OBSERVATION - LIFE-HISTORY OF SPOTTED KNAPWEED

Spotted knapweed (Centaurea maculosa Lam,) is a non-indigenous weed in festing large areas of rangeland in western North America. Life histor y models have been used to identify key processes regulating weed popu lation dynamics and may be valuable in developing and testing integrat ed weed management strategies. Our objective was to characterize the l ife history of spotted knapweed, Demographic attributes were monitored monthly during snow free periods beginning August 1994 through Octobe r 1996 on 2 sites. Data were arranged into life history tables, and se nsitivity analysis was performed to determine key transition phases af fecting seed output. Spotted knapweed seed production ranged from 998 to 7815 viable seeds/m(2) at both sites during the study. Seeds reachi ng the soil averaged 41 and 50% of seed output at sites 1 and 2, respe ctively. Less than 6% of seeds reaching the soil germinated in the fal l 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 kn apweed juvenile density peaked August 1995 and June 1996 at both sites . Peaks corresponded with the beginning of the summer dry period. Plan ts bolted beginning June 1995 and May 1996, Sensitivity analysis ident ified early-summer juvenile survivorship, late-summer adult survivorsh ip, transition from juvenile to adult, and seeds produced per adult as critical stages for spotted knapweed seed output. Management strategi es 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 demog raphic data was developed and can be used to design and test integrate d spotted knapweed strategies.