In the United States, a conservation easement (also called a conservation basement, conservation covenant, conservation restriction or conservation servitude) is a power invested in a qualified private land conservation organization (often called a "land trust") or government (municipal, county, state or federal) to constrain, as to a specified land area, the exercise of rights otherwise held by a landowner so as to achieve certain conservation purposes. It is an interest in real property established by agreement between a landowner and land trust or unit of government. The conservation easement "runs with the land," meaning it is applicable to both present and future owners of the land. As with other real property interests, the grant of conservation easement is recorded in the local land records; the grant becomes a part of the chain of title for the property.

http://en.wikipedia.org/wiki/Conservation_easement

http://geography.berkeley.edu/documents/sayre/sayre_rissman_conservation_easements.pdf