Yellow starthistle (Centaurea solstitialis L.) is an introduced Asteraceae that has become established on 10 million acres in the Pacific Northwest and California. This weed functions as an annual or short-lived perennial and depends on seeds for reproduction. Strategies of control that reduce plant fitness or lower seed production or viability may help limit the rate of spread of yellow starthistle. Previous work has shown that grazing and mowing can influence seed production. This study tested the hypothesis that proper timing and frequency of defoliation can reduce the number and viability of seeds produced. The study was conducted in Umatilla County, Oregon using a randomized block design with 4 replications of each of 4 defoliation treatments: (1) single defoliation at the bolting stage; (2) single defoliation at the bud stage; (3) two defoliations, once at the bolting stage and again at the bud stage; (4) non-defoliated control. Each of 4 blocks consisted of a 12 x 12 m area, with 16 plots measuring 3 x 3 m. Plants were defoliated at ground level using a gas-powered string-type mower. Response measurements were collected at the end of the growing season (September) following potential regrowth and included: (1) number of seedheads per plant; (2) number of seeds per seedhead; (3) number of seeds per plant; (4) number of seeds m⁻², (5) seed viability (% germination rates). Supporting measurements included: seedhead diameter; plant height, number of branches per plant; pre-dawn xylem pressure; soil moisture; and documentation of 5 biological control insect species. A single defoliation at bolting resulted in fewer seeds per seedhead, and fewer seeds per plant than non-defoliated controls. A single defoliation at the floral bud stage or repeated defoliation (bolting and again at the bud stage) resulted in equally fewer seeds per plant and fewer seeds m⁻² compared to non-defoliated controls. There was no statistical difference in percent germination of seeds among treatments. Defoliation had no effect on the infestation rates of seedheads by biological control insects. A second study examined nutrient content of yellow starthistle during 6 phenological stages from sites in Union, Baker and Umatilla Counties, Oregon during each of 2 years. Acid detergent fiber, lignin, cellulose and neutral detergent fiber contents increased through phenological development. Crude protein ranged from 16.7 to 5.0%. In Vitro dry matter digestibility ranged from 84.8% to 57.0%. Mineral nutrients P, K, CA, Mg, Mn, Fe, Cu, Zn, and Na were analyzed and determined to be adequate for maintenance needs of ewes. / Graduation date: 2002

In the canyon grasslands of Garden Creek Ranch Preserve in Idaho, where the threatened plant Silene spaldingii occurs and invasion by the exotic species Centaurea solstitialis and Bromus tectorum is proceeding rapidly, I examined environmental and community patterns of site invasion, and evaluated the apparent influence of invasion on Silene population vigor. In addition, two separate lightning fires at the preserve presented the opportunity to examine the short-term influence of late-season fire on this species and its associated bunchgrass plant community. I found that Silene-supporting sites most often invaded by exotics were on relatively gentle slopes that received more incident radiation. This pattern may relate, in part, to light requirements of Centaurea solstitialis. Invaded sites were also typically at higher elevations, which may indicate they were moister and therefore more productive. The plant communities in invaded Silene-supporting sites were similar to plant communities in uninvaded sites, although invaded sites tended to have greater legume and exotic annual grass cover. Exotic species invasion did not appear to influence negatively the vigor of Silene populations, as indicated by similar plant height and comparable levels of flowering, fruit and seed set in invaded and uninvaded populations. The similarity in Silene vigor between invaded and uninvaded sites may reflect a moderating influence of site productivity in invaded populations, or may indicate that mature Silene plants and the exotic species partition space or resources differently, potentially reducing competition between them. However, Silene recruitment may be limited by competition from weeds; my data did not allow a rigorous test of this possibility. Fire apparently decreased cover of Festuca idahoensis and increased cover of Lupinus sericeus in the first year after burning, while cover of Pseudoroegneria spicata, exotic grasses, and most other forb species did not differ between burned and unburned areas. Silene cover and abundance within populations were similar before and after fire. Burning did not appear to influence levels of flowering, change the number of flowers or capsules produced per stem, or alter the number of seeds per capsule. Burning decreased plant size slightly, and decreased the proportion of flowers that matured to seed-filled capsules. Silene and the plant communities that support this species appear well suited to late season fire, however the response to burning in other seasons or at higher frequencies remains unknown in this study area. / Graduation date: 2003