Elton’s hypothesis revisited: an experimental test using cogongrass

Reduced Invasiveness of Common Ragweed (Ambrosia artemisiifolia) Using Low-Dose Herbicide Treatments for High-Efficiency and Eco-Friendly Control

Common ragweed (Ambrosia artemisiifolia) is an invasive annual weed that invades heavily disturbed habitats and natural habitats less disturbed by human activities with native plant species in need of protection. Achieving effective control of A. artemisiifolia for the protection of native organisms and the local ecological environment is an ongoing challenge. Based on the growth and development characteristics of A. artemisiifolia, we examined the effectiveness of herbicides in controlling this species and the optimal time for application in the field with the aim of reducing herbicide dosage. Additionally, we analyzed whether the efficiency of low-dose applications for controlling this species might improve with increasing native plant species richness. Our findings indicate that aminopyralid (33 g ai ha^-1) was the most suitable herbicide for chemical control of A. artemisiifolia, with optimum application time being during vegetative growth (BBCH 32-35). Application of aminopyralid was found to kill approximately 52% of A. artemisiifolia plants, and more than 75% of the surviving plants did not bloom, thereby reducing seed yield of the population by more than 90%. Compared with the application of high-dose herbicide, the phytotoxicity of aminopyralid to native plants at the applied dose was substantially reduced. After 2 years of application, the relative coverage of A. artemisiifolia significantly decreased, with few plants remaining, whereas the relative coverage of native plants more than doubled, representing an eco-friendly control. Further, there was an increase in the A. artemisiifolia control rate in the plant community with higher native plant species richness at the same herbicide rates and a reduction in seed yield of A. artemisiifolia. Our findings help toward developing control measures to reduce the invasiveness of A. artemisiifolia with low-dose herbicides meanwhile protecting native plants, and then using the species richness of native plant communities to indirectly promote the effectiveness of low-dose herbicide application.

Dam-induced difference of invasive plant species distribution along the riparian habitats

Riparian ecosystem is structurally unstable due to the frequent disturbances from water fluctuation. Moreover, dams on large rivers tend to trigger fundamental changes of the composition and structure of riparian plant communities, which provides high odds for invasive species to colonize. Yet, how the invasive species distribute along a dam-induced riparian habitat, and how the native species resist to plant invasion are still puzzles. In this study, we investigated spatial distribution of invasive floral species and its correlation with the distance from dam and the dam-triggered flooding stresses, as well as the resistance of native species to plant invasion in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) along the Yangtze River. By our investigation, a total of 43 alien plant species belonging to 14 families and 34 genera were found, including 20 existed and 23 newly discovered alien species recorded. Most of the new invasive species are annual herbs of the Asteraceae family. At the current successional stage, the new invasive species had not yet fully occupied the habitats of the existed invasive species. Longitudinally, number and coverages of the new invasive species showed an opposite distribution pattern to the existed invasive species, but vertically they demonstrated similar pattern. Currently, the new dominant invasive species are mainly concentrated at the intermediate elevation of WLFZ in the middle section of the reservoir, whereas the existed dominant invasive species have proliferated across the whole WLFZ. Additionally, native species showed a weak resistance to plant invasion, and water fluctuation along the elevation exerted the most significant influence on plant invasion. The results indicated that, after a decade of riparian community succession, the invasiveness of alien species remain persisted. The potential penetration site of the invasion may locate at the intermediate section along the vertical and longitudinal dimension.

Allelopathic exudates of cogongrass (Imperata cylindrica): implications for the performance of native pine savanna plant species in the southeastern US

We conducted a greenhouse study to assess the effects of cogongrass (Imperata cylindrica) rhizochemicals on a suite of plants native to southeastern US pine savanna ecosystems. Our results indicated a possible allelopathic effect, although it varied by species. A ruderal grass (Andropogon arctatus) and ericaceous shrub (Lyonia ferruginea) were unaffected by irrigation with cogongrass soil "leachate" (relative to leachate from mixed native species), while a mid-successional grass (Aristida stricta Michx. var. beyrichiana) and tree (Pinus elliottii) were negatively affected. For A. stricta, we observed a 35.7 % reduction in aboveground biomass, a 21.9 % reduction in total root length, a 24.6 % reduction in specific root length and a 23.5 % reduction in total mycorrhizal root length, relative to the native leachate treatment. For P. elliottii, there was a 19.5 % reduction in percent mycorrhizal colonization and a 20.1 % reduction in total mycorrhizal root length. Comparisons with a DI water control in year two support the possibility that the treatment effects were due to the negative effects of cogongrass leachate, rather than a facilitative effect from the mixed natives. Chemical analyses identified 12 putative allelopathic compounds (mostly phenolics) in cogongrass leachate. The concentrations of most compounds were significantly lower, if they were present at all, in the native leachate. One compound was an alkaloid with a speculated structure of hexadecahydro-1-azachrysen-8-yl ester (C23H33NO4). This compound was not found in the native leachate. We hypothesize that the observed treatment effects may be attributable, at least partially, to these qualitative and quantitative differences in leachate chemistry.