For ad valorem tax purposes, real property that is subject to an easement, or a written cooperative agreement, for purposes of ORS § 390.950 to 390.989 and 390.995 (2) shall be valued at its real market value, less any reduction in value caused by the easement or the written cooperative agreement, and assessed in accordance with ORS § 308.232. The easement shall be exempt from assessment and taxation the same as any other property owned by the state. [1971 c.614 § 13; 1981 c.804 § 100; 1991 c.459 § 395]