I think I have correctly identified this tree. According to our local conservation authority, common buckthorn is an invasive species originating in Europe and is spread widely by birds that ingest the berries which has an unfortunate laxitive effect on them. I am eliminating it from our property to try to replace it with more friendly varieties. Preferably native trees. I have just cut it off at ground level but I suspect that will just make it angry come spring and it will sprout up with a vengence. Any suggestions on how to keep it at bay while I try to nurse seedlings of other trees in its place. I assume I will just be going after it with clippers.