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Novel Weapons: Allelopathy

The success of some invasive plants may be due to the production of chemicals that inhibit the growth of other plants. This is called allelopathy. This inhibition can occur through direct toxicity to other plants, or secondarily through altering soil microbial communities.

These weeds may not have evolved these characteristics for the sole purpose of poisoning other plants. Plants produce chemicals to help them acquire nutrients, defend against herbivores and harmful microbes, and to send messages between different plant parts. It may merely be circumstantial that one or more of these naturally occurring chemicals inhibits other plants once invasive weeds arrive in a new home. Regardless of intended use, these chemicals help morph some weeds, of little consequence in their home range, into aggressive invaders in new environments.

Russian knapweed flowerhead
Russian knapweed
Several allelopathic compounds have been found in Russian knapweed. These compounds can be released from roots or leached into the soil from decomposing leaves. As a result, both germination and root growth of other plants are hindered.




Diffuse and spotted knapweed
Spotted knapweed flowerhead Diffuse knapweed flowerhead There is still confusion about the role allelopathy plays in the success of diffuse and spotted knapweed. They do possess allelopathic compounds, though in relatively minimal concentrations. Despite this, studies suggest that grasses growing naturally with these knapweeds in Europe fair better than similar grasses growing where the knapweeds have been introduced. Further studies indicate that European grasses may be used to the allelopathic compounds, whereas grasses where knapweeds have been introduced are not.

Bull thistle flowerhead


Bull thistle
The litter of bull thistle inhibits the growth of other plants. Most of this effect comes from immobilization of nutrients during the decomposition process. This prevents other plants from using much needed nutrients.




Halogeton flowers

Halogeton
Halogeton collects salts from the soil, and accumulates them in its leaves. These salts build up at the soil surface. In dense stands, this can make it harder for other plants to draw moisture and nutrients from the soil, and infiltration rates are lower.



Images:
-Russian knapweed: Eric Coombs, Oregon Department of Agriculture, (http://www.forestryimages.org, 26 September 2006).
-Diffuse knapweed: USDA APHIS Archives, USDA APHIS, (http://www.forestryimages.org, 26 September 2006).
-Spotted knapweed: Steve Schoenig CDFA.
-Bull thistle: Christopher L. Christie © 2005.
-Halogeton: Public domain.

Text references:
-Callaway, Ragan M.; Aschehoug, Erik T. 2001. Mechanisms for the success of invaders: diffuse knapweed interacts differently with new neighbors than with old ones. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 69. Abstract.
-Eckert, R.E. and F.E. Kinsinger. 1960. Effects of Halogeton glomeratus leachate on chemical and physical properties of soil. Ecology. Vol. 41 No. 4 pp. 764-772.
-Zouhar, Kris 2001. Acroptilon repens. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).
-Zouhar, Kris 2001. Centaurea diffusa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).
-Zouhar, Kris 2002. Cirsium vulgare. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).