Observation: Comparative Live-History of Cheat-Grass and Yellow Starthistle

Abstract

The objective of this research was to characterize the life-histories of cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) growing in association. Biweekly demographic attributes were monitored during 1991 (moist spring) and 1992 (dry spring). Data were arranged into life-history tables, and sensitivity analysis was performed to determine key transition phases. The entire cheatgrass seed crop reached the soil surface, 41% of yellow starthistle's seed output was lost during seed rain. Frost heaving reduced winter seedling populations of cheatgrass (53%) more than yellow starthistle (40%). All cheatgrass seedlings surviving the frost heaving period became adults. Yellow starthistle density was reduced by 75% during the juvenile phase. Cheatgrass adults appeared about 6 weeks before yellow starthistle adults. Cheatgrass seed output remained about 7,000 m2 with moist and dry spring conditions. Yellow starthistle seed output was about 21,600 m2 and 5,200 m2 with moist and dry spring conditions, respectively. Reduction of yellow starthistle seed output with dry spring conditions suggest oscillatory community dynamics. Key processes associated with life-history transitions were interference (competition), resource acquisition rates and duration, and reproductive allocation.