Every conveyance which includes lands within the boundaries of a national forest shall, as to such lands, contain reservations that:

(a) Until December 18, 1976, the sale of any timber from the land is subject to the same restrictions relating to the export of timber from the United States as are applicable to national forest lands in Alaska under rules and regulations of the Secretary of Agriculture; and,

(b) Until December 18, 1983, the land shall be managed under the principles of sustained yield and under management practices for protection and enhancement of environmental quality no less stringent than such management practices on adjacent national forest lands.