It is the declared policy of the state to encourage reforestation of cutover lands and timber culture generally; and to that end and in consideration of the public benefits arising therefrom, the timber growing on lands which shall hereafter be designated by the State Forestry Commission as auxiliary state forests under the provision of this article shall not be taxable or assessed for taxation by any authority from the time that said lands are so designated until they are withdrawn as auxiliary state forests, and only the land on which said timber grows may be taxed or assessed for taxation as if the ownership of the timber growing thereon had been severed from the ownership of the land, and the valuation of the timber growing on auxiliary state forests shall not be included in the valuation of the shares of stock in any domestic corporation owning such timber in arriving at the valuation of the shares of stock of such corporation for taxation; provided, that said land shall be appraised jointly by the Department of Revenue and the State Forestry Commission with view to its use for timber production purposes, such appraisal being made with due regard to the fact that the timber yields from such lands require a considerable period of years for maturing and that the valuation determined by such appraisal for the purposes of taxation of the land independently of the timber shall not be increased during the continuance of such land as auxiliary state forests; and provided further, that when the land embraced within an auxiliary state forest does not exceed 160 acres, the land shall not be taxed or assessed for taxation.