Testing for nativity, competitive, or allelopathic effects in interspecific interactions of Centaurea pratensis (meadow knapweed)
Comparisons of invasive species in their native and introduced ranges are rare, but offer insights into which mechanisms may contribute to their invasion potential in novel environments. Though competition is considered
Comparisons of invasive species in their native and introduced ranges are rare, but offer insights into which mechanisms may contribute to their invasion potential in novel environments. Though competition is considered a major mechanism for invasive species success in novel environments, allelopathy (the negative effect of one plant on another through the release of chemical compounds) has also been suggested as a mechanism which may promote success of the exotic invader. The novel weapons hypothesis suggests that invasive species produce unique allelopathic chemicals which enable success against native species which have not evolved tolerance to the novel biochemicals.
Some of the best examples of evidence for the novel weapons hypothesis come from research on Centaurea species. Several Centaurea species are among the most devastating noxious plants in North America. While C. pratensis is known to be invading many habitats throughout the Pacific Northwest, no studies have addressed the mechanism of this invasion. A preliminary laboratory study using fragments of roots from C. pratensis suggested that this species may have more allelopathic potential than either C. stoebe or C. diffusa. A quantitative study is needed to increase understanding of C. pratensis and potential interactions with native species in the Pacific Northwest.
To test C. pratensis’ allelopathic capability, we created several greenhouse experiments. C. pratensis was grown alongside species native to the Pacific Northwest, species native to Eurasia (known home range of Centaurea), as well as with itself to explore the roles of allelopathy, competition, and nativity on C. pratensis success.
Through our experiements, we found no evidence of C. pratensis producing allelopathic chemicals at this growth stage. Though our study did not indicate production of allelopathic chemicals, there is potential that C. pratensis may create these chemicals at a later growth stage. Preliminary laboratory analyses using mature root fragments of C. pratensis suggested production of allelopathic chemicals, thus future studies of later growth stages are needed to increase understanding of potential interactions.
Trends suggest that competition may be a mechanism mediating interaction at this growth stage. We found both a nativity and competitor effect when C. pratensis was grown with multiple competitors, however both of these relationships are likely the reflection of an interaction of C. pratensis with Phaseolus coccineus, an exotic legume. Further investigations of different species combinations and with wild-collected or limited production seed would increase understanding of the interactions of C. pratensis. Another factor limiting interpretation of our results is the time frame of the studies. These experiments were harvested while plants were in the seedling to young-rosette stages. Future studies of later growth stages would increase understanding of how these intra- and interspecific interactions may change over time.
This study increases understanding of the mechanisms mediating interspecific interaction at the seedling stage. Centaurea pratensis poses a major threat to native and agricultural systems throughout Oregon. Though our study indicates that C. pratensis does not produce allelopathic chemicals at this growth stage, future studies focusing on interactions of mature plants would greatly increase understanding of potential interactions over time. The ability of C. pratensis to create monocultures in invaded areas demonstrates that this species is highly competitive, likely over a longer time frame than encompassed by our study.
