The Secretary of Agriculture is authorized to accept lands contained within the exterior boundaries of national forests that are chiefly valuable for national forest purposes, and in return patent an equal value of other national forest land or authorize removal of an equal value of timber. [16 U.S.C. §485]
Either party to such an exchange may reserve timber, minerals, or easements, the value of which shall be considered in determining the value of the land. [16 U.S.C. §486]
The statute is available from the U.S. Government Publishing Office: https://www.gpo.gov/fdsys/pkg/USCODE-2017-title16/html/USCODE-2017-title16-chap2-subchapI-sec485.htm and https://www.gpo.gov/fdsys/pkg/USCODE-2017-title16/html/USCODE-2017-title16-chap2-subchapI-sec486.htm