Allelopathy is an interspecific antagonistic
strategy adopted by plant
s to maximize their chances of survival in a competitive
environment. The plants produce noxious chemical
(s) to limit the production
of neighbouring competitive species.
There are three principle allelopathic mechanisms observed in the plant kingdom: volatilization, exudation and leaching. Volatilization is used mostly by trees, which release the noxious chemicals as a gas, which are then absorbed by competing species. Plants which exude their allelopathic chemicals release them into the soil from their roots, and thus more directly impact their immediate neighbours. Leaching is a more indirect method of allelopathy, where the leaves or needles lost by a plant release noxious chemicals as they decompose on the ground.
The chemicals released by allelopathic species are quite varied. They may inhibit the establishment of young plants, the germination of seeds, slow or stop photosynthesis of competitors or they may even reduce the chlorophyll concentration in their neighbours' leaves.
A good example of an allelopathic species is production of inhibitory chemicals by fennel. The roots of fennel plants produce a suite of chemicals which can reduce the root elongation, root hair growth and germination of neighbouring plants, like lettuce. This is an example of allelopathic exudation.
A second example is the pine tree. Forests in the northern Boreal forest are often monocultures, dominated by a single species of conifer. The understory is often completely denuded of other plant life, due to the fact that pine needles, when they decompose, acidify the soil to such an extent that seeds of other species are unable to germinate.
Help from Begon, Harper and Townsend's book Ecology: Individuals, Populaitons and Communities, 2nd ed. Blackwell Scientific, 1990. and Raven, Evert and Eichhorn's Biology of Plants, 5th ed., Worth Publishers, 1992. Also, see http://koning.ecsu.ctstateu.edu/Plant_Biology/biotic.html