(1) If any part of the principal consists of property in lands from which may be taken timber, minerals, oils, gas, or other natural resources, and the trustee or tenant is authorized by law or by the terms of the transaction by which the principal was established to sell, lease, or otherwise develop such natural resources, and no provision is made for the disposition of the net proceeds thereof after the payment of expenses and carrying charges on such property, such net proceeds, if received as rent on a lease, shall be deemed income but, if received as consideration, whether as royalties or otherwise, for the permanent severance of such natural resources from the lands, shall be deemed, to the extent provided in this section, principal to be invested to produce income, and the remainder of such net proceeds shall be deemed income. Of the net proceeds received during any period as consideration for permanent severance of a natural resource, the amount to be considered as principal for that period shall be the greater of the following:

(a) The amount that bears the same ratio to the fair inventory value of such natural resource, which in the case of a testamentary trust, unless a contrary intention appears in the will, shall be the value at date of death, or, in default thereof, its market value at the time the principal was established, or its cost if purchased later, as the number of units of the natural resource severed during the period bears to the total number of severable units of the natural resource estimated as having existed at the time the principal was established;

(b) The amount that bears the same ratio to the estimated value of the natural resource at the time of commencement of severance as the number of units of the natural resources severed during the period bears to the total number of severable units of the natural resource estimated as having existed at the time of commencement of such severance;

(c) An amount equal to that percentage of the net proceeds received as consideration for such permanent severance that is allowable as a deduction from gross income for depletion purpose under the federal income tax law then in effect at the time of severance or, if the federal income tax law then in effect makes no provision for the deduction of any stated percentage for depletion, or for any reason is not applicable to such natural resource, then fifteen percent of such net proceeds. Such disposition of net proceeds shall apply whether permanent severance commenced before or after the time the principal was established and without regard to the time whenthe instrument under which severance is being made was executed.

(2) Nothing in this section shall be construed to abrogate or extend any right which may otherwise have accrued by law to a tenant to develop or work such natural resources for his or her own benefit.