|Liow, Pui sze|
Submitted to: Invasive Plant Science and Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/23/2011
Publication Date: 7/1/2011
Citation: Spencer, D.F., Enloe, S.F., Ksander, G.G., Liow, P., Carruthers, R.I. 2011. Does superior competitive ability explain yellow starthistle's invasion of annual grasslands?. Journal of Invasive Plant Science and Management. 4(3):284-295. Interpretive Summary: Yellow starthistle has invaded western grasslands within the last 100 years. The mechanism for this successful invasion is not known for certain, even though it would be useful in managing this troublesome weed. We grew yellow starthistle in competition experiments with wild oats. The results indicate that yellow starthistle is not a strong competitor in relation to wild oats. Soil water measurements indicate that yellow starthistle used water from deeper in the soil profile than wild oats, suggesting that these species avoid competition in this way. We also measured the level of plant compounds believed to be involved in defending the plant from being consumed by animals. These measurements indicated that levels in yellow starthistle plants were similar to those in resident species. These results are contrary to a current hypothesis which suggests that plant invaders are stronger competitors because they do not have to defend themselves in new habitats. Results from this study support the “empty niche hypothesis” which implies that yellow starthistle has successfully invaded western grasslands because there are unused resources available to it in these systems.
Technical Abstract: Centaurea solstitialis represents one of the most spectacular examples of biological invasion in the Western United States. However, the mechanisms leading to its success have not been clearly elucidated. While its success has been attributed to “superior competitive ability,” few competition studies have been performed with C. solstitialis to test this assertion. Centaurea solstitialis and Avena fatua (a dominant component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition. A companion study revealed temporal separation in the phenology of these plants explaining the weak role of interspecific competition. Additional measurements of growth and soil moisture dynamics in large 270 x 50 cm PVC columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Avena fatua reduced soil moisture to 5% but only to a depth of approximately 150 cm. Centaurea solstitialis depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when A. fatua and C. solstitialis were grown individually or together in the columns, indicating that C. solstitialis had a greater impact on soil moisture and to greater depths. Contrary to expectations, total phenolic acid content of C. solstitialis was either equal to or greater than that of A. fatua. Centaurea solstitialis’ invasion of grasslands in the western United States does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture. These results do not support a current hypothesis that implies that invasive species are successful in new habitats because of their superior competitive abilities resulting from reallocation of resources from the production of defense chemicals.