Study Site Location: Fort McCoy, Wisconsin; Yakima Training Center, Yakima, WA; Fort Collins, CO.
For more information, see the SERDP summary website.
Executive Summary
Allelopathy is a relatively controversial concept in the ecological literature as it is hard to quantify in the field. Interest in allelopathy has increased in recent years with reports of invasive plants using allelopathy as an invasion mechanism. The present project aimed to understand this process under a variety of conditions including natural habitats and to utilize this basic knowledge in strategies to control invasive plant species affecting military testing and training. We reasoned that greater knowledge of allelopathy in the context of invasion biology could lead to more sustainable measures of invasive plant control and management. In a series of studies we characterized new allelochemicals produced by a variety of invasive plants. We screened large numbers of native plants for resistance to allelochemicals, and in the process highly competitive species were identified. Additionally, we tested new ideas about using native allelopathic smother crops against invaders. The selection of native plant species capable of establishing within invasive plant infestations was a modest success as shown in greenhouse and field studies.
The project also generated basic knowledge to contribute 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 has contributed basic knowledge on the plasticity of invasive species and how their intrinsic biochemistry related to defense and aggression (invasion) is control by the presence of other neighboring individuals. These basic and applied studies and others described in this report are likely to hold promise for more fully understanding and managing plant invasions.
Our main objective was to develop an understanding of the allelopathic properties of several invasive plants in order to improve methods for controlling these and other invasives. Accordingly, we sought to learn more about the nature of allelopathy and invasiveness, and weak links in the allelopathic chain that might be exploited in order to control the spread of invasive plants. In pursuit of this goal, we 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’s removal. We also worked to determine and describe the mechanisms used by allelopathic plants to neutralize the effects of their own toxins. Our ultimate purpose was to develop useful products and practical information for direct transmission to participating installations for in-site use. Specific objectives associated with our multidisciplinary project are summarized below.
– Isolation and characterization of allelochemicals from invasive plants found on military bases.
– Using allelopathy for the control of invasive plants.
– Biological degradation of allelochemicals.
– Identify native plants that are resistant to allelochemicals.
– Explore how various control strategies for spotted knapweed impact the amount of allelochemicals released into the soil and revegetation efforts.
– Understand the mechanisms of allelochemical detoxification.
– Integrate allelochemical control of invasive species with other proven control strategies.
Key findings include:
1. Allelopathy is conditional.
2. Different metals in soils determine at least one aspect of the conditional effect of catechin.
3. Plants may use mixtures of compounds as allelochemicals rather that single compounds as we have found for Russian knapweed.
4. The defense and aggressive strategies of invasive plants are not entirely determined by intrinsic physiology and biochemistry but also by the presence of specific plant neighbors surrounding the invasive species.
5. Native plant species have the potential to evolve resistance to invaders.
6. Genomics and metabolomics will contribute to the future of understanding plant invasion and this project has generated seminal information on this topic.
7. Screening for successful competitors by direct application of allelochemicals or by direct competition with a given invasive plant taxa is a cost-effective way to find native competitors to revegetate areas invaded by invasives.
Abstract of the Mester’s Thesis of Matthew J. Schultz. Submitted Spring, 2008.
SOIL ECOLOGICAL INTERACTIONS OF SPOTTED KNAPWEED AND NATIVE PLANT SPECIES
Effective, long-term management methods are lacking for many invasive exotic plants in North America. Planting some native species as “nurse” or “cover” crops may ameliorate adverse conditions, thus improving the performance of other native species. Allelopathy, the production of phytotoxins, has been suggested as a mechanism of invasion for Centaurea stoebe (spotted knapweed) in North American grasslands. Some native plants may resist the Centaurea allelochemical (±)-catechin by secreting organic acids that detoxify (±)-catechin. My objective was to evaluate whether (±)-catechinresistant native species can reduce effects of (±)-catechin on (±)-catechin-sensitive native species and thus facilitate their growth in interactions with Centaurea. A (±)-catechinsensitive native species, Festuca idahoensis, was grown with or without Centaurea or exogenous (±)-catechin, and with or without the (±)-catechin-resistant plants Gaillardia aristata and Lupinus sericeus or exogenous oxalic acid. Activated carbon, which adsorbs organic compounds, was added to separate effects of resource and interference competition (i.e. allelopathy). However, Gaillardia, Lupinus, and oxalic acid treatment did not improve Festuca biomass when in competition with Centaurea or when treate with (±)-catechin. Several lines of evidence suggest that (±)-catechin did not influence Centaurea-Festuca interactions in my experiment. Activated carbon did not improve Festuca growth in the presence of Centaurea, suggesting that organic compounds from Centaurea did not inhibit Festuca. Exogenous (±)-catechin appeared to degrade rapidly and had no phytotoxic effect, and Centaurea (±)-catechin production was episodic, both of which limited opportunities to observe facilitation. Given the episodic nature of soil (±)-catechin and the conditional nature of (±)-catechin phytotoxicity, the potential benefits for (±)-catechin-sensitive species of planting (±)-catechin-resistant species to detoxify (±)-catechin are unlikely to counterbalance the costs of additional resource competition.
Exotic plant invasion consists of multiple stages each with specific mechanisms for invader success. The mechanisms behind effects of establishment order (i.e. priority effects) on competitive outcomes are unclear. Priority effects could result from interference competition, but this mechanism has never been tested. I conducted a greenhouse experiment to examine effects of establishment order on competition between Centaurea stoebe, an allelopathic invasive species, and two native species, Festuca idahoensis and Gaillardia aristata. Advantages due to prior establishment were analyzed by comparing plant responses to competition when the native species were established before, simultaneously, and after Centaurea. Activated carbon amendments were used to separate interference competition from resource competition. Early establishment conferred a strong competitive advantage regardless of species identity. The species that established earlier strongly inhibited the later colonizing species. These results indicate that Centaurea requires competitors to be absent for invasion, and suggest that rapid colonization of disturbed areas, not superior competitive ability, may contribute to the establishment phase of Centaurea invasion. Priority effects were similar for the native species and Centaurea, perhaps offering an explanation for relictual native species populations and persistent Centaurea infestations. Activated carbon did not alter priority effects for any species, which suggests that resource competition was more important than chemical interference competition when the species were established at different times. In contrast, when the species were established simultaneously there was some evidence of Centaurea allelopathy. These results suggest that allelopathy is not responsible for Centaurea invasion of intact native vegetation, but may play a role during competition between establishing seedlings of Centaurea and some native species. However, the priority effects in this experiment might have been weaker under field conditions with more spatial heterogeneity in vegetation or belowground resources.
Understanding priority effects in interactions between native and exotic species may indicate ways that establishment order can be manipulated to improve community composition and diversity in restored ecosystems and maximize biotic resistance to Centaurea invasion. The rhizosphere dynamics of Centaurea (±)-catechin under natural conditions are poorly understood. (±)-Catechin is often absent from Centaurea soil and occurs only episodically. Pulses in allelochemical concentrations could have phytotoxic effects on neighboring plants, but measuring episodic (±)-catechin concentrations will require new sampling approaches that can capture pulses throughout a sampling period. Current methods for measuring (±)-catechin involve solvent extraction of soil, which is timeconsuming, expensive, and might miss a (±)-catechin pulse. My objectives were to determine the ability of adsorbent and sorbent materials to consistently recover (±)- catechin under laboratory conditions and, based on those results, evaluate (±)-catechin degradation after adsorption to gauge applicability in long-term field studies. Three materials, polydimethylsiloxane (PMDS) tubing, nylon bags containing Amberlite™ XAD-7HP resin, and polyester capsules containing 50% IRN-150 resin and 50% Ambersorb® 563 resin, were exposed to (±)-catechin in aqueous solution and later extracted to determine recovery. Of the three materials tested, Amberlite™ XAD-7HP resin bags recovered the greatest amount of (±)-catechin. Amberlite™ XAD-7HP resin bags were then impregnated with (±)-catechin, and placed outside underground for a variable number of weeks to assess the degradation of adsorbed (±)-catechin over time. Even though Amberlite™ XAD-7HP readily adsorbed (±)-catechin, variable losses occurred over time. The reliability of Amberlite™ XAD-7HP for (±)-catechin recovery after one week is questionable. Deploying resin bags for longer than two weeks risks loss of adsorbed catechin as little (±)-catechin recovery was observed after the third week of incubation. When designing future experiments, it may be necessary to replace resin bags every two weeks for continuous sampling of (±)-catechin which involves considerable sampling effort. Amberlite™ XAD-7HP offers promise for recovering (±)- catechin from various experimental designs over limited periods of time, but numerous field conditions could affect adsorption and loss. Given the small number of materials tested, screening of other materials in the future could be beneficial. The use of adsorbent materials could elucidate allelochemical dynamics and clarify the role of (±)- catechin in Centaurea invasion.