COMPARATIVE LIVE-HISTORY OF CHEATGRASS AND YELLOW STARTHISTLE - OBSERVATION

The objective of this research was to characterize the life-histories of cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea s olstitialis L.) growing in association. Biweekly demographic attribute s were monitored during 1991 (moist spring) and 1992 (dry spring). Dat a were arranged into life-history tables, and sensitivity analysis was performed to determine key transition phases. The entire cheatgrass s eed crop reached the soil surface, 41% of yellow starthistle's seed ou tput was lost during seed rain. Frost heaving reduced winter seedling populations of cheatgrass (53%) more than yellow starthistle (40%). Al l cheatgrass seedlings surviving the frost heaving period became adult s. Yellow starthistle density was reduced by 75% during the juvenile p hase. Cheatgrass adults appeared about 6 weeks before yellow starthist le adults. Cheatgrass seed output remained about 7,000 m-2 with moist and dry spring conditions. Yellow starthistle seed output was about 21 ,600 m-2 and 5,200 m-2 with moist and dry spring conditions, respectiv ely. Reduction of yellow starthistle seed output with dry spring condi tions suggest oscillatory community dynamics. Key processes associated with life-history transitions were interference (competition), resour ce acquisition rates and duration, and reproductive allocation.