For fiscal year 2014 and thereafter, the National Forest Foundation may hold Federal funds made available but not immediately disbursed and may use any interest or other investment income earned (before, on, or after January 17, 2014) on Federal funds to carry out the purposes of Public Law 101–593: Provided further, That such investments may be made only in interest-bearing obligations of the United States or in obligations guaranteed as to both principal and interest by the United States.