National Forest Small Tracts Act Amendments Act of 2015 Amends the Small Tract Act of 1983 (the Act) to permit the sale, exchange, or interchange under such Act of National Forest System lands the sale or exchange of which is not practicable under any other authority of the Department of Agriculture (USDA) which have a value determined to be not more than $500,000. Includes as those lands permitted to be sold, exchanged, or interchanged under the Act: parcels of 40 acres or less that are determined to be physically isolated, to be inaccessible, or to have lost their National Forest character; parcels of 10 acres or less which are not eligible for conveyance under such Act, but which are encroached on by a permanent habitable improvement for which there is no evidence that the encroachment was intentional or negligent; or parcels used as a cemetery, a landfill, or a sewage treatment plant under a special use authorization issued by the USDA. Requires any proceeds under this Act to be deposited in the special fund established under the Sisk Act and made available for: the acquisition of land or interests in land for administrative sites for the National Forest System (NFS) in the state from which the amounts were derived; the acquisition of land or interests in land for inclusion in the NFS in that state, including those which enhance opportunities for recreational access; the performance of deferred maintenance on administrative sites for the NFS in that state or other deferred maintenance activities in that state which enhance opportunities for recreational access; and the reimbursement of the USDA for costs incurred in preparing a competitive sale conducted under the authority of the Act.