Observation: Life History of Spotted Knapweed

Abstract

Spotted knapweed (Centaurea maculosa Lam.) is a non-indigenous weed infesting large areas of rangeland in western North America. Life history models have been used to identify key processes regulating weed population dynamics and may be valuable in developing and testing integrated weed management strategies. Our objective was to characterize the life history of spotted knapweed. Demographic attributes were monitored monthly during snow free periods beginning August 1994 through October 1996 on 2 sites. Data were arranged into life history tables, and sensitivity analysis was performed to determine key transition phases affecting seed output. Spotted knapweed seed production ranged from 998 to 7815 viable seeds/m2 at both sites during the study. Seeds reaching the soil averaged 41 and 50% of seed output at sites 1 and 2, respectively. Less than 6% of seeds reaching the soil germinated in the fall at both sites. Recruitment peaked in April at 36% and in June at 20% of seeds reaching the soil on sites 1 and 2, respectively. Spotted knapweed juvenile density peaked August 1995 and June 1996 at both sites. Peaks corresponded with the beginning of the summer dry period. Plants bolted beginning June 1995 and May 1996. Sensitivity analysis identified early-summer juvenile survivorship, late-summer adult survivorship, transition from juvenile to adult, and seeds produced per adult as critical stages for spotted knapweed seed output. Management strategies that reduce spotted knapweed populations at these stages are likely to have the greatest impact on spotted knapweed population growth and spread. A weed population dynamics model using the life history demographic data was developed and can be used to design and test integrated spotted knapweed strategies.