Fire and Invasive Annual Grasses in Western Ecosystem

A project funded by the Joint Fire Science Program

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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.