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Fire and Invasive Annual Grasses
in Western Ecosystem
Many species of invasive annual grasses, including cheatgrass (Bromus
tectorum), red brome (Bromus rubens), and medusahead (Taeniatherum
caput-medusae) increase in dominance after fire and establish grass/fire
cycles. Although annual grass invasion is commonly attributed to fire there
are areas of minimal disturbance in which alien annual grasses occur and
areas of high disturbance, which remain invasion-free. Soil characteristics
may be the most likely controlling factor in this variance. Soil nutrient availability is one of the most important soil characteristics
affecting plant community composition. Annual plants generally require
higher levels of most nutrients than perennial plants, and native plants
that grow well on infertile soils respond less to increased nutrient
levels than weedy species. Annual plants generally respond positively
to increased levels of nitrogen and phosphorus, especially in low nutrient
ecosystems such as the Great Basin Shrub-Steppe, Mojave Desert Scrub
and Sierra Nevada Yellow Pine Forests, making these ecosystems more susceptible
to invasion. However, the relative importance of these two nutrients
varies depending on concentrations of soil carbonates, which are very
common in high pH desert soils. In low calcium carbonate areas nitrogen
is the most common limiting nutrient for plant use, but in water limited
regions calcium carbonate binds phosphorus, limiting its availability
for plant use and making it the limiting nutrient in soils that are high
in calcium carbonates.
Fire can either increase or decrease soil nutrient availability which
should affect the relative productivity of invasive annual grasses. Theoretically
various temperatures will affect specific nutrients differently, which
will affect invasive annual grasses that depend on these nutrients. Soil
properties such as texture, chemistry, and moisture as well as the amount
and duration of soil heating strongly influence nutrient availability.
Very intense fires can kill seeds of invasive annual grasses located
in the soil in addition to reducing soil nutrient levels. Less intense
fires can have the same effect if fire spread is slow and against the
wind, fuel consumption is complete, and relative humidity is low. Spring
fires can be more effective than summer fires at controlling invasive
annual grasses if timed to burn when seeds are still attached to inflorescences
and have a high moisture content. A less intense fire may also increase
availability of soil nutrients, thus increasing the invasibility of postfire
landscapes even if a significant number of seeds are killed.
The positive relationships between fire, soil nutrient availability,
and the invasion of annual grasses is likely to be strongest in nutrient
limited soils, where fires should generate the greatest increase in available
nutrients, thus favoring invasive aliens over native plants. Identifying
what soil and other site factors confer natural resistance to annual
grass invasion, and understanding how these factors are altered by fire,
is a critical first step in prediction, prevention and mitigation of
these invasions. With such information, managers could 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 and restoration techniques could be better targeted.
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URL http://www.werc.usgs.gov/fire/lv/fireandinvasives/ background.htm
Contact: Webmaster
Last Modification: July 21, 2004
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