| Centaurea diffusa, a non-native semelparous forb of Eurasian origin, is currently invading and dominating prairies of western North America that were historically dominated by perennial C3 and C4 grasses. I conducted research to examine mechanisms related to soil resource availability and use that might explain the ability of C. diffusa to obtain and maintain dominance in western grasslands. In a Front Range Colorado, USA, mixed-grass prairie invaded by C. diffusa, I compared nutrient concentrations and rates of cycling, timing of use and depletion of pools of available nutrients and water, and standard physicochemical parameters in soils beneath C. diffusa and two native and previously dominant grasses. I conducted field experiments to determine the effects of altering soil resource availability on C. diffusa and the rest of the grassland community, and I monitored the effects of removal of C. diffusa from the grassland community on soil resource availability and on the rest of the community. In addition, I evaluated use of C. diffusa by five biocontrol insect species under a range of soil fertility conditions. Available nitrogen and labile phosphorus pools, rates of in-situ nitrogen mineralization, texture, and micro- and other macronutrient concentrations did not differ significantly beneath C. diffusa and the native grasses, and C. diffusa did not alter soil conditions in areas that it invaded. Removal of C. diffusa from the community did not induce a strong “release” response in terms of soil resource availability or in terms of the remaining community biomass, cover, or composition. Centaurea diffusa is not an aggressive competitor, but instead is benefited by removal of competitors. Centaurea diffusa appears to be unresponsive to soil nutrient status, fairly tolerant of a wide range of nutrient availability, and functionally similar to the native prairie grasses. The wide range of tolerance of C. diffusa to resource availability, its ability to function like the native species, its prolific seed production and germination rate, and its escape from the species-specific herbivory that may control its population in its native range explain its ability to invade and dominate western North American rangelands. |
