In order to encourage the growing of such trees, now existing or hereafter produced, as will at the proper age be suitable for merchantable forest products, whether such be of natural reproduction or from seed sown, or trees planted out, or all combined, all surface land which may be set apart according to the provisions of this act, and exclusively used for growing such trees, is hereby constituted a separate and distinct class of land, to be known as auxiliary forest reserves.
32 P.S. § 61