There are several kinds of chemical allelopathy. In one kind, a plant releases protective chemicals into the soil from it's roots. Other plants growing next to the allelopathic plant or seeds germinating near by absorb these chemicals and die. Another way allelopathic plants keep other plants from growing too close is the leaching of chemicals from decomposing leaves into the soil with the same results. An example of an allelopathic plant easily observed is the pine tree.
Few plants grow where a pine tree has shed it's needles.
The decomposing needles contain acid which leaches into the soil stifling the germination of other plants. Weedy field plants and some trees competing in the intermediate stages of succession such as pines, tree of heaven, some ferns and sagebrush are more likely to produce these chemicals. Even large trees like the black walnut are allelopathic. The exotic species, purple loosestrife, is rapidly colonizing the wetlands of North America. It easily invades an area that is disturbed and quickly out populates the indigenous wetland species such as cattail, sedges and rushes.