Apparently Black Walnut trees can put shit in the soil inside their drip line, that will discourage other tree seeds from germinating there. They also discourage birds from nesting on them, although I don't see the benefit unless it cuts down on birds shitting seeds near the tree.
Plants can be vicious too. If to don't believe it I've got acres of nettles, briars, and Atomic Grade Poison Ivy, to prove it.
