The rate of any tax levied on the activity of harvesting or severing from the ground row crops, timber or other plants shall be equal and uniform in every county in the state. However, any such tax levied by private act or otherwise prior to July 30, 1997, shall remain valid and in effect, but the rate of tax shall not be increased by private act after July 30, 1997. No such tax shall be levied by any city or county after July 30, 1997, unless authorized by general law.