Mont. Code Ann. § 15-24-1103

When the property is held under lease, other interest, or estate therein that is less than the fee, except under contract of sale, the property must be assessed and taxed for the value, as defined in Title 15, chapter 6, part 1, of the leasehold, interest, or estate in the property. The lien for the tax must attach to and be enforced against only the leasehold, interest, or estate in the property. Whenever the interest in the property is acquired through foreclosure, the lessor is liable for property taxes. When the United States authorizes the taxation of the property for the full assessed value of the fee, the property must be assessed for full assessed value as defined in 15-8-111.