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Impact of Fire and Grazing
on Diversity and Invasion in Sierran Forests | |
| Two important disturbance factors in the Western United States are grazing and fire, and both have been linked to plant invasions. Disturbances often create an imbalance in distribution of resources, which may alter species composition through shifts in resource availability. These in turn may create conditions favoring invasion of nonnative species and deletions of native species. |  |
| In the foothill grasslands and savannas of California, nonnative grasses and forbs dominate due to a history of over-grazing coupled with extreme droughts. These ecosystems have already been so heavily invaded by nonnative grasses and forbs that present livestock grazing at low to moderate stocking densities is not tied to shifts in the native/nonnative dominance. However, recent U.S. Geological Survey (USGS) research in California’s southern Sierra Nevada shows that grazing by different types of livestock in foothill woodlands may alter species composition and distribution of plant functional types or growth forms. | |
| In the higher elevation coniferous forests, invasive plants have not previously posed a significant threat. However, the recent reintroduction of fire to these forests may provide conditions conducive to the invasion and spread of some weeds. USGS research in the southern Sierra Nevada has shown that gaps created by high-intensity fires are particularly susceptible to invasion by nonnative plant species. Such invasions may be controlled by managing for low-intensity fires, but this may not be desirable because native species richness and tree seedling recruitment were also found to be favored by high-intensity fires. | |
| In these Sierran ecosystems, the threat of invasive species is most profound in the lower elevations and decreases with elevation. Part of the explanation for this pattern is the fact that most invasives in this region are annual plants and this growth form declines in both species number and dominance with elevation. The lower elevation ponderosa pine forests are potentially most susceptible to new invasions, and particularly troublesome is the apparently recent expansion of cheatgrass (Bromus tectorum) in these forests in Kings Canyon National Park. |  |
| As is the case with species diversity in general, the invasion of cheatgrass is strongly correlated with localized gaps produced by high-intensity burning. Because of this apparent relationship between fire and cheatgrass, prescribed burning has been temporarily halted in these forests. Early control of this apparent invasion is of concern to resource managers in Sierra Nevada parks, and a more detailed study of fire and other concerns regarding cheatgrass invasion are currently being investigated by the USGS. | |
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USGS scientists are conducting research through the Joint Fire Science Program to determine if fire contributes most to plant invasion in low-nutrient soils by making available increased nutients that invasive grasses may exploit more effectively than native flora. Soil nutrient changes can vary widely depending on soil properties and the amount and duration of soil heating. In forested ecosystems of the Sierra Nevada such impacts of fire on soil nutrients and light solar radiation are potentially more profound now than under historic conditions because of unnaturally high fuel loads resulting from fire suppression.
The researchers will use intensive field manipulations at sites in Kings Canyon National Park that will couple burning, nutrients, fuels and light treatments. In addition, extensive surveys will be conducted to assess the soil nutrient status associated with invaded and non-invaded sites. These field studies will be coupled with laboratory studies to examine in detail the relationship between soil heating and nutrient availability for invasive grasses. With this information, managers would be able to determine in advance if habitats are naturally vulnerable or resistant to invasions, enabling limited resources to be more effectively deployed both during and after fires. Fire prescriptions could be designed to avoid creating conditions susceptible to invasion, plus, restoration techniques could be better targeted, saving both time and money. | |
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Jon E. Keeley Research Ecologist Sequoia and Kings Canyon Field Station Sequoia and Kings Canyon Natl. Parks 47050 Generals Highway Three Rivers, CA 93271-9651 Phone: (559) 565-3170 Fax: (209) 565-3177 Email: jon_keeley@usgs.gov |
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