Allelopathy

Allelopathy refers to the inhibition of growth of a plant due to biomolecules released by another.  Conceptually, biomolecules (specifically termed allelochemicals) produced by a plant are released into the environment and subsequently influence the growth and development of neighbouring plants. Casuarina equisetifolia  litter, for example, can completely suppress germination of other understory plants, even if the canopy is relatively open and there is ample rainfall. It is important to keep in mind that allelopathy involves the addition of a chemical compound or compounds (secondary metabolites) into the environment, while resource competition involves the removal or reduction of some factor or factors in the environment (such as nutrients, water, or light).

Although allelopathic science is a relatively new field of study, there exists convincing evidence that allelopathic interactions between plants play a crucial role in determining species distribution and abundance within some plant communities. Allelopathic interactions are also thought to be an important factor in the success of many invasive plants, for example, Spotted Knapweed  (Centaurea maculosa), Garlic Mustard (Alliaria petiolata), and Nutsedge.

[Source:  Wikipedia]