All counties are hereby authorized to enter into covenants with owners of land designated as farm, forest, water supply, or open space land on an adopted municipal, county or regional plan for the purpose of preserving the land as open space. Such covenants and extensions thereof shall take effect upon recording in the office of recorder of deeds. The land owner may voluntarily covenant for himself and his successors and assigns in right, title and interest that the land will remain in open space use as designated on the plan for a period of ten years commencing with the date of the covenant. The county shall covenant that the real property tax assessment, for a period of ten years commencing with the date of the covenant, will reflect the fair market value of the land as restricted by the covenant. The board to which assessment appeals are taken shall take into consideration the covenant's restriction upon the land in fixing the assessment.