Buckthorn (Rhamnus cathartica)
Where did it come from?
Buckthorn is a species of plant that was brought to North America as a landscaping plant, it had been used in Europe for hundreds of years. It was thought to make wonderful hedges, as it is fast growing and, grows in thick, making an impenetrable hedge. Additionally Buckthorn flowers in the spring, providing berries for birds, and even holds onto them into winter.
Is it really issue?
Buckthorn has a vast root system which out competes native plants for water and nutrients. As it grows quickly to create a canopy shading out the other species. This pushes out many natives species that had been supporting the local wildlife. Causing the area under the Buckthorn to become dark and barren.
Once a stand of Buckthorn is established it is hard to remove it. The berries that the birds contain seeds that the birds conveniently spread enveloped in a bit of fertilizer to help the Buckthorn seed get off to a good start. Once the seed is spread it has a long life and can stay in the soil for up to five years. This one one reason why Buckthorn is not removed from an area just by cutting it down, in fact it gives the new seed a perfect opportunity to start sprouting.
One of the other reasons Buckthorn is hard to remove is its ability to come back from roots left in the ground, or even from the stump. The stump will use some of the energy stored in root system to send up multiple new shoots, enabling the trees to aggressively start over. It can be beat though, through repetitive cuttings or grazing wearing away the energy stored in the root system.