The creation of a park district in an area composed of a portion or all of the townships of Mission and Shawnee in Johnson county, including not less than all of the cities, the boundaries of which are within the boundaries of such park district, is hereby authorized, and the government operation and financing set up as herein provided: Provided, That no lands or improvements located within such district which are otherwise exempt by law from ad valorem taxes, shall be subject to payment of any taxes levied by said district.