico, in 2002
(left) and in
2004
(right).
Images courtesy of D.
Allen, USGS.The "Four Corners" region where New Mexico, Arizona, Colorado, and Utah meet is not known
for being particularly wet. But the drought that descended on the region from 2000 to 2003 was
abnormally severe because it combined low precipitation—25–50 percent less than the long-term
average—with unusually high temperatures. By the end of 2003, a large number of the piñons
in the region were dead. The main cause of death was infestation by the pine bark beetle, which
often targets trees that have been weakened by other stresses—in this case, heat and drought. The
widespread loss of these pine trees caused a major ecological change over a large area.
In general, we do not know the thresholds for such major changes before they occur. This
example highlights the threat that a stressful event that would not normally trigger a dramatic
ecological change may do so when an ecosystem is subject to many interacting stresses.
This web page is based on the National Academies' educational booklet
Ecological Impacts of Climate Change.
