Invasive plant species are a persistent problem for land managers in the western United States. Department of Defense (DoD) installations have special difficulty due to military training activities, which frequently disturb large areas of ground and lead to infestations of invasive plants that degrade environmental quality. The DoD strives to manage its lands responsibly and in line with federal environmental regulations while continuing to test equipment and train troops. To continue to perform large-scale training operations, DoD is in need of an effective, economical, and ecological-based method for combating various invasive plants. The knapweeds (Centaurea maculosa , Acroptilon repens , and C. diffusa ) are among the worst invasive offenders in the U.S., infesting over 4.3 million hectares in 14 western states and two Canadian provinces. Along with leafy spurge (Euphorbia esula) and Canada thistle (Cirsium arvense), these invasive rangeland plants are notorious for their ability to negatively affect soil quality through the release of natural plant toxins known as allelochemicals among other qualities that make these plants good invaders.

The main objective of this project was to develop an understanding of the allelopathic properties of several invasive plants to improve methods for controlling these and other invasives. Accordingly, this project investigated the impact of various management strategies on allelopathic invasive species, as well as the duration and long-term effect of allelopathic chemicals in the soil after the invasive plant’s removal. This study also sought to determine and describe the mechanisms used by allelopathic plants to neutralize the effects of their own toxins. The ultimate purpose was to develop useful products and practical information for direct transmission to participating installations for on-site use.

In a series of studies, this project characterized new allelochemicals produced by a variety of invasive plants. Large numbers of native plants were screened for resistance to allelochemicals, and in the process highly competitive species were identified. Additionally, new ideas about using native allelopathic smother crops against invaders were tested.  Experiments were conducted in which areas were seeded with collections of native species that were developed to maximize their invasive plant resistance. This approach is potentially powerful, yet underutilized, in restoration contexts. In additional, to improve understanding of the nature of invasiveness the first cDNA library of invasive spotted knapweed was created, which enables investigation of the genomic basis for invasiveness to an extent that was previously impossible.

Allelochemicals from several invasive plants were successfully isolated and characterized indicating that allelopathy may potentially play a role in the invasion biology of these species. Some of these allelochemicals constitute tools to screen for successful plant competitors. This research provides evidence that some native species, which are either resistant to allelochemicals or are superior competitors, can be used to compete with invasive allelopathic plants. Specifically, planting annual ragweed (Ambrosia artemisiifolia) and common sunflower (Helianthus annuus)as cover crops in western grassland restorations may reduce cheatgrass (Bromus tectorum), Japanese brome (B. japonicus), Canada thistle (Cirsium arvense), and whitetop (Lepidium draba) invasion and may improve desired species growth in competition with cheatgrass and Japanese brome.

Results from this research show that allelopathy is conditional, allelopathy is not due to a single compound, the stability of an allelochemical in the soil is of crucial importance, allelopathy depends on the presence of given neighbors and on plant density, and allelopathy might be indirect and act through a negative effect on soil microbial populations.

This project generated basic knowledge that contributes to the ecological literature by demonstrating that allelopathy is a conditional biological occurrence and that certain biological or environmental triggers must exist to make allelopathy apparent in the field. Similarly, this project contributed basic knowledge on the plasticity of invasive species and how their intrinsic biochemistry related to defense and aggression (invasion) is controlled by the presence of other neighboring individuals. These basic and applied studies are likely to hold promise for more fully understanding and managing plant invasions.