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 sixteen macro-plots (containing twenty, 1 meter square quadrats) were established in eight 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 was the dominant weed. For the macro-plots with spotted knapweed, there were 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 adult knapweed. However, for both types, there was a negative relationship between fire severity and number of knapweed seedlings. This implies that fires of all severities had little impact on established plants, but reduced the 2000 weed seed crop that could lead to the invasion and expansion of exotic species in native plant communities. The full impact of wildfire on exotic weed invasion had yet to be realized.

In 2002, two years after the fire, all macro-plots were re-sampled. Weeds had not only increased in density on macro-plots that previously had weeds, but weeds had colonized burned macro-plots that were previously weed free. While bunchgrass, ponderosa pine and Douglas-fir communities were most impacted by weeds, spotted knapweed had also invaded riparian areas. Although spotted knapweed remained the dominant weed, bull thistle had increased in numbers and both species had colonized weed-free Douglas fir macro-plots. Knapweed densities remained highest in bunchgrass communities and changes in knapweed densities were related to fire severity with a greater increase on burned sites than unburned sites. Ponderosa pine sites showed a similar pattern with a greater increase in knapweed densities on sites with crown fires than surface fires and burned sites than unburned sites. Most of the increase in knapweed densities was due to seed germination and seedling establishment.