The department is further authorized to purchase, lease, set aside, or exchange any public lands which are deemed to be natural areas: PROVIDED, That the appropriate state land trust receives the fair market value for any interests that are disposed of: PROVIDED, FURTHER, That such transactions are approved by the board of natural resources.
An area consisting of public land designated as a natural area preserve shall be held in trust and shall not be alienated except to another public use upon a finding by the department of natural resources of imperative and unavoidable public necessity.