Heat oven to 350°F. In large bowl, break up cookie dough. Stir or knead in flour and orange peel until well blended. Work with half of dough at a time; refrigerate remaining dough until needed.
On floured surface with rolling pin, roll dough to 1/8-inch thickness (about 11-inch round). Cut with floured 3- to 3 1/2-inch tree-shaped cookie cutter; place 1 inch apart on ungreased cookie sheets. Repeat with remaining dough.
Bake 6 to 9 minutes or until cookies are set and edges just begin to brown. Cool 1 minute; remove from cookie sheets. Cool completely, about 10 minutes.
In small bowl, mix powdered sugar and enough orange juice until smooth and desired spreading consistency. Stir in green food color until well blended. Frost cooled cookies; sprinkle with nonpareils. Let stand until icing is set, about 15 minutes. Store between sheets of waxed paper in tightly covered container.