PARENT SESSION
Oral Contributed Papers: Case Studies
Moderator: Seastedt, Tim¹, ¹ University of Colorado, Boulder, CO
Wednesday, November 5, 2003, 10:00 AM to 12:00 PM
Key West

Effects of insect herbivores, soil nutrients, and plant competition on diffuse knapweed.

Seastedt, T.^*,1, LeJeune, K.², Suding, K.¹, ¹ University of Colorado, Boulder, CO² Stratus Consulting Inc., Boulder, CO

ABSTRACT- Diffuse knapweed(Centaurea diffusa) is an invasive aster of Eurasian origin that has colonized tens of thousands of hectares of the Colorado Front Range and over a million hectares of land in the western US. A study begun in 1997 demonstrated the extent to which insect herbivores, soil nutrient characteristics, and plant competition can affect the abundance of this invasive species. Five species of insects, including Larinus minutus, a seedhead weevil, reduced relative cover of diffuse knapweed from about 30% of all vegetation in 2000 to less than 1% of cover in 2002. Insect herbivory impacts were reduced under conditions of higher soil nitrogen and phosphorus levels. Knapweed showed the greatest growth potential under the absence of plant competition, and seed production of this weed was substantially reduced in the presence of native species. The ability of knapweed to compete with native species was not enhanced by increased soil N and P. Reductions of soil N and P by using amendments to induce N, P, or N+P limitations showed that while the biomass of the weed was affected, its competitive abilities remained largely unchanged. The ability of this species to function as an aggressive invasive weed appears largely related to its reproductive output and not its ability to manipulate or exploit soil resources.

Key words: biocontrols, Centaurea diffusa, nutrients, competition