Heat oven to 350°F. Grease bottom only of 15x10x1-inch pan with shortening. Make cake mix as directed on box using water, oil and eggs. Spread batter in pan.
Bake 20 to 25 minutes or until toothpick inserted in center comes out clean. Cool completely on cooling rack, about 45 minutes.
Spread vanilla frosting on top of cake. With toothpick, draw tree trunk on bottom 1/3 of cake. With chocolate frosting, fill in trunk and branches, using tines of fork to create bark on tree. Place candy leaves over top 2/3 of cake, leaving space between leaves.
With sharp knife, cut small slit in side of each gumdrop half. Insert one end of paper leaf in slit; place on cake.