(1) The department shall assign all forest land to one of the following forest site productivity class designations:

(a) forest site productivity class IV (25 through 44.9 cubic feet of wood per acre per year) ;

(b) forest site productivity class III (45 through 64.9 cubic feet of wood per acre per year) ;

(c) forest site productivity class II (65 through 84.9 cubic feet of wood per acre per year) ;

(d) forest site productivity class I (85 and over cubic feet of wood per acre per year) .