|
|
Fire and Invasive Annual Grasses
in Western Ecosystem
Great Basin Sagebrush Steppe |
|
|
Historic fire return intervals have been shown to vary
in sagebrush communities.
All estimates of sagebrush recovery have been greater than 10 years. |
This type-converted grassland has fire recurrence intervals
of less than 10 years, which is not enough time for recovery of
sagebrush vegetation. |
The ecology and effects of invasive annual grasses
have been studied most extensively in this ecoregion, especially
focusing on cheatgrass (Bromus tectorum) and more recently
including medusahead (Taeniatherum caput-medusae). The invasion
of cheatgrass is the
best documented
example of how annual grasses can cause major changes in natural
fire regimes. Vast areas have been type-converted from semi-arid
shrubland to invasive annual grassland, which is highly flammable
and promotes recurrent fire. The wildfires of 1999 demonstrated the
extreme threat that invasive annual grasses present to the protection
of property and natural resources in this region. Although the threats
are undisputable, land managers cannot reliably predict the conditions
under which invasive annual grasses will dominate and manifest their
negative effects nor do they have any reliable basis for comparing
the relative effects of methods to control them. |
Sierra Nevada Yellow Pine Forest |
|
|
An area in Kings Canyon National Park where the litter
of pine needles has kept cheatgrass from establishing. |
Cheatgrass has established in this area of the
park making it more vulnerable to fire. |
Within the last decade it has become obvious that
cheatgrass has made significant inroads in the
lower elevation yellow pine forests of the southern Sierra Nevada.
As is the case with other forest types, annual grass invasion does
not post a threat where long term fire exclusion has been effective,
but it dose post a potential problem in forests subjected to a
reintroduction of fire. This has been studied most closely in the
Cedar Grove area of Kings Canyon National Park where an alarming
increase in cheatgrass has been noted in recent years and is correlated
with an active prescription burning program in these forests. So
alarming is this relationship that the National Park Service has
suspended prescription burning in these low elevation forests of
Kings Canyon. Similar cheatgrass invasion has been reported from
other lower elevation yellow pine forests in other parts of California,
Washington, and Idaho. Currently the fuels in these forests are
woody debris and needles, however, it is widely held that under
historic conditions, grasses were and integral part of the fuel
structure, and thus the potential exists for invasive grasses to
alter fire behavior.
Relative to other coniferous forest types the yellow pine ecosystem
is distributed at the more arid and low nutrient end of the gradient.
Preliminary soil comparisons of cheatgrass dominated sites in Kings
Canyon National Park show that nitrogen levels are high relative
to non-invaded sites with the same fire history. These soil comparisons
also showed K/Mg (potassium/magnesium) ratios were not a likely
explanatory factor, as may be the case in some desert soils. Recent
studies have shown out of 60 sites studied throughout the southern
Sierra Nevada, cheatgrass invasion was most predictable in forest
patches that were burned with high intensity. This is troublesome
in that such high intensity gaps are often the only sites where
the dominant tree species demonstrate successful seedling recruitment.
Circumstantial evidence suggests that such patches require extended
fire-free conditions for two to three decades for successful pine
recruitment. Thus, cheatgrass, through direct competition with
tree seedlings as well as accelerating the fire frequency, may
have long range impacts on forest dynamics |
Mojave Desert Scrub |
|
|
|
Blackbrush (Coleogyne ramosissima) vegetation
type near the Beaver Dam Wash in SW Utah. |
Type converted invasive annual grassland dominated
by red brome (Bromus madritensis ssp. rubens) |
The understory of this blackbrush
is dense with invasive annual grasses. |
Invasive annual grasses have been studied very little
in this ecoregion, largely because managers have not considered them
a significant ecological threat until the past decade. It was generally
thought that the hotter and drier conditions in the Mojave Desert
would not allow species such as cheatgrass to establish the
large amounts of continuous biomass that had caused significant fire
threats in the Great Basin. So far this has been generally true,
with cheatgrass producing significant amounts of fuels only
at very high elevations and the more mesic margins of the Mojave
Desert. However, red brome emerged as a dominant component of
the flora during the 1970’s and was strongly associated with
increased fire frequency across this ecoregion during the 1980 and
1990’s. This species now dominates postfire landscapes in regional
hotspots where fire return intervals are now 5-10 years, similar
to what has been observed in the Great Basin. If methods to mitigate
the effects of invasive annual grasses are not developed, then the
Mojave Desert may be destined for the extreme wildfire problems that
currently exist in the Great Basin. |
|
|
|
URL http://www.werc.usgs.gov/fire/lv/fireandinvasives/ study_ecosystems.htm
Contact: Webmaster
Last Modification: July 21, 2004
|
|