Any tract of land while remaining in the class of auxiliary forest reserves as above provided, may, nevertheless, be sold or incumbered by or through the owner thereof, but no sale or incumbrance, whether voluntary by the owner or involuntary under any statutory or judicial proceeding whatsoever, whether of any State or of the United States, shall effect a discharge of any obligation imposed under this act, and said land shall be removed from said class only in accordance with the provisions hereof.