PARENT SESSION
Poster Session #57: Invasive Species Ecology: Grasslands.
Thursday, August 8. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Post-fire weed response in western Montana.

Sutherland, Steve^*,1, ¹ Fire Sciences Laboratory, Missoula, MT

ABSTRACT- In 2000, wildfires burned more than 200,000 acres on the Bitterroot National Forest of Montana and nearly 1.5 million acres in the Northern and Intermountain Regions. These fires increased light and nutrient levels, reduced plant competition, and increased exposure of bare soil. These conditions favor the invasion and expansion of exotic species in native plant communities. In 2001, a five-year study was initiated to determine the impact of the fires on exotic weed invasion. One hundred and ten macro-plots (containing twenty, 1 meter square quadrats) were established in seven vegetation types at four fire severities in the Bitterroot National Forest. Data were collected on the number of seedlings, number of adults, and percent cover of weed species and compared to physiographic and biological data. One year after the fire, there were significant differences in the susceptibility of vegetation types to weed expansion. The warmer and drier types (bunchgrass, ponderosa pine and Douglas-fir) were the only communities with significant weed populations, and spotted knapweed (Centaurea biebersteinii) was the dominant weed. For the macro-plots with spotted knapweed, there were significantly higher densities of knapweed stems (seedlings plus adults) in bunchgrass compared to forested types. Within bunchgrass or ponderosa pine types, there was no significant relationship between fire severity and densities of knapweed stems. However, for both types, there was a significant relationship between fire severity and proportion of knapweed stems that were seedlings. Specifically, unburned bunchgrass and ponderosa pine macro-plots had a higher proportion of seedlings than burned macro-plots. This implies that fire of all severities reduced the weed seed bank and that the full impact of wildfire on exotic weed invasion has yet to be realized. The impact of fire suppression and post-fire restoration on weed invasion was also examined.

KEY WORDS: Centaurea biebersteinii, spotted knapweed , fire, exotic species