Common Buckthorn
Rhamnus cathartica (GoBotany, Wikipedia, USDA) is a small tree growing up to 30 ft. tall, with small black berries. It may or may not have thorns.
It isn't very common on Acton Conservation Lands, although occasional trees are scattered about.
Removal
Plants with stems less than half an inch in diameter may be pulled.
Larger plants should be girded, although it will sprout vigorously below the gird and will need cutting back several times before killing the root system.
Common Mis-identifications
Common Dogwood (Cornus sanguinea a.k.a. Swida sanguinea, Wikipedia, USDA) is listed as a possible mis-identification. This is NOT a native Dogwood in North America, although it is listed as introduced to Massachusetts. Unlike Common Buckthorn, it has bright red twigs in the winter.
Glossy Buckthorn is frequently listed as a mis-identification, but it has smooth leaf edges. It is also invasive.
Dahurian Buckthorn (Rhamnus davurica) is listed as a mis-identification. It is also invasive.