Life-history, growth and interference of cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.)

Graduation date: 1994Cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea\ud soistitialis L.) have invaded over 3 million ha of grasslands in the western U.S.\ud Future management will require an understanding of the conditions,\ud mechanisms, and processes which control species dominance, community\ud dynamics, and desirable plant establishment in these alien dominated annual\ud rangelands. Life-history models of cheatgrass and yellow starthistle were\ud developed. Sensitivity analysis indicated that cheatgrass and yellow starthistle\ud seed output was most sensitive to population reductions during the transition\ud from juvenile to adult and adult survivorship phase of their life history. Key\ud processes associated with these transition phases are interference (competition),\ud growth rates and duration, and reproductive allocation. The objective of this\ud research was to investigate those key processes.\ud Several addition series experiments, with total densities ranging from 20-\ud 20000 plants m⁻², were conducted to quantify the intensity of interference\ud between cheatgrass and yellow starthistle. In unrestricted soil depths,\ud intraspecific interference was approximately twice as important as interspecific\ud interference for both species with respect to predicting plant weight throughout\ud the growing season and seed production. Resource partitioning via rooting\ud depth was evident. The ecological importance of strong intraspecific\ud interference was associated with being strong competitors, the advantages of\ud self-thinning, and regulating community composition. Decreasing soil depth\ud altered the competitive balance toward the relatively shallow and fibrous rooted\ud cheatgrass.\ud In a growth chamber study, isolated seedling growth rates of both species\ud were similar, however yellow starthistle roots grew more geotropically than\ud those of cheatgrass. In field experiments, yellow starthistle grew more rapidly\ud than cheatgrass after the seedling stage. We believe that rapid and geotropic\ud growth allowed deeper soil penetration, continued growth and increased seed\ud output of yellow starthistle over that of cheatgrass. As densities increased\ud and/or soil depth decreased, the growth rates and maturation dates of\ud cheatgrass and yellow starthistle became increasingly similar and lower