Why is buckthorn a problem? It outcompetes natives and non-natives and a key to this is the fact that it leaves out early in Spring and it keeps its leaves late in fall which gives it a biological advantage in shading out other plants. It produces thousands of berries a year which can grow into new trees. The berries are quite toxic to birds providing few nutrients and buckthorn is allelopathic which means it produces chemicals that restrict the growth of other plants (among other things). These chemicals been associated with birth defects in amphibians.
The Buckthorn problem will not go away by itself!Morton Arboretum – read their full tree census report here.