Fire and Invasive Annual Grasses in Western Ecosystem
A project funded by the Joint
Fire Science Program
Great Basin Sagebrush Steppe |
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| 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 |
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| 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 |
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Mojave Desert Scrub |
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| 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 |
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